Seminar history    

2018-11-15 Thu Matt Allcock (University of Sheffield) SP2RC seminar
10:00 LT 10 Asymmetric Solar Waveguides: theory and observations
Are solar MHD waveguides symmetric? It is convenient to assume that they are. The solar physics community is familiar with the traditional notion of sausage and kink waves, which propagate along waveguides in the solar atmosphere that we assume are symmetric. In this talk, we drop this assumption and motivate the study of MHD wave propagation in asymmetric waveguides from theoretical and observational viewpoints. We discuss the implications that asymmetric waveguides have for mode identification, highlighting the observational ambiguity between waves in symmetric and asymmetric waveguides, which becomes a crucial consideration when implementing magneto-seismology diagnostics. We present a novel technique for solar magneto-seismology that utilises the observed asymmetry of MHD waves to diagnose background parameters of the solar atmosphere that are difficult to measure using traditional methods. We present a preliminary application of this technique to chromospheric fibrils as a proof-of-concept and discuss the potential further application to prominences, elongated magnetic bright points, and sunspot light walls.

2018-11-15 Thu Istvan Ballai (Sheffield) Plasma Dynamics Group
15:00 Room K14 (Hicks Building) Introduction to multiple scaling methods to solve differential equations with applications to plasma physics. Part I: Ordinary linear differential equations
Many of the equations we encounter in our research in solar and space plasma physics dynamics contain essential physical constraints (non-linearity, singularities, complex domains of interest, complex boundary conditions, etc.) that makes difficult to find exact solutions. Therefore, in order to obtain information about solutions of governing equations, we are forced to use analytical approximate methods, numerical solutions, or both. The most important analytical approximation methods are perturbation methods, where the solutions are represented by the first few terms of an expansion. In this seminar I will review perturbation methods used to solve ordinary differential equations, highlighting their advantages and shortcomings. The presentation will revolve around simple examples of differential equations, presenting methods of finding approximate analytical solutions of differential equations applicable to plasma physics.

2018-11-14 Wed Lukasz Grabowski (Lancaster) Pure Maths Colloquium
14:00 J11 Approximation of groups with respect to the rank metric.
I'll talk about an ongoing joint work with Gabor Elek about approximation of groups with respect two the rank metric. The basic question is the following variant of the Halmos problem about commuting matrices: if A and B are large matrices such that the rank of the image of the commutator is small, is it true that A and B can be perturbed with small rank matrices in such a way that the resulting matrices commute? There are interesting connections to classical notions of commutative algebra, in particular we develop what are perhaps some new (or forgotten) variants of Nullstellensatz for primary ideals.

2018-11-14 Wed James McLaughlin (Northumbria) Applied Mathematics Colloquium
14:00 Hicks, LT 9 Modelling Quasi-Periodic Pulsations in Solar and Stellar Flares
Solar flare emission is detected in all EM bands and variations in flux density of solar energetic particles. Often the EM radiation generated in solar and stellar flares shows a pronounced oscillatory pattern, with characteristic periods ranging from a fraction of a second to several minutes. These oscillations are referred to as quasi-periodic pulsations (QPPs), to emphasise that they often contain apparent amplitude and period modulation. We review the current understanding of quasi-periodic pulsations in solar and stellar flares. In particular, we focus on the possible physical mechanisms, with an emphasis on the underlying physics that generates the resultant range of periodicities. These physical mechanisms include MHD oscillations, self-oscillatory mechanisms, oscillatory reconnection/reconnection reversal, wave-driven reconnection, two loop coalescence, MHD flow over-stability, the equivalent LCR-contour mechanism, and thermal-dynamical cycles. We also provide a histogram of all QPP events published in the literature at this time. The occurrence of QPPs puts additional constraints on the interpretation and understanding of the fundamental processes operating in flares, e.g. magnetic energy liberation and particle acceleration. Therefore, a full understanding of QPPs is essential in order to work towards an integrated model of solar and stellar flares. Based on McLaughlin et al., 2018, Space Science Review, 214, 45,

2018-11-14 Wed Jordan Williamson (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks Equivariant Topology and Commutative Algebra
Equivariant topology is the study of spaces with a group action, and some invariants for studying these objects are equivariant cohomology theories. In this talk, we will explain how algebraic techniques can be used to study equivariant cohomology theories, and we will give a sketch-proof of a theorem of Greenlees-Shipley which classifies the equivariant cohomology theories on free G-spaces over the rational numbers. This will involve a discussion of Borel cohomology and its relation to representation theory, and the algebraicization theorem of Shipley, which provides a bridge between algebra and topology.

2018-11-13 Tue Philip G. Judge (High Altitude Observatory ) SP2RC seminar
13:00 LT 09 Restoring the observational basis for solar physics
SUMMARY: A simple near-UV polarimeter on board a spacecraft that is more than 0.1 radians away from the Earth-Sun line will, with the suite of terrestrial solar observatories, resolve all ambiguities in vector field measurements, permitting us to restore studies of solar magnetic fields to its proper, observationally-based place. I will show how the spectrum of Fe~I at UV and IR wavelengths can strengthen the foundations of solar physics with consequences for all subjects involving magnetic activity. MOTIVATION: In recent years, research in solar physics has arguably become divorced from genuinely penetrating measurements. The idea of refuting theoretical pictures with critical observations seems to be losing ground to the development and application of computer models as a prime tool, indeed some models appear to have superceded the Sun itself in terms of ``reality''. Funding agencies and peer review enable what I call "institutionalized science" which is designed from the outset to optimize the number of publications, leading to vast numbers of, at best, incremental advances. I will argue that our collective "institutions" need to reward bold new ideas that are risky. I will present one such idea that will enable us to place measurements of solar magnetism at stage center, recognizing that the variable magnetism lies at the core of essentially all problems of interest in solar physics.

2018-11-12 Mon George Constable (Bath) Mathematical Biology Seminar Series
14:00 Hicks LT10

2018-11-09 Fri Alexander Shukhobodskiy (Sheffield) SP2RC seminar
14:00 F28 Kink Oscillations of Expanding Coronal Loops in the Presence of Bulk Flow
Transverse coronal loop oscillations were first observed by TRACE in 1998 and reported by Aschwanden et al. (1999) and Nakariakov et al. (1999). One important property of transverse coronal loop oscillations is that they are usually strongly damped with the damping time being comparable with the oscillation period. However, sometimes this is not the case. At present, a generally accepted mechanism of this damping is resonant absorption. Observations show that very often oscillating coronal loops are in a highly dynamic state. In particular, they can cool quickly with a characteristic cooling time of the order of a few periods of kink oscillation. It was later showed theoretically that cooling causes amplification and may result in existence of oscillations for which amplitude does not vary in time. Although the coronal loop expansion is relatively small, the ratio of the loop cross-section radii at the apex and at the foot-points still can be about 1.5. These leads to particular interest the effect of expansion on kink oscillations. A coronal loop is modeled as a cylindrical magnetic flux tube. The tube consists of a core region and a thin transitional region at the tube boundary. The plasma density monotonically decreases from its value in the core region to the value outside the tube. Both the plasma density and velocity of background flow vary along the tube and in time. Using multiscale expansions, the system of two equations describing the kink oscillations was derived. This model is then studied both analytically and numerically.

2018-11-06 Tue Vlad Serban (Vienna) Number Theory seminar
14:00 J11 A finiteness result for families of Bianchi modular forms
We develop a p-adic "unlikely intersection” result and show how it can be used to examine which Hida families over imaginary quadratic fields interpolate a dense set of modular forms for GL2 over an imaginary quadratic field. In this way we arrive at the first proven examples where only finitely many classical automorphic forms are on a p-adic family.

2018-11-02 Fri Professor Craig J. Rodger (University of Otago) SP2RC seminar
14:00 Sir Henry Stephenson Building, LT01 And then the Sun went "Bang": An Overview of Space Weather Research
The Sun is the main provider of energy for the Earth; without it we would surely die. However, the Sun is not just a huge light bulb sending heat and light to us - it is a gigantic fiery ball of burning gas on which the largest explosions in our solar system take place. The highly dynamic Sun affects the Earth in multiple ways. We are only just starting to understand how the Sun drives "Space Weather" - changes in the environment on and around the Earth which affect our technological systems. In my colloquia I will give an overview of this research field, and provide some specific examples around hazards to Earth-orbiting satellites and electrical transmission networks.

2018-11-01 Thu David Kuridze (Aberystwyth University) SP2RC seminar
10:00 LT 10 Spectropolarimetric Inversions of the Ca II 8542 Å Line in an M-class Solar Flare
We study an M1.9-class solar flare (SOL2015-09-27T10:40 UT) using high-resolution full Stokes imaging spectropolarimetry of the Ca II 8542 Å line obtained with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the non-LTE code NICOLE are used to construct semi-empirical models of the flaring atmosphere to investigate the structure and evolution of the flare temperature and magnetic field. A comparison of the temperature stratification in flaring and nonflaring areas reveals strong heating of the flare ribbon during the flare peak. The polarisation signals of the ribbon in the chromosphere during the flare maximum become stronger when compared to its surroundings and to pre- and post-flare profiles. Furthermore, a comparison of the response functions to perturbations in the line-of-sight magnetic field and temperature in flaring and nonflaring atmospheres shows that during the flare, the Ca II 8542 Å line is more sensitive to the lower atmosphere where the magnetic field is expected to be stronger. The chromospheric magnetic field was also determined with the weak-field approximation, which led to results similar to those obtained with the NICOLE inversions.

2018-11-01 Thu Sam Marsh and Simon Willerton (Sheffield) Teaching Lunch
13:00 LT6 Things we've tried in 115.
In MAS115 Mathematical Investigation Skills -- where the students learn programming and LaTeX amongst other things -- we have experimented with various ideas including peer assessment, video marking, group work, students creating mathematical websites, in-class marking of homework. Usually we try to think of things which will benefit the students, but not increase our workload overly. We will present a smorgasbord of things we've tried and comment on how successful they've been, hopefully giving other people ideas along the way.

2018-11-01 Thu Samuel Skirvin (Sheffield) Plasma Dynamics Group
15:00 Room K14 (Hicks Building) Properties of Alfvénic waves in the solar chromosphere
In the first part of my talk I will discuss the results of investigation of the properties of transverse waves existing in spicules using the automated wave tracking code NUWT. Analysing a distance-time diagram at an altitude of 7 Mm relative to the solar limb produces the measured distribution of properties such as wave amplitude, period and velocity amplitude. In the second part of the talk I will provide an overview of the recent studies on the effect of initial flow profiles on the dynamics of solar jets and introduce the work I will be doing as part of my PhD project

2018-11-01 Thu Markus Szymik (NTNU) Topology seminar
16:00 J11 Quandles, knots, and homotopical algebra
Knots and their groups are a traditional topic of geometric topology. In this talk I will explain how aspects of the subject can be approached using ideas from Quillen’s homotopical algebra, rephrasing old results and leading to new ones.

2018-10-31 Wed Miguel Teixeira (Reading) Applied Mathematics Colloquium
14:00 Hicks, LT 9 A physically-based model for the wind-driven current in the wavy oceanic surface layer
A simple analytical model is developed for the current induced by the wind and modified by surface wind-waves in the oceanic surface layer, based on a first-order turbulence closure and including the effect of a vortex force representing the Stokes drift of the waves. The shear stress is partitioned between a component due to shear in the current and a wave-induced component, which decays over a depth proportional to the wavelength. The model reproduces the apparent reduction of the friction velocity and enhancement of the roughness length estimated from current profiles, detected in a number of studies. The current profile becomes flatter for strong wave forcing owing to a smaller fraction of the total shear stress being supported by the current shear. These effects are entirely attributed to non-breaking surface waves, and predicted to increase with wave forcing. A version of the model where the shear stress decays to zero with depth is able to adequately predict the surface current speed.

2018-10-30 Tue Adel Betina (Sheffield) Number Theory seminar
14:00 J11 On the p-adic periods of semi-stable modular curves
I will present a joint work with E.Lecouturier in which we prove a variant of Oesterlé's conjecture about $p$-adic periods of the modular curve $X_0(p)$, with an additional $Γ(2)$-structure. We use de Shalit's techniques and $p$-adic uniformization of Mumford curves whose reduction is semi-stable.

2018-10-29 Mon Elaine Ferguson (Glasgow) Mathematical Biology Seminar Series
14:00 Hicks F41

2018-10-25 Thu Anna Marie Bohmann (Vanderbilt) Topology seminar
16:00 J11 Graded Tambara Functors
Let G be a finite group. The coefficients of G-equivariant cohomology theories naturally form a type of structure called a Mackey functor, which incorporates data coming from each subgroup of G. When the cohomology theory is a G-ring commutative spectrum---meaning that is has an equivariant multiplication---interesting new structures arise. In particular, work of Brun and of Strickland shows that the zeroth homotopy groups have norm maps which yield the structure of a Tambara functor. In this talk, I discuss joint work with Vigleik Angeltveit on the algebraic structure induced by norm maps on the higher homotopy groups, which we call a graded Tambara functor.

2018-10-24 Wed Christian Voigt (Glasgow) Pure Maths Colloquium
14:00 J11 $C^*$-algebras, the Baum-Connes conjecture and quantum groups
A central theme of research in operator algebras is the Baum-Connes conjecture, which predicts the K-theory of group $C^*$-algebras and crossed products. In this talk I will give a leisurely introduction to this conjecture, explain what it is good for, and discuss some recent connections with the theory of quantum groups.

2018-10-24 Wed Istvan Cziegler (York) Applied Mathematics Colloquium
14:00 Hicks, LT 11 Turbulence and phase transitions in tokamak plasmas
The transition from the low- to the high-confinement operation is one of the most important phenomena in magnetic confinement fusion. The high-confinement regime, known as H-mode, leads to a vastly increased plasma density and temperature, which equates to a significant gain in fusion power. Since the dominant transport across the confining magnetic field is due to turbulence, the L-H transition can be thought of as a phase transition to suppressed turbulence. It is known that both the quality of global confinement and the threshold of the transition depend on macroscopic parameters, such as plasma density, magnetic topology and geometry near particle exhaust areas called divertors. The talk will connect the various scales of dynamics with this phenomenology and some broader context in physics.

2018-10-24 Wed James Brotherston, Callum Reader, Sadiah Zahoor ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks
What is Forcing? (Reader)

I will give an introduction to the main themes behind Cohen's forcing method in ZFC, as framed by Scott through the lens of Boolean algebras. Given time there will also be some discussion of why, contrary to Godel's belief, forcing shows that the inclusion of large cardinal axioms does not solve the continuum hypothesis

A Calculation of Some Group Cohomology (Brotherston)

I will give a very brief introduction to group cohomology and why we care in relation to algebraic K-theory. I will then move on to outline a mostly geometric calculation of the former in the case where $G = GL_n(\mathbb{F}_q)$ with coefficients in $\mathbb{Z}/l$ for $l$ coprime to the characteristic of the field $\mathbb{F}_q$. There will hopefully be a little of number theory, algebraic topology and algebraic geometry mentioned.

Tate Shafarevich Groups - The mysterious objects attached to Elliptic Curves (Zahoor)

One of the main unsolved mystery about elliptic curves is the size of the Tate Shafarevich Group (Sha), that is conjectured to be finite. The lack of proof of this particular fact is one of the main barriers in giving an algorithm to compute rank of an elliptic curve. Intuitively, Sha measures the obstruction to the Hasse (local-global) principle. This talk aims at understanding Tate Shafarevich Groups using torsors of elliptic curves and is deigned for anyone having a first look at the Sha.

2018-10-23 Tue Kim Klinger-Logan (Minnesota) Number Theory seminar
14:00 J11 The Riemann Hypothesis and periods of Eisenstein series
This summer at Perspectives on the Riemann Hypothesis, Bombieri and Garrett discussed modifications to the invariant Laplacian $\Delta=y^2(\partial_x^2+\partial_y^2)$ on $SL_2(\mathbb{Z})\backslash\mathfrak{H}$ possibly relevant to RH. We will present a $GL(2)$ $L$-function as a period of Eisenstein series which can, in turn, be thought of as a linear functional on an Eisenstein series and we will discuss how such functionals may be use to analyze the zeros of the $L$-function. This idea is an extension of recent work of Bombieri and Garrett and uses techniques from functional analysis and spectral theory of automorphic forms.

2018-10-23 Tue Nebojsa Pavic (Sheffield) Algebra / Algebraic Geometry seminar
16:00 J11 Grothendieck groups and singularity categories of quotient singularities
We study the K-theory of the Buchweitz-Orlov singularity category for quasi-projective algebraic schemes. Particularly, we show for isolated quotient singularities with abelian isotropy groups that the Grothendieck group of the singularity category is finite torsion and that rational Poincare duality is satisfied on the level of Grothendieck groups. We consider also consequences for the resolution of singularities of such quotient singularities and study dual properties in this setting, more concretely we prove a conjecture of Bondal and Orlov in the case of quotient singularities.

2018-10-18 Thu Gary Verth (Sheffield) Plasma Dynamics Group
15:00 Room K14 (Hicks Building) Introduction to the Sun
This talk will be an introduction to the science required to understand the Sun and its atmosphere. It is primarily intended for students starting their postgraduate research in plasma, solar, or magnetospheric physics. Due to the introductory nature of the talk, it would also be suitable for any interested non-specialists.

2018-10-18 Thu Simon Willerton (Sheffield) Topology seminar
16:00 J11 The Legendre-Fenchel transform from a category theoretic perspective
The Legendre-Fenchel transform is a classical piece of mathematics with many applications. In this talk I'll show how it arises in the context of category theory using categories enriched over the extended real numbers $\overline{ \mathbb{R}}:=[-\infty,+\infty]$. It turns out that it arises out of nothing more than the pairing between a vector space and its dual in the same way that the many classical dualities (eg. in Galois theory or algebraic geometry) arise from a relation between sets.
I will assume no knowledge of the Legendre-Fenchel transform and no knowledge of enriched categories.

2018-10-17 Wed Kevin Buzzard (Imperial) Pure Maths Colloquium
14:00 Hicks Lecture Theatre C Pure mathematics in crisis?
I argue that pure mathematics is walking inexorably towards a cliff edge, and that anyone who believes that current pure mathematics is rigorous, or a science, needs to wake up and look at the facts, which there will be plenty of in this talk, and they are not pretty. Are our results reproducible? Does it matter? What *is* mathematics? Can computer scientists save us? Can *undergraduates* save us? I hope so. This talk is about pure mathematics but will be accessible to undergraduates, mathematicians both pure and applied/applicable and computer scientists.

2018-10-16 Tue Kohei Kikuta (Osaka) Algebra / Algebraic Geometry seminar
16:00 J11 On categorical entropy
The categorical entropy of triangulated endofunctors was defined by Dimitrov-Haiden-Katzarkov-Kontsevich motivated by classical dynamical theory. In this talk, I'll explain relations to classical entropy theory, basics on categorical entropy, many examples and future directions.

2018-10-15 Mon Helen Kettle (BIOSS) Mathematical Biology Seminar Series
14:00 Hicks F41

2018-10-09 Tue Angelo Rendina (Sheffield) Number Theory seminar
14:00 J11 Nearly holomorphic Siegel modular forms and applications
Nearly holomorphic modular forms were introduced by Shimura as a generalization of modular forms to study a more general class of Eisenstein series. I will introduce some of the tools that we use to work with them, such as the Shimura-Maass differential operator and holomorphic projection, and present some applications: some formulae for the sum of divisor $s_r$ and Ramanujan $\tau$ functions and then congruences of critical $L$-values attached to Siegel modular forms, the latter being part of my research project.

2018-10-03 Wed Priya Subramanian (Leeds) Applied Mathematics Colloquium
14:00 Hicks, LT 9 Pattern formation in systems with multiple scales

2018-10-01 Mon Rachel Bearon (Liverpool) Mathematical Biology Seminar Series
14:00 Hicks F41 Revisiting Jeffery orbits; the importance of shape for micro-organism transport

2018-09-26 Wed Yang Zhang (Sheffield (Mech Eng)) Applied Mathematics Colloquium
14:00 Hicks, LT 9 Imaging based flame diagnostics and its quantitative analysis
In this talk, a brief history of photography and their application in combustion studies will be given, which actually goes back to the 19th century. Then case studies will be shown on digital flame imaging, especially high speed imaging for the tracking of the fast flame dynamics. It will demonstrate that selective digital imaging enhancement is essential in observing the drastically different flame light intensities of the soot and chemical species. Through digital image processing, quantitative and useful information can be extracted from the flame image database. The simultaneous imaging of visible and infrared light emissions from the ignition of a flame using a single high speed colour digital camera will also be demonstrated which also poses a challenge in spectroscopic study on how to identify this infrared only emissions.

2018-08-07 Tue Kirill Mackenzie (Sheffield) Differential geometry seminar
14:00 LT 7 Quotients of Lie algebroids
For a transitive Lie algebroid $A$, and an ideal in the adjoint bundle (= kernel of the anchor), there is a simple construction of a quotient Lie algebroid over the same base, and this has the usual properties. When the base manifold is also quotiented, the situation is more complicated. This talk will describe the general quotient construction, starting with the case of vector bundles. I'll assume a basic familiarity with Lie algebroids. All are welcome.

2018-07-17 Tue David Spencer (Sheffield) Number Theory seminar
14:00 J11 Congruences of local origin for higher levels
There are many kinds of congruences between different types of modular forms. The most well known of which is Ramanujan's mod 691 congruence. This is a congruence between the Hecke eigenvalues of the weight 12 Eisenstein series and the Hecke eigenvalues of the weight 12 cusp form (both at level 1). This type of congruence can be extended to give congruences of ''local origin''. In this talk I will explain what is meant by such a congruence while focusing on the case of weight 1. The method of proof in this case is very different to that of higher weights and involves working with Galois representations and ray class characters.

2018-07-06 Fri Christian Voigt (Glasgow) K-Theory, Hecke Algebras and Representation Theory
09:30 Lecture Theatre C Categorification and Hecke algebras
The idea of categorification is to replace set theoretic constructions and theorems by category theoretic analogues, recovering the original constructions via taking isomorphism classes or K-groups. In this talk I’ll discuss some examples of this procedure related to Hecke algebras and connections to noncommutative geometry.

2018-07-06 Fri Maarten Solleveld (Nijmegen) K-Theory, Hecke Algebras and Representation Theory
11:00 Lecture Theatre C Topological K-theory of affine Hecke algebras

An affine Hecke algebra can be completed to a C*-algebra. These algebras appear in the theory of reductive p-adic groups, and they are of interest in representation theory and in relation with the Baum--Connes conjecture.They provide typical examples of C*-algebras which are close to commutative.

In this talk I will discuss results about the K-theory of such C*-algebras, and techniques used to study it. In particular, I will show that the K-theory does not depend on the deformation parameter of the Hecke algebra. In the end all calculations will be reduced to equivariant K-theory of topological spaces, with respect to certain nice actions of finite groups. I will show that under mild conditions these equivariant K-groups are torsion-free.

2018-07-06 Fri Sergio Mendes (ISCTE Lisbon) K-Theory, Hecke Algebras and Representation Theory
14:00 Lecture Theatre C On L-packets and depth for SL2(K) and its inner forms

An invariant that makes sense on both sides of the local Langlands correspondence is depth. In this talk we survey the notion of depth and study depth-preservation under the local Langlands correspondence. Examples will be provided with special emphasis to the group SL2 over a local field with characteristic 2.

This talk is based on joint work with Anne-Marie Aubert, Roger Plymen and Maarten Solleveld.

2018-07-06 Fri Anne-Marie Aubert (Paris 6) K-Theory, Hecke Algebras and Representation Theory
15:30 Lecture Theatre C Affine Hecke algebras on the Galois side

We will explain a way to attach affine Hecke algebras to certain Langlands parameters on Levi subgroups of a given p-adic reductive group in relation with the ABPS-conjecture.

This is joint work with Ahmed Moussaoui and Maarten Solleveld.

2018-07-05 Thu Peter Hochs (Adelaide) K-Theory, Hecke Algebras and Representation Theory
09:30 Lecture Theatre C K-types of tempered representations and index theory
Let G be a semisimple Lie group. Tempered representations of G are the ones occurring in the Plancherel decomposition of $L^2$(G). They are also relevant to the Langlands classification of the more general admissible representations. In joint work with Yanli Song and Shilin Yu, we realise the restriction of any tempered representation to a maximal compact subgroup K as an equivariant index. This is a concrete expression of Kirillov's orbit method. A consequence of this realisation is a geometric expression for the multiplicities of the irreducible representations of K in that restriction. (The irreducible representations that occur are the K-types of the representation.) This helps to study the general behaviour of those multiplicities. As an example, we show that admissible representations of SU(p,1) and SO_0(p,1) restrict multiplicity-freely to maximal compact subgroups. That was proved earlier by Koornwinder, but now illustrates our multiplicity formula.

2018-07-05 Thu Henrik Schlichtkrull (Copenhagen) K-Theory, Hecke Algebras and Representation Theory
11:00 Lecture Theatre C Harmonic analysis on real spherical spaces
Let G be a real reductive Lie group. A homogeneous space Z of G is called real spherical if the minimal parabolic subgroups of G have only finitely many orbits on Z. For example, the Bruhat decomposition of G implies that Z=G is real spherical for the two-sided action of G$\times$G. A survey will be given of some recent progress (by F. Knop, B. Krötz, and others) on the generalization of Harish-Chandra's harmonic analysis to such spaces.

2018-07-05 Thu Beth Romano (Cambridge) K-Theory, Hecke Algebras and Representation Theory
14:00 Lecture Theatre C The local Langlands correspondence in small residue characteristic
Through explicit examples, I'll discuss why the local Langlands correspondence becomes mysterious for small residue characteristic. I'll focus on examples and conjectures related to ``epipelagic" representations, which have minimal positive depth.

2018-07-05 Thu Tyrone Crisp (Nijmegen) K-Theory, Hecke Algebras and Representation Theory
15:30 Lecture Theatre C Parabolic induction over the p-adic integers
For p-adic reductive groups like GL(n,Q_p), the right-hand side of the Baum-Connes conjecture --- i.e., the K-theory of the group C*-algebra --- is in many respects better understood than the left-hand side. This unusual state of affairs is due to the extremely complicated representation theory of compact p-adic groups like GL(n,Z_p). In this talk I shall present an ongoing program, joint with Ehud Meir and Uri Onn, that aims to understand the representations of these compact groups in terms of parabolic induction from Levi subgroups, analogously to the way one usually studies representations of real, complex, p-adic, and finite reductive groups.

2018-07-04 Wed Hang Wang (Adelaide / East China Normal) K-Theory, Hecke Algebras and Representation Theory
09:30 Lecture Theatre C Role of local Langlands correspondence in K-theory of group C*-algebras
K-theory of C*-algebras associated to a Lie group can be understood both from the geometric point of view via Baum-Connes assembly map and from the representation theoretic point of view via harmonic analysis of Lie groups. Inspired by the local Langlangds correspondence and work by Plymen and collaborators, one can study relations between two groups, where their L-parameters are related in a nice way, from the aspects of K-theory and index theory of invariant elliptic operators. I will introduce two examples I investigated with Peter Hochs (when the two groups are inner forms to each other) and with Kuok Fai Chao (when there is a base change involved between the L-parameters of the two groups).

2018-07-04 Wed Alexandre Afgoustidis (Paris 9) K-Theory, Hecke Algebras and Representation Theory
11:00 Lecture Theatre C On the tempered dual of a real reductive group and that of its Cartan motion group

Given a reductive Lie group G and a maximal compact subgroup K, one can consider the isometry group of the (flat) tangent space to G/K at the identity coset: this is a first-order approximation of G near K, called the Cartan motion group of G. George Mackey’s early work on semi-direct products describes its unitary representations in very simple and concrete terms.

In the 1970s, Mackey noticed that his parametrization for the representations of the motion group showed unexpected similarities with Harish-Chandra’s more subtle parametrization for the tempered representations of G. Motivated by quantum-mechanical considerations related with the existence of a one-parameter family of Lie groups interpolating between both groups, he suggested that a kind of rigidity of representation theory along the deformation may be observed in general. Alain Connes and Nigel Higson later pointed out that the Baum-Connes-Kasparov isomorphism in operator K-theory can be viewed, for real reductive groups, as a cohomological reflection of Mackey’s ideas. For the special case of complex semisimple groups, Nigel Higson gave in 2008 a precise form to Mackey’s analogy and its relationship with the Baum-Connes-Kasparov isomorphism.

For real reductive groups, I will describe a natural one-to-one correspondence between the tempered and admissible duals of both groups, and discuss some geometrical (or topological) aspects of the rigidity revealed by the correspondence along the deformation from one group to the other.

2018-07-04 Wed Pierre Clare (William&Mary) K-Theory, Hecke Algebras and Representation Theory
14:00 Lecture Theatre C On the reduced C*-algebra of real reductive groups
I will report on joint work with Nigel Higson regarding the description up to Morita equivalence of the reduced C*-algebra of a class of real reductive groups. The results build on previous work, joint with Tyrone Crisp, and are related to the approach to the Connes-Kasparov isomorphism promoted by Roger Plymen and others.

2018-07-04 Wed Nigel Higson (PennState) K-Theory, Hecke Algebras and Representation Theory
15:30 Lecture Theatre C On (some of) the work of Roger Plymen
I shall give an appreciation of some of the fundamental contributions of Roger Plymen to the themes of this conference, focusing on his studies of the C*-algebras of real and p-adic groups, and their K-theory groups.

2018-07-03 Tue Bram Mesland (MPIM Bonn) K-Theory, Hecke Algebras and Representation Theory
14:30 Lecture Theatre C A Hecke module structure on the KK-theory of arithmetic groups

Let G be a locally compact group, H a discrete subgroup and C(G,H) the commensurator of H in G. The cohomology of H is a module over the Shimura Hecke ring of the pair (H,C(G,H)). This construction recovers the action of the Hecke operators on modular forms for SL(2,Z) as a particular case. In this talk I will discuss how the Shimura Hecke ring of a pair (H, C(G,H)) maps into the KK-ring associated to an arbitrary H-C*-algebra. From this we obtain a variety of K-theoretic Hecke modules. In the case of manifolds the Chern character provides a Hecke equivariant transformation into cohomology, which is an isomorphism in low dimensions. We discuss Hecke equivariant exact sequences arising from possibly noncommutative compactifications of H-spaces. Examples include the Borel-Serre and geodesic compactifications of the universal cover of an arithmetic manifold, and the totally disconnected boundary of the Bruhat-Tits tree of SL(2,Z).

This is joint work with M.H. Sengun (Sheffield).

2018-07-03 Tue Heath Emerson (Victoria) K-Theory, Hecke Algebras and Representation Theory
16:00 Lecture Theatre C Noncommutative Lefshetz fixed-point formulas via KK-theory
Using the idea of K-theoretic Poincaré duality, it is possible to formulate an analogue of the classical Lefschetz fixed-point formula from basic algebraic topology, which applies to KK-endomorphisms (e.g. ordinary C*-algebra endomorphisms) of a C*-algebra equipped with such a duality. We discuss the general methodology and apply it in the example of the C*-algebra crossed-product of a discrete group acting properly, smoothly and co-compactly on a smooth manifold. The result is an `orbifold' Lefschetz formula of some interest; our hope is that many other examples should exist.

2018-06-28 Thu Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Hida families (a la Pilloni)

2018-06-27 Wed Ariel Weiss (Sheffield) Number Theory Learning Seminar
14:00 J-11 Hida families (classical treatment)

2018-06-25 Mon Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Classicality of overconvergent modular forms

2018-06-21 Thu Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Classicality

2018-06-18 Mon Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 The canonical subgroup

2018-06-14 Thu Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Introduction to rigid analytic geometry- Part (3)

2018-06-11 Mon Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Introduction to rigid analytic geometry- Part (2)

2018-06-07 Thu Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Introduction to rigid analytic geometry- Part (1)

2018-06-06 Wed Ciaran Schembri (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks Fuchsian group
Fuchsian groups are discrete subgroups of the special projective linear group. They act as isometries on the hyperbolic plane and are studied because of their role in generating Riemann surfaces among other things. In this talk I will outline their geometric properties and if time permits will discuss how they relate to modern number theory.

2018-06-04 Mon Ariel Weiss (Sheffield) Number Theory Learning Seminar
14:00 J-11 Hasse invariant

2018-05-31 Thu S. Shelyag (Northumbria University)
10:00 LT 10 Do photospheric non-magnetic bright points exist?
Recent high-resolution simulations of non-magnetic solar photospheric convection suggest the presence of a separate class of small-scale photospheric brightenings, which coincide with intergranular vortex tubes. In contrast to well-known magnetic bright points, these brightenings are not related to magnetic fields. In our presentation, using high-resolution simulations with MURaM and detailed radiative diagnostics of the simulated models, I will analyse the physical characteristics of these brightenings and their observability with current and future instruments for solar observations.

2018-05-31 Thu Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Modular curve- Part 2

2018-05-30 Wed Nebojsa Pavic (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks Intersection theory in algebraic geometry: History and motivation.
In this talk I'm going to motivate the notion of intersection theory in algebraic geometry by considering the example of Riemannian surfaces and only requiring a basic knowledge of complex analysis and a little bit of complex differential geometry. If time permits, I will give a rigorous definition of intersection groups, so called Chow groups, and relate them to the example.

2018-05-29 Tue Scott Balchin (Sheffield) Magnitude Homology
13:00 J11 Mangitude homology and persistent homology??

2018-05-29 Tue Elisa Postinghel (University of Loughborough) Algebra / Algebraic Geometry seminar
14:00 J11

2018-05-29 Tue Jessie Durk (Queen Mary UL) Cosmology, Relativity and Gravitation
16:00 LT09, Hicks Black hole lattices as inhomogeneous cosmological models
The standard model of cosmology, ΛCDM, is based on the assumption that the Universe can be described by the homogeneous and isotropic FLRW solution to the Einstein field equations. The need to test whether the large-scale expansion of space is that of FLRW, or is instead modified by the presence of inhomogeneities, has lead to this assumption being relaxed. An interesting subset of inhomogeneous cosmologies include those dubbed black hole lattices. These are exact, fully-relativistic treatments of universes with a discretised matter content. We generalise an existing family of these to include a cosmological constant, structure formation and electric charge. For each new generalisation, we find a common behaviour of tending towards FLRW-like as the number of masses is increased, and for the addition of structures, we investigate the effect of gravitational interaction energies between clustered masses.

2018-05-24 Thu Adel Betina (Sheffield) Number Theory Learning Seminar
14:00 J-11 The modular curve

2018-05-23 Wed Ciaran Meachan (University of Glasgow) Algebra / Algebraic Geometry seminar
13:00 Hicks LT09 Derived equivalent Hilbert schemes of points on K3 surfaces which are not birational
Starting with two non-birational derived equivalent K3 surfaces, one can ask whether their Hilbert schemes of points are birational. In this talk, we will show that in some cases they are but in most cases they are not. This is joint work with Giovanni Mongardi and Kota Yoshioka.

2018-05-23 Wed Daniil Proskurin (Kiev Taras Shevchenko University) Pure Maths Colloquium
14:00 J11 $C^*$-algebras generated by quonic commutation relations and extensions of non-commutative tori
We consider $C^*$-algebras $A_{q_i}\Theta$ generated by relations of the following form $$a_i^*a_i=1+q_i a_ia_i^* a_i^*a_j=e^{2\pi\theta_{ij}} a_ja_i^*, \quad i\ne j i, \quad j=1,\ldots,d $$ where $-1 \lt q_i \lt 1$, $\theta_{ij}=-\theta_{ji}$, $i\ne j$.
We show that $A_{q_i}\Theta \simeq A_{0}\Theta$ is an extension of higher-dimensional non-commutative tori and study its properties.

2018-05-23 Wed Xiaolei Zhao (Northeastern University) Algebra / Algebraic Geometry seminar
15:00 Hicks LT09 Twisted cubics on cubic fourfolds and stability conditions
It is a classical result of Beauville and Donagi that Fano varieties of lines on cubic fourfolds are hyper-Kahler. More recently, Lehn, Lehn, Sorger and van Straten constructed a hyper-Kahler eightfold out of twisted cubics on cubic fourfolds. In this talk, I will explain a new approach to these hyper-Kahler varieties via moduli of stable objects on the Kuznetsov components. Along the way, we will derive several properties of cubic fourfolds as consequences. This is based on a joint work with Chunyi Li and Laura Pertusi.

2018-05-23 Wed Chunyi Li (University of Warwick) Algebra / Algebraic Geometry seminar
16:15 Hicks LT09 Bogomolov type inequality for Fano varieties with Picard number 1
I will talk about some basic facts about slope stable sheaves and the Bogomolov inequality. New techniques from stability conditions will imply new stronger bounds on Chern characters of stable sheaves on some special varieties, including Fano varieties, quintic threefolds and etc. I will discuss the progress in this direction and some related open problems.

2018-05-22 Tue Paolo Stellari (Universita' degli studi di Milano) Algebra / Algebraic Geometry seminar
14:00 J11 TBA

2018-05-22 Tue Joseph Fernandez (Liverpool John Moores) Cosmology, Relativity and Gravitation
16:00 J11, Hicks Tidal encounters of compact binaries with massive black holes as a source of gravitational waves
Massive black holes (MBHs) are ubiquitous in galactic centres. The extreme potential due to these objects dominates the surrounding dynamics, giving rise to physics not possible in other regions. These regions are of particular interest for gravitational wave astronomy, as several dynamical processes which can give rise to black hole (BH) binary mergers have been postulated. We show that compact binaries can survive close encounters with the MBH without being disrupted, and that they tend to become hard and eccentric. Since the GW merger time of binaries is sensitive to the semi-major axis length and eccentricity, we find that this leads to a reduction of the merger time by several orders of magnitude in some cases. Therefore, tidal encounters of stellar mass BH binaries with a MBH at the centre of galaxies can provide a new formation channel of BH mergers. We use Monte Carlo simulations to evaluate the effective spin of the binaries after the encounter We also provide a description of a simple scenario to understand how this process could take place the a larger astrophysical context.

2018-05-21 Mon Angelo Rendina (Sheffield) Number Theory Learning Seminar
14:00 J-11 Geometric definition of modular forms

2018-05-17 Thu Frazer Jarvis (Sheffield) Number Theory seminar
14:00 F24 $p$-adic periods of genus 2 curves via the AGM
The arithmetic-geometric mean provides the fastest way to compute periods of elliptic curves, both over the complex and $p$-adic numbers. There is an isogeny of genus 2 curves which looks like it might play the same role to compute periods for curves of genus 2. In this talk, we will discuss progress in developing an algorithm for the $p$-adic case, where $p$-adic periods were defined and first investigated in Teitelbaum's thesis. It is as yet incomplete, but the only missing step is an explicit Tate uniformisation for genus 2 curves. This is joint work with Rudolf Chow, and relates to the final chapter of his PhD thesis.

2018-05-16 Wed Shaun Stevens (University of East Anglia) Pure Maths Colloquium
14:00 J11 Towards an explicit local Langlands correspondence for classical groups
The local Langlands correspondence is a web of sometimes conjectural correspondences between, on the one hand, irreducible representations of reductive groups over a p-adic field F and, on the other hand, certain representations of the absolute Weil group of F (which is almost the absolute Galois group). I will try to explain what the objects involved here are, some of what the correspondence predicts and what is known/unknown, as well as work (particularly due to Bushnell and Henniart for general linear groups) towards making the correspondence explicit. Hopefully I will also explain some joint work (with Blondel and Henniart) where we describe the "wild part'' of the correspondence for symplectic groups.

2018-05-16 Wed Ati Sharma (Southampton) Applied Mathematics Colloquium
14:00 Hicks, LT 9 Some recent developments in the low-order modelling of fluid flows
Modelling fluid flows is a difficult problem. Fluid flows are well described by the Navier-Stokes equations, but these are nonlinear PDEs, which are difficult to solve in a general way. Much recent work has focused on finding low-dimensional approximations to fluid flow systems, either by abstracting them from data generated from experiment and simulation or by finding suitable approximations to the equations. This talk will discuss two such approaches, Dynamic Mode Decomposition (DMD) and resolvent analysis. The approaches will be explained, and a variety of recent applications to flow analysis and estimation will be presented.

2018-05-16 Wed Arend Bayer (University of Edinburgh) Algebra / Algebraic Geometry seminar
15:30 J11 Families of Hyperkaehler varieties via families of stability conditions
Abstract: Stability conditions on derived categories of algebraic varieties and their wall-crossings have recently been used extensively to study the geometry of moduli spaces of stable sheaves. In work in progress with Macri, Lahoz, Nuer, Perry and Stellari, we are extending this toolkit to the "relative" setting, i.e. for a family of varieties. Our construction comes with relative moduli spaces of stable objects; this gives additional ways of constructing new families of varieties from a given family, thereby potentially relating different moduli spaces of varieties. Our main application is for families of cubic fourfolds; in particular, this produces many new examples of algebraically constructed families of Hyperkaehler varieties over a base of maximal dimension 20.

2018-05-16 Wed *No talk* ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks

2018-05-15 Tue Simon Willerton (Sheffield) Magnitude Homology
13:00 J11 A graph with torsion in magnitude homology (after Kaneta and Yoshinaga)

2018-05-15 Tue Damiano Testa (University of Warwick) Algebra / Algebraic Geometry seminar
14:00 J11 Plane quartics and their inflection lines
Let C be a general plane quartic curve. It follows from the Plücker formulas that C admits 24 inflection lines. We address the question of finding all the plane quartics D, having the same 24 inflection lines as C. In joint work with Marco Pacini, we show that, over fields of characteristic coprime with 6, the curve C is uniquely determined by its configuration of inflection lines.

2018-05-15 Tue Damien Trinh (Manchester) Cosmology, Relativity and Gravitation
16:00 J11, Hicks The dark sector in light of GW170817
Recent observations in the propagation of gravitational waves have seemingly severely restricted viable alternatives to the cosmological constant. Indeed, the stringent constraint that gravitational waves must propagate at the speed of light is not something which many modified gravity theories predict. Using the Equation of State approach, I will discuss its implications to some theories and also highlight some caveats to the recent observations which may yield a slightly more optimistic outlook for this field.

2018-05-14 Mon Kayla King (Oxford) Mathematical Biology Seminar Series
13:00 Alfred Denny LT1

2018-05-11 Fri Siung Ghai (University of Sheffield) SP2RC seminar
12:00 LT 11 A statistical study of ionopause perturbation and associated boundary wave formation at Venus.

2018-05-10 Thu Fredrik Stromberg (Nottingham) Number Theory seminar
14:00 F24 Spectral theory and Maass forms for noncongruence subgroups
The spectral theory for congruence subgroups of the modular group is fairly well understood since Selberg and the development of the Selberg trace formula. In particular it is known that congruence subgroups has an infinite number of discrete eigenvalues (corresponding to Maass cusp forms) and there is extensive support towards Selberg’s conjecture that there are no small eigenvalues for congruence subgroups. In contrast to this setting, much less is known for noncongruence subgroups of the modular group even though these groups are clearly arithmetic. In fact, it can be shown that under certain circumstances small eigenvalues must exist. And even the existence of infinitely many “new” discrete eigenvalues is not known for these groups. The main obstacle for developing the spectral theory here is that there is in general no explicit formula for the scattering determinant. In this talk I will present sufficient conditions for an “odd” discrete spectrum to exist and I will also give experimental support for the conjecture that these conditions are also necessary. I will also present an experimental version of Turing’s method for certifying correctness of the spectral counting.

2018-05-09 Wed Shunsuke Takagi (University of Tokyo) Pure Maths Colloquium
14:00 J11 General hyperplane sections of 3-folds in positive characteristic
Miles Reid proved that in characteristic zero, a general hyperplane section of a canonical (resp. klt) 3-hold has only rational double points (resp. klt singularities). His proof heavily depends on the Bertini theorem for free linear series, which fails in positive characteristic. Thus, it is natural to ask whether the same statement holds in positive characteristic or not. In this talk, I will present an affirmative answer to this question when the characteristic is larger than 5. This is joint work with Kenta Sato.

2018-05-09 Wed Riccardo Vanon (Sheffield) Applied Mathematics Colloquium
14:00 Hicks, LT 9 The role of zonal flows in self-gravitating astrophysical disc turbulence
Astrophysical discs whose mass is not negligible compared to their central object (ie. self-gravitating discs) can be induced to fragment or settle into a turbulent-like state due to the de-stabilising action of self-gravity. Although the fragmentation process is well studied, the mechanism allowing the gravitational turbulent state to sustain itself is poorly understood, as this requires a continuous extraction of energy from the background flow to prevent its decay. In this talk I will present numerical simulations, carried out using a bespoke pseudo-spectral code, that will show how the gravitational turbulent state and the formation of zonal flows in the disc are strongly connected thanks to the action of axisymmetric and non-axisymmetric instabilities.

2018-05-09 Wed Matthew Bisatt (King's College) Number Theory seminar
15:00 LT 9 The generalised Birch--Swinnerton-Dyer conjecture and twisted L-functions
The Birch and Swinnerton-Dyer conjecture famously connects the rank of an elliptic curve to the order of vanishing of its L-function. We combine this with a conjecture of Deligne to study twisted L-functions and derive several interesting properties of them using tools from representation theory. We show that, under certain conditions, these conjectures predict that the order of vanishing of the twisted L-function is always a multiple of a given prime and provide analogous statements for L-functions of modular forms.

2018-05-09 Wed Davide Spriano (ETH Zurich) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks What is geometric group theory (and why people care about it)
Geometric group theory is an area of mathematics that focuses on understanding a group by understanding its geometric properties, which are typically expressed in terms of actions on metric spaces. The advantage of the geometric group theory approach is that it provides a better understanding of groups of great interest on which more classical approach would fail. A prominent example is given by "automorphism groups of certain objects". They can be easily defined, but it is not at all clear when they are, for instance, finitely generated. The first part of the talk will be concerned in providing example of such groups motivating why are they interesting and which are the main difficulties that arises in the study of them. In the second part, we will introduce some of the fundamental concepts and tools of geometric group theory and coarse geometry.

2018-05-08 Tue Johannes Nicaise (Imperial College (London)) Algebra / Algebraic Geometry seminar
14:00 J11 A motivic Fubini theorem for the tropicalization map
Abstract: This talk is based on joint work with Sam Payne (arXiv:1703.10228). Kontsevich and Soibelman's motivic upgrade of Donaldson-Thomas theory produces refined curve counting invariants by means of motivic vanishing cycles of potential functions. In order to get a coherent theory, Kontsevich-Soibelman and Davison-Meinhardt have conjectured formulas for the motivic vanishing cycles of special types of functions. I will explain how one can deduce these formulas from a combination of Hrushovski-Kazhdan motivic integration and tropical geometry.

2018-05-04 Fri Mathew Owens (University of Reading) SP2RC seminar
13:00 LT 09 The changing heliosphere.
Knowledge of long-term solar variability underpins understanding of the solar dynamo and quantification of potential climate and space weather implications. Prior to direct spacecraft measurements of the heliospheric magnetic field (HMF) and solar wind, accurate annual reconstructions are possible using geomagnetic and sunspot records. On longer timescales, information about the HMF can be extracted from cosmogenic radionuclide records, particularly 14C in ancient trees and 10Be in ice sheets. I'll discuss these proxies and what they reveal about the HMF and solar wind, both in terms of space climate and space weather.

2018-05-03 Thu Salvo Guglielmino (Osservatorio Astrofisico di Catania) SP2RC seminar
10:00 K14 Interactions between pre-existing and emerging magnetic flux systems observed with IRIS
We report multi-wavelength ultraviolet observations taken with the IRIS satellite, concerning the emergence phase in the upper chromosphere and transition region of an emerging flux region (EFR) embedded in the unipolar plage of active region NOAA 12529. IRIS data are complemented by full-disk, simultaneous observations of the Solar Dynamics Observatory satellite, relevant to the photosphere and the corona. The photospheric configuration of the EFR is also analysed by measurements taken with the spectropolarimeter onboard the Hinode satellite, when the EFR was fully developed. Recurrent intense brightenings that resemble UV bursts, with counterparts in all coronal passbands, are identified at the edges of the EFR. Jet activity is also found at chromospheric and coronal levels, near the observed brightness enhancement sites. Analysis of the IRIS line profiles reveals heating of dense plasma in the low solar atmosphere and the driving of bi-directional, high-velocity flows with speeds up to 100 km/s at the same locations. Comparing these signatures with previous observations and numerical models, we suggest evidence of several long-lasting, small-scale magnetic reconnection episodes between the emerging bipole and the ambient field. This process leads to the cancellation of a pre-existing photospheric flux concentration of the plage with the opposite polarity flux patch of the EFR. Moreover, the reconnection appears to occur higher in the atmosphere than usually found in UV bursts, explaining the observed coronal counterparts.

2018-05-03 Thu Abhishek Saha (Queen Mary) Number Theory seminar
14:00 J11 On the critical values for the standard L-function of a Siegel modular form
I will talk about some joint work with Pitale and Schmidt where we prove an explicit pullback formula that gives an integral representation for the twisted standard L-function for a holomorphic vector-valued Siegel cusp form of degree n and arbitrary level. In contrast to all previously proved pullback formulas in this situation, our formula involves only scalar-valued functions despite being applicable to L-functions of vector-valued Siegel cusp forms. Further, by specializing our integral representation to the case n=2, we prove an algebraicity result for the critical L-values in that case (generalizing previously proved critical-value results for the full level case). Time permitting, I will also talk of some further applications and works in progress.

2018-05-02 Wed Owen Jones (Cardiff) Probability in the North East
13:30 LT D Runoff processes on trees
The volume of catchment discharge that reaches a stream via the overland flow path is critical for water quality prediction, because it is via this pathway that most particulate pollutants are generated and transported to the stream channel, via surface erosion processes. When it rains, spatial variation in the soil infiltration rate leads to the formation and reabsorption of rivulets on the surface, and local topography determines the coalescence of rivulets. We consider the question of how coalescence facilitates overland flow using a highly abstracted version of the problem, in which the drainage pattern is represented by a Galton-Watson tree. We show that as the rate of rainfall increases there is a distinct phase-change: when there is no stream coalescence the critical point occurs when the rainfall rate equals the infiltration rate, but when we allow coalescence the critical point occurs when the rainfall rate is less than the infiltration rate, and increasing the amount of coalescence increases the total expected runoff.

2018-05-02 Wed David O'Sullivan (Sheffield Hallam University) Pure Maths Colloquium
14:00 J11 Isomorphism conjectures and assembly maps via topological categories

The Baum-Connes conjecture is the "commutative" part of Alain Connes' noncommutative geometry programme, since it forms the bridge to classical geometry and topology. In its classical form, the conjecture identifies two object associated with a countable discrete group: one analytical and one topological.

In their 1997 paper, Davis and Lück utilised a (then little known) category theoretic variant of an operator algebra to present a unified approach to this conjecture, and to the Isomorphism Conjecture of Farrell and Jones on the algebraic K- and L-theory of integral group rings.

In this talk we will look at how this machinery fits together, what information the machinery gives us about the identifying maps, and how we might go about extending the scope of the techniques to topological groups and groupoids.

2018-05-02 Wed Vittoria Silvestri (Cambridge) Probability in the North East
14:20 LT D Recent progress on Laplacian growth models
The Hastings-Levitov planar aggregation models describe growing random clusters on the complex plane, built by iterated composition of random conformal maps. A striking feature of these models is that they can be used to define natural off–lattice analogues of several fundamental discrete models, such as the Eden model or Diffusion Limited Aggregation, by tuning the correlation between the defining maps appropriately. In this talk I will discuss shape theorems and fluctuations of large clusters in the weak correlation regime. Joint work with James Norris and Amanda Turner.

2018-05-02 Wed David Penman (Essex) Probability in the North East
15:30 LT D Probabilistic aspects of comparable pairs and linear extensions of partial orders
Given a finite partially ordered set $(P,\prec)$, a linear extension of it is a total order on the set which is compatible with the original partial order. A comparable pair of elements is two elements $x,y\in P$ for which we have $x\prec y$ or $y\prec x$. It is reasonable to ask about the extent of any relationship between the number of comparable pairs and the number of linear extensions, with an initial rough intuition being that fewer comparable pairs should correspond to more linear extensions. We will focus in this talk on the case of dense posets where a strictly positive proportion of pairs are comparable, though other cases will be considered as well. I shall focus on probabilistic aspects in this version of the talk, including the original motivating problem of estimating how many linear extensions a random interval order (which has roughly two-thirds of pairs comparable) possesses. This talk is based on joint work with Vasileios Iliopoulos and Colin McDiarmid.

2018-05-02 Wed *No talk* ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks

2018-05-02 Wed Andreas Kyprianou (Bath) Probability in the North East
16:20 LT D Entrance and exit at infinity for stable jump diffusions
In a seminal work from the 50s of the last century, William Feller classified all one-dimensional diffusions on $-\infty\leq a1$ and $\alpha= 1$ differ significantly in the details by virtue of non-existence of local times for $\alpha=1$.

2018-05-01 Tue Jordan Williamson (Sheffield) Magnitude Homology
13:00 J11 Magnitude homology equals Hochschild homology II

2018-04-26 Thu Emmanuel Lecouturier (Paris) Number Theory seminar
14:00 LT 6 Higher Eisenstein elements in weight 2 and prime level
In his classical work, Mazur considers the Eisenstein ideal $I$ of the Hecke algebra $T$ acting on cusp forms of weight $2$ and level $\Gamma_0(N)$ where $N$ is prime. When $p$ is an Eisenstein prime, i.e. $p$ divides the numerator of $\frac{N−1}{12}$, denote by $\mathbf{T}$ the completion of $T$ at the maximal ideal generated by $I$ and $p$. This is a $\mathbb{Z}_p$-algebra of finite rank $g_p ≥ 1$ as a $\mathbb{Z}_p$-module. Mazur asked what can be said about $g_p$. Merel proved a criterion for when $g_p \geq 2$. We will give criteria for $g_p \geq 3, 4$ and prove higher Eichler formulas.

2018-04-25 Wed Andrew Granville (University College London) Pure Maths Colloquium
14:00 J11 An alternative approach to analytic number theory
Ever since Riemann's seminal paper in 1859, analytic approaches to number theory have developed out of an understanding of the zeros of the Riemann zeta-function. In 2009, Soundararajan and I proposed an alternative approach (the "pretentious approach"), piecing together many "ad hoc" ideas from the past into a coherent theory. This new theory has taken on a life of its own in the last few years, providing the framework for some impressive new results by Matomaki, Radziwill, Tao and others. In this talk we will explain the main ideas and try to give some sense of how these new works fit in.

2018-04-25 Wed Caitlin McAuley (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks Introducing stability conditions
The space of stability conditions is a complex manifold associated to a triangulated category. The definition of a stability condition was motivated by work in string theory and as such, an understanding of the stability manifold will have important consequences in mirror symmetry. I'll introduce stability conditions on an arbitrary triangulated category and discuss some of their most important features, as well as discussing some examples.

2018-04-24 Tue Jordan Williamson (Sheffield) Magnitude Homology
13:00 J11 Magnitude homology equals Hochschild homology I

2018-04-19 Thu Martine Barrons (Warwick) Statistics Seminar
14:00 LT3

2018-04-19 Thu Gary McConnell (Imperial) Number Theory seminar
14:00 J11 Empirical connections between "crystals" of complex equiangular lines and Hilbert's twelfth problem for real quadratic fields
Let K be a real quadratic field of discriminant D or 4D, and set d to be one of the infinitely many integers for which the square-free part of $(d-1)^2 - 4$ is D. Over the past ten years it has become evident from many calculations that there is a profound connection between certain maximal sets of equiangular lines in complex d-dimensional Hilbert space on the one hand, and the ray class field of K of conductor d on the other. I will give a short outline of what has been discovered and where we believe it may be heading.

2018-04-18 Wed Ilke Canakci (University of Newcastle) Pure Maths Colloquium
14:00 J11 Cluster algebras and continued fractions
I will report on a new connection between cluster algebras and continued fractions given in terms of so-called 'Snake graphs'. Snake graphs are planar graphs first appeared in the context of cluster algebras associated to marked surfaces. In their first incarnation, they were used to give formulas for generators of cluster algebras. Along with further investigations and several applications, snake graphs were also studied from a more abstract point of view as combinatorial objects. This talk will focus on a combinatorial realisation of continued fractions in terms of 'perfect matchings' of snake graphs. I will also discuss applications to Cluster algebras, to elementary Number theory and, time permitting, to Knot theory. This is joint work with Ralf Schiffler.

2018-04-18 Wed Elena Marensi (Sheffield) Applied Mathematics Colloquium
14:00 Hicks, LT 9 Calculation of minimal seeds in stabilised pipe flows
Turbulent wall flows exert a much higher friction drag than laminar flows, with consequent increase in energy consumption and carbon emissions. Considerable research effort is thus directed towards the design of control strategies to reduce the turbulent drag or delay the transition to turbulence. Of fundamental interest from this viewpoint is the so-called minimal seed, i.e. the initial perturbation of lowest energy capable to trigger transition. In this talk, variational methods are used to construct fully nonlinear optimisation problems that seek the minimal seed in stabilised pipe flows. The question of how representative the minimal seed is of typical ambient disturbances is addressed here for the first time by performing a statistical study of the critical initial energies for transition with different initial perturbations. A set of initial conditions are thus generated to investigate the stabilising effect of a simple model for the presence of a baffle in the core of the flow. Significant increases in the critical energy and drag reductions are found to be possible. The relevance of the minimal seed in realistic scenarios will be further discussed, as well as a closely-related variational problem for the control of transition.

2018-04-18 Wed David Spencer (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks Visualising the values of a binary quadratic form
The solution of Diophantine equations is still a thriving area in number theory. In this talk I will consider binary quadratic forms, those of the form $ax^2 + bxy + cy^2$ with $a,b,c \in\mathbb Z$. I will show how to construct a graph which allows us to see the possible values such a quadratic form can take. This will allow us to determine when the Diophantine equation $ax^2 + bxy + cy^2 = k$ is solvable in integers $(x,y)$, and to find such integers when it is solvable.

2018-04-17 Tue Igor Sikora (Sheffield) Magnitude Homology
13:00 J11 Euler Characteristic II

2018-04-16 Mon Natasha Savage (Liverpool) Mathematical Biology Seminar Series
14:00 Hicks LT6 Where to draw one’s theoretical boundary: One closed question, one experimental data set, two published models, two opposing answers.

2018-03-27 Tue Prof. Manolis Georgoulis (Academy of Athens) SP2RC seminar
13:00 LT 10 From Physical Understanding to Forecasting of Solar Flares and Coronal Mass Ejections
The imperative and pressing nature of an efficient space weather forecasting has spurred multiple efforts around the world to address this problem. A recent realization is that the problem's tackling should not be restricted to heliophysics, but should utilize help provided by the big data and machine learning communities, in a complementary and reinforcing role. This said, we argue that without the lead of solar and heliospheric physicists, these interdisciplinary efforts would be ill-fated. This is because a successful forecasting effort should be driven by an in-depth understanding of the solar pre-eruption phase(s) and evolution of solar source regions. We will provide a few examples of this enhanced-understanding process put to work in the framework of the EU FLARECAST project on solar flare prediction. From flares, we will then take a conceptual step toward an efficient CME forecasting that can be facilitated by what has been already achieved through FLARECAST and other EU projects. Concluding, we will touch on another necessary aspect of efficient space weather forecasting, namely, the input data from solar monitoring. With continuous, near-realtime monitoring of the Sun attempted predominantly from satellites and spacecraft, we discuss a potentially viable, longer-term and better managed alternative such as a strategically built network of ground-based monitoring stations. Such networks can be served, maintained, and expanded / upgraded at will, better adapting to the problem at hand while reflecting progress in its physical understanding.

2018-03-26 Mon Hans Werner Henn (Strasbourg) Topology seminar
16:00 J11 The centralizer resolution of the K(2)-local sphere at the prime 2.
In the last few years two different resolutions of the K(2)-local sphere at the prime 3 have been used very successfully to settle some basic problems in K(2)-local stable homotopy theory like the chromatic splitting conjecture, the calculation of Hopkins' K(2)-local Picard group and determining $K(2)-local Brown-Comentz duality. The focus is now moving towards the prime 2 where one can hope for similar progress. In this talk we concentrate on one of these two resolutions, the centralizer resolution at the prime 2.

2018-03-22 Thu Petros Syntelis (ESPOS Seminar) (University of St Andrews) SP2RC seminar
10:00 E39 Recurrent CME-like Eruptions in Flux Emergence Simulations
We report on three-dimensional MHD simulations of recurrent small-scale Coronal Mass Ejection (CME)-like eruptions using flux-emergence simulations and study their formation and eruption mechanism. These eruptions have the size and energies of small prominence eruptions. The erupting flux ropes are formed due to the reconnection of J-loops (formed by shearing and rotation) and are located inside magnetic envelope field favouring torus instability. The flux rope eruptions are triggered by the action of a tension removal mechanism, such as the typical tether-cutting where the envelope field reconnects with itself. Another side tether-cutting is also found. There, the envelope field reconnected with the J-loops. The two tether-cutting mechanisms transfer hot plasma differently inside the erupting structures. We report similar mechanisms creating three more eruptions in a recurrent manner.

2018-03-22 Thu Netan Dogra (Imperial) Number Theory seminar
14:00 J11 Unlikely intersections and the Chabauty-Kim method over number fields
Chabauty's method is a method for proving finiteness of rational points on curves under assumptions on the rank of the Jacobian. Recently, Kim has shown that one can extend this to prove finiteness of rational points on curves over Q, under slightly weaker assumptions on the dimension of certain Galois cohomology groups. A conjecture of Beilinson-Bloch-Kato implies these assumptions are always satisfied. In this talk I will explain Kim's construction, and how to extend his results to general number fields by proving an 'unlikely intersection' result for the zeroes of p-adic iterated integrals.

2018-03-21 Wed Evgeny Shinder (Sheffield) Pure Maths Colloquium
14:00 J11 Rationality in families of algebraic varieties
I will talk about the following problem in algebraic geometry: given a family of algebraic varieities, if general fibers are rational, are all fibers rational? The talk will be based on recent joint work of myself and Nicaise, and a development by Kontsevich and Tschinkel.

2018-03-21 Wed Sam Morgan (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 F20 Lie groupoids and their application in symplectic geometry
The aim of this talk is to introduce the theory of Lie groupoids and Lie algebroids to a broad audience. It is hoped that the subject can be well motivated, without many prerequisites. In the second part of the talk, we will see why Lie groupoids were first introduced into symplectic and Poisson geometry, and what role they play here.

2018-03-20 Tue Igor Sikora (Sheffield) Magnitude Homology
13:00 J11 Euler characteristics

2018-03-19 Mon Cameline Orlendo (Glasgow) Mathematical Biology Seminar Series
14:00 Hicks F41

2018-03-15 Thu Matthew Bisatt (King's College) Number Theory seminar
14:00 LT A The generalised Birch--Swinnerton-Dyer conjecture and twisted L-functions- CANCELLED
The Birch and Swinnerton-Dyer conjecture famously connects the rank of an elliptic curve to the order of vanishing of its L-function. We combine this with a conjecture of Deligne to study twisted L-functions and derive several interesting properties of them using tools from representation theory. We show that, under certain conditions, these conjectures predict that the order of vanishing of the twisted L-function is always a multiple of a given prime and provide analogous statements for L-functions of modular forms.

2018-03-15 Thu Dr Robert Massey (Royal Astronomical Society)
15:00 Hicks LT A Funding Blue Skies Research In The Age Of Austerity

2018-03-15 Thu Simon Wood (Cardiff) Topology seminar
16:00 J11 Questions in representation theory inspired by conformal field theory
Two dimensional conformal field theories (CFTs) are conformally invariant quantum field theories on a two dimensional manifold. What distinguishes two dimensions from all others is that the (Lie) algebra of local conformal transformations become infinite dimensional. This extraordinary amount of symmetry allows certain conformal field theories to be solved by symmetry considerations alone. The most intensely studied type of CFT, called a rational CFT, is characterised by the fact that its representation theory is completely reducible and that there are only a finite number isomorphism classes of irreducibles. The representation categories of these CFTs form so called modular tensor categories which have important applications in the construction of 3-manifold invariants. In this talk I will discuss recent attempts at generalising this very rich structure to CFTs whose representation categories are neither completely reducible nor finite.

2018-03-14 Wed Igor Sikora (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks Could the Philosophy of Mathematics be interesting for mathematicians
Philosophical reflections of mathematics are concerned with the fundamental problems about mathematics, such as existence of mathematical objects, subject of research of mathematics, how do we extend our mathematical knowledge, what are relations of mathematics with other sciences etc. In this talk I will attempt to describe several classical and modern problems in philosophy an approaches to solve them.

2018-03-08 Thu Dr Krishna Prasad (Queen's University, Belfast)
10:00 LT 6 Frequency-dependent Damping of Slow Magneto-acoustic Waves in Sunspots
Propagating slow magneto-acoustic waves are regularly observed in the solar corona, particularly in sunspot related loop structures. These waves exhibit rapid damping as they propagate along the loops. Several physical and geometrical effects were found to produce the observed decay in the wave amplitude. It has also been shown that the damping is frequency dependent. A majority of the observed characteristics have been attributed to damping by thermal conduction in the solar corona. Although it is believed that these waves originate in the photosphere, their damping behaviour in the sub-coronal layers is relatively less studied. Using high spatial and temporal resolution images of a sunspot, we investigated propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. The major conclusions from this study will be discussed in the talk which include: 1) The energy flux in slow waves estimated from the relative amplitudes decays gradually right from the photosphere even when the oscillation amplitude is increasing. 2) The damping displayed by slow waves is frequency dependent well below coronal heights. 3) A spatial comparison of power spectra across the umbra highlights enhancement of high-frequency waves near the umbral center.

2018-03-08 Thu David Spencer (Sheffield) Number Theory seminar
14:00 F35 Congruences of local origin for higher levels- CANCELLED
There are many kinds of congruences between different types of modular forms. The most well known of which is Ramanujan's mod 691 congruence. This is a congruence between the Hecke eigenvalues of the weight 12 Eisenstein series and the Hecke eigenvalues of the weight 12 cusp form (both at level 1). This type of congruence can be extended to give congruences of ''local origin''. In this talk I will explain what is meant by such a congruence while focusing on the case of weight 1. The method of proof in this case is very different to that of higher weights and involves working with Galois representations and ray class characters.

2018-03-05 Mon Justin Travis (Aberdeen) Mathematical Biology Seminar Series
14:00 Hicks F41

2018-03-02 Fri Dr. Jiajia Liu (University of Sheffield) SP2RC seminar
12:00 LT 11 A New Tool for CME Arrival Time Prediction Using Machine Learning Algorithms: CAT-PUMA
Coronal Mass Ejections (CMEs) are arguably the most violent eruptions in the Solar System. CMEs can cause severe disturbances in the interplanetary space and even affect human activities in many respects, causing damages to infrastructure and losses of revenue. Fast and accurate prediction of CME arrival time is then vital to minimize the disruption CMEs may cause when interacting with geospace. In this paper, we propose a new approach for partial-/full-halo CME Arrival Time Prediction Using Machine learning Algorithms (CAT-PUMA). Via detailed analysis of the CME features and solar wind parameters, we build a prediction engine taking advantage of 182 previously observed geo-effective partial-/full-halo CMEs and using algorithms of the Support Vector Machine (SVM). We demonstrate that CAT-PUMA is accurate and fast. In particular, predictions after applying CAT-PUMA to a test set, that is unknown to the engine, show a mean absolute prediction error ~5.9 hours of the CME arrival time, with 54% of the predictions having absolute errors less than 5.9 hours. Comparison with other models reveals that CAT-PUMA has a more accurate prediction for 77% of the events investigated; and can be carried out very fast, i.e. within minutes after providing the necessary input parameters of a CME. We have also designed a publicly free User Interface (, allowing the community to perform their own applications for prediction using CAT-PUMA.

2018-03-02 Fri Mladen Dimitrov (Lille) Number Theory seminar
14:00 LT-5 $p$-adic L-functions for nearly finite slope Hilbert modular forms and the Exceptional Zero Conjecture
We attach $p$-adic L-functions and improved variants theoreof to families of nearly finite slope cohomological Hilbert modular forms, and use them to prove the Greenberg-Benois exceptional zero conjecture at the central point for forms which are Iwahori spherical at $p$. This is a joint work with Daniel Barrera and Andrei Jorza.

2018-03-01 Thu Mladen Dmitrov (Université de Lille ) Pure Maths Colloquium
14:00 J11 L-functions of GL(2n): p-adic properties and nonvanishing of twists

A crucial result in Shimura's work on the special values of L-functions of modular forms concerns the existence of a twisting character to ensure that a twisted L-value is nonzero at the center of symmetry. Even for simple situations involving L-functions of higher degree this problem is open: for example, if $\pi$ is the automorphic representation attached to a holomorphic cusp form, then it has been an open problem to find a character such that the twisted symmetric cube L-function of $\pi$ does not vanish at the center.

We will present a recent joint work with F. Januszewski and A. Raghuram in which purely arithmetic methods involving studying p-adic distributions on Galois groups are used to tackle this problem.

Given a cohomological unitary cuspidal automorphic representation $\Pi$ on GL(2n) over a totally real field, under a very mild regularity assumption on the infinity type that ensures two critical points for the standard L-function of $\Pi$, supposing $\Pi$ admits a Shalika model, then for any ordinary prime p for $\Pi$, we prove that for all but finitely many Hecke characters the twisted central L-value of $\Pi$ does not vanish.

For example, with a classical normalization of $L$-functions, it follows from our results that there are infinitely many Dirichlet characters $\chi$ such that $L(6, \Delta \otimes \chi) L(17, {\rm Sym}^3\Delta \otimes \chi) \neq 0$ for the Ramanujan $\Delta$-function.

2018-03-01 Thu Christian Wimmer (Bonn) Topology seminar
16:00 J11 A model for equivariant commutative ring spectra away from the group order
Stable homotopy theory simplifies drastically if one consider spectra up to rational equivalence. It is a classical result that taking homotopy groups induces an equivalence $$G \text{-} \mathcal{SHC} \simeq_{\mathbb{Q}} \text{gr.} \prod_{(H \leq G)} \mathbb{Q} [WH] \text{-mod}$$ between the genuine $G$-equivariant stable homotopy category ($G$ finite) and the category of graded modules over the Weyl groups $WH$ indexed by the conjugacy classes of subgroups of $G$. However, this approach is too primitive to be useful for the comparison of highly structured ring spectra in this setting.

Let $R \subset \mathbb{Q}$ be a subring such that $|G|$ is invertible in $R$. I will explain how geometric fixed points equipped with additional norm maps related to the Hill-Hopkins-Ravenel norms can be used to give an $R$-local model: They induce an equivalence $$\text{Com}(G\text{-Sp}) \simeq_R \text{Orb}_G \text{-Com}(\text{Sp})$$ between the $R$-local homotopy theories of genuine commutative $G$-ring spectra and $\text{Orb}_G$-diagrams in non-equivariant commutative ring spectra, where $\text{Orb}_G$ is the orbit category of the group $G$. As a corollary this gives an algebraic model $$\text{Com}(G\text{-Sp})_\mathbb{Q} \simeq \text{Orb}_G \text{-CDGA}_\mathbb{Q}$$ for rational ring spectra in terms of commutative differential algebras. I will also try to indicate the analogous global equivariant statements.

2018-02-28 Wed Peter Millington (Nottingham) Applied Mathematics Colloquium
14:00 Hicks, LT 9 Energy-parity from a bicomplex algebra
There is a long history of attempts to alleviate the sensitivity of quantum field theory to vacuum fluctuations and ultraviolet divergences by introducing states of negative norm or states of negative energy. This history involves early works by Dirac, Pauli, Pontrjagin and Krein, as well as more recent suggestions by Linde, Kaplan and Sundrum, and ‘t Hooft and Nobbenhuis. In this talk, we will attempt to construct viable scalar quantum field theories that permit positive- and negative-energy states by replacing the field of complex numbers by the commutative ring of bicomplex numbers. The two idempotent zero divisors of the bicomplex numbers partition the algebra into two ideal subalgebras, and we associate one with positive-energy modes and the other with negative-energy modes. In so doing, we avoid destabilising, negative-energy cascades, while realising a discrete energy-parity symmetry that eliminates the vacuum energy. The probabilistic interpretation is preserved by associating expectation values with the Euclidean inner product of the bicomplex numbers, and both the positive- and negative-energy Fock states have positive-definite Euclidean norms. We consider whether this construction can yield transition probabilities consistent with the usual scattering theory and highlight potential limitations. We conclude by commenting on the extension to spinor, vector and tensor fields.

2018-02-27 Tue Alice Rizzardo (University of Liverpool) Algebra / Algebraic Geometry seminar
14:00 J11

2018-02-27 Tue Christos Aravanis (Sheffield) ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks Hopf algebras in categories of complexes
I will discuss about a generalization of the notion of a Hopf algebra in monoidal categories due to Brugieres and Virelizier. Of particular interest will be the derived category of coherent sheaves on a smooth complex projective variety.

2018-02-26 Mon Dwight Barkley (Warwick) Applied Mathematics Colloquium
15:00 Hicks, LT E Recent Advances in the subcritical transition to turbulence
Explaining the route to turbulence in wall-bounded shear flows has been a long and tortuous journey. After years of missteps, controversies, and uncertainties, we are at last converging on a unified and fascinating picture of transition in flows such as pipes, channels, and ducts. Classically, subcritical transition (such as in a pipe), was thought to imply a {\em discontinuous} route to turbulence. We now know that this is not the case -- subcritical shear flows may, and often do, exhibit continuous transition. I will discuss recent developments in experiments, simulations, and theory that have established a deep connection between transition in subcritical shear flows and a class of non-equilibrium statistical phase transitions known as directed percolation. From this we understand how to define precise critical points for systems without linear instabilities and how to characterize the onset of turbulence in terms of non-trivial, but universal power laws. I will discuss the physics responsible for the complex turbulent structures ubiquitously observed near transition and end with thoughts on outstanding open questions.

2018-02-26 Mon Laurette Tuckerman (ESPCI Paris) Applied Mathematics Colloquium
15:45 Hicks, LT E Exotic patterns of Faraday waves
When a fluid layer is vibrated at a sufficiently high amplitude, a pattern of standing waves appears at its surface. Because of the imposed periodicity, this is a Floquet problem, but we explain how to easily solve it. Classically, the pattern takes the form of stripes, squares or hexagons, but we also look at more exotic patterns like quasipatterns, heteroclinic orbits, supersquares, and Platonic polyhedra. (Longer version) A standing wave pattern appears on the free surface of a fluid layer when it is subjected to vertical oscillation of sufficiently high amplitude. Like Taylor-Couette flow (TC) and Rayleigh-Benard convection (RB), the Faraday instability is one of the archetypical pattern-forming systems. Unlike TC and RB, the wavelength is controlled by the forcing frequency rather than by the fluid depth, making it easy to destabilize multiple wavelengths everywhere simultaneously. Starting in the 1990s, experimental realizations using this technique produced fascinating phenomena such as quasipatterns and superlattices which in turn led to new mathematical theories of pattern formation. Another difference is that the Faraday instability has been the subject of surprisingly little numerical study, lagging behind TC and RB by several decades. The first 3D simulation reproduced hexagonal standing waves, which were succeeded by long-time recurrent alternation between quasi-hexagonal and beaded striped patterns, interconnected by spatio-temporal symmetries. In a large domain, a supersquare is observed in which diagonal subsquares are synchronized. A liquid drop subjected to an oscillatory radial force comprises a spherical version of the Faraday instability. Simulations show Platonic solids alternating with their duals while drifting.

2018-02-22 Thu Adel Betina (Sheffield) Number Theory seminar
14:00 F24 Classical and overconvergent modular forms - CANCELLED
I will explain the proof of Kassaei of Coleman’s theorem via analytic continuation on the modular curve.

2018-02-22 Thu Luca Pol (Sheffield) Topology seminar
16:00 J11 On the geometric isotropy of a compact rational global spectrum
In this talk I will explain a way to detect groups in the geometric isotropy of a compact rational global spectrum. As an application, I will show that the Balmer spectrum of the rational global stable homotopy category exhibits at least two different types of prime: group and multiplicative primes.

2018-02-21 Wed Scott Balchin, Caitlin McAuley, Ariel Weiss ShEAF: postgraduate pure maths seminar
16:00 J11 Hicks What is...?
Three fifteen minute introductory talks on some important themes from different areas of pure maths, namely Tiling Spaces, Mirror Symmetry, and the Langlands Program.

2018-02-20 Tue Daniel Graves (Sheffield) Magnitude Homology
13:00 J11 Hochschild homology of enriched categories

2018-02-20 Tue Thanasis Bouganis (Durham) Number Theory seminar
14:00 F24 On the standard L function attached to Siegel-Jacobi modular forms of higher index
The standard L function attached to a Siegel modular form is one of the most well-studied L functions, both with respect to its analytic properties and to the algebraicity of its special L-values. Siegel-Jacobi modular forms are closely related to Siegel modular forms, and it was Shintani who first studied the standard L function attached to them. In this talk, I will start by introducing Siegel-Jacobi modular forms and then discuss joint work with Jolanta Marzec on the analytic properties of their standard L function, extending results of Murase and Sugano, and on the algebraicity of its special L values. I will also discuss some open questions.

2018-02-20 Tue RSS Local Group / Michael Wallace (Sheffield) RSS Seminar Series
16:30 Hicks Room I19 A tribute to the life and work of Nick Fieller
Join the Sheffield Local Group in a tribute to Nick Fieller, a member of staff at the University of Sheffield from 1974 until his retirement in 2012, a long-standing fellow of the RSS, and an active member of the local RSS committee until just prior to his death in 2017.

The meeting will start with memories of Nick, continue with a seminar given by Dr Michael Wallace from the Department of Archaeology at The University of Sheffield, and end with a drinks reception.

A geometric morphometric view of early agriculture - Michael Wallace

Nick Fieller had a long and rich history of collaboration with several colleagues in the Department of Archaeology, and Michael was fortunate enough to work with him on the ERC project 'The Evolutionary Origins of Agriculture' (PI: Prof. Glynis Jones).

The switch from a mobile hunter-gatherer way of life to one based on settled agriculture was perhaps the most fundamental change in the development of our species, and the subsequent spread of agriculture required the use of crops in environments far outside their natural distribution. A key element of the ERC project was to pioneer the use of geometric modern morphometrics (GMM) for the study of ancient crop remains (primarily cereal grains). GMM allows us to enhance our exploration of past crop remains by quantifying the variation within a crop species, which in turn can offer new insights into ancient crop selection.

In this seminar, Michael will discuss some of the key research themes to which GMM can contribute in archaeobotany, the implementation of morphometrics using Vincent Bonhomme's "Momocs' (which was expanded as part of the ERC project), and some of the ongoing research that explores the origins and spread of agriculture.

2018-02-19 Mon David Miller (St Andrews) Mathematical Biology Seminar Series
14:00 Hicks F41 Accounting for detectability in spatially-explicit abundance models of cetaceans

2018-02-15 Thu Neil Dummigan (Sheffield) Number Theory seminar
14:00 J11 Automorphic forms on Feit's Hermitian lattices
This is joint work with Sebastian Schoennenbeck. Feit showed, in 1978, that the genus of unimodular hermitian lattices of rank 12 over the Eisenstein integers contains precisely 20 classes. Complex-valued functions on this finite set are automorphic forms for a unitary group. Using Kneser neighbours, we find a basis of Hecke eigenforms, for each of which we propose a global Arthur parameter. This is consistent with several kinds of congruences involving classical modular forms and critical L-values, and also produces some new examples of Eisenstein congruences for U(2,2).

2018-02-15 Thu Jeremy Colman (Sheffield) Statistics Seminar
15:00 F41 Stan: better faster MCMC - A user review

2018-02-15 Thu David Barnes (Queen's University Belfast) Topology seminar
16:00 J11 Cohomological dimension of profinite spaces
I will introduce the notion of rational cohomological dimension of topological spaces and show a simple way to calculate it when we restrict ourselves to a certain class of topological spaces. Very roughly, the r.c.d of a space X is the largest p such that the pth rational cohomology of X is non-zero. This invariant can be calculated in terms of the more geometric notion of sheaves on X. The category of sheaves on X is an abelian category and the injective dimension of this category is the r.c.d of X. This is a standard way to calculate the the r.c.d. of a space, but can be rather difficult. In this talk, I will describe how for profinite spaces, this injective dimension is related to a simpler notion: the Cantor-Bendixson dimension of the space. There will be a number of pictures and some nice examples illustrating the calculations.

2018-02-14 Wed Lassina Dembele (University of Sheffield) Pure Maths Colloquium
14:00 J11 Hilbert modular forms and arithmetic applications

Hilbert modular forms were introduced by David Hilbert in 1892 in an attempt to generalise so called elliptic modular forms to other settings. Considered to be a notoriously difficult topic, it wasn't until the mid 1970s that they were seriously studied, notably by Goro Shimura. Since then, they have become very central objects to modern number theory.

In this talk, we will start with a gentle introduction to Hilbert modular forms. Then, we will discuss various applications to number theory and arithmetic geometry.

2018-02-13 Tue Scott Balchin (Sheffield) Magnitude Homology
13:00 J11 Magnitudes of enriched categories and metric spaces

2018-02-13 Tue Tommi Tenkanen (Queen Mary UL) Cosmology, Relativity and Gravitation
16:00 J11, Hicks Primordial Black Holes as Dark Matter
I revisit the cosmological and astrophysical constraints on the fraction of dark matter (DM) in primordial black holes (PBHs). I consider a variety of production mechanisms and mass functions for PBHs and discuss whether they can constitute the observed DM abundance or not. I also discuss how one can constrain the physics of the early Universe with the constraints on PBHs, presenting e.g. constraints on the running of the inflaton spectral index which are comparable to those from the Planck satellite.

2018-02-12 Mon Malte Heuer (Sheffield) Differential geometry seminar
14:00 LT 11 Decompositions of Triple Vector Bundles
I will prove that any triple vector bundle is non-canonically isomorphic to a decomposed one. The method relies on del Carpio-Marek's construction of local splittings of double vector bundles. Our method yields a useful definition of triple vector bundles via atlases of triple vector bundle charts. This is joint work with Madeleine Jotz Lean.

2018-02-08 Thu Christopher Williams (Imperial) Number Theory seminar
14:00 F24 p-adic Asai L-functions of Bianchi modular forms
The Asai (or twisted tensor) L-function attached to a Bianchi modular form is the 'restriction to the rationals' of the standard L-function. Introduced by Asai in 1977, subsequent study has linked its special values to the arithmetic of the corresponding form. In this talk, I will discuss joint work with David Loeffler in which we construct a p-adic Asai L-function -- that is, a measure on Z_p* that interpolates the critical values L^As(f,chi,1) -- for ordinary weight 2 Bianchi modular forms. The method makes use of techniques from the theory of Euler systems, namely Kato's system of Siegel units, building on the rationality results of Ghate. I will start by giving a brief introduction to p-adic L-functions and Bianchi modular forms.

2018-02-07 Wed Ben Ashby (Bath) Mathematical Biology Seminar Series
13:00 Hicks F20

2018-02-07 Wed Dan Lucas (Keele) Applied Mathematics Colloquium
14:00 Hicks, LT 9 A dynamical systems perspective on layers and mixing in stratified turbulence
Stably stratified flows, with dense fluid underlying lighter fluid, are commonly observed in nature and industry. In the oceans the behaviour of turbulence when the fluid is strongly stratified is of great importance if we are to understand fundamental processes such as layer formation and mixing. In this work we approach these issues from the so-called ‘dynamical systems perspective’ where we seek unstable simple solutions, or “exact coherent structures”, which are embedded in the chaos of the turbulent flow. First we show that when forcing the flow with a horizontal shear, spontaneous layers form. We are able to associate the coherent structures responsible for the layers with steady states which a bifurcation analysis shows are the finite amplitude product of a sequence of stratified linear instabilities [1]. Secondly we attack the problem of mixing in stratified turbulence by locating unstable periodic orbits embedded in the turbulence in two parameter regimes; one where the mixing is quite efficient and another where the mixing is weak. The periodic orbits represent a reduced description of the flow which we are able to examine in detail, and compare the processes involved in rearranging the buoyancy field in each case [2]. [1] Lucas, Caulfield & Kerswell 2017 J. Fluid Mech. 832 pp 409-437 [2] Lucas & Caulfield 2017 J. Fluid Mech. 832 R1,

2018-02-06 Tue Simon Willerton (Sheffield) Magnitude Homology
13:00 J11 Graph magnitude homology + organization

2018-02-06 Tue Timothy Logvinenko (University of Cardiff) Algebra / Algebraic Geometry seminar
14:00 J11 P^n-functors and cyclic covers
I will begin by reviewing the geometry of a cyclic cover branched in a divisor. I will then explain how it gives the first ever example of a non-split P^n-functor. This is a joint work with Rina Anno (Kansas).

2018-02-02 Fri Dr Jie Chen (National Astronomical Observatories) SP2RC seminar
13:00 LT 11 Study of solar coronal jets

2018-02-01 Thu Scott Balchin Chromatic homotopy theory reading seminar
14:00 J11 Flat modules over M_FG

2018-01-30 Tue Yu Qiu (Chinese University of Hong Kong) Algebra / Algebraic Geometry seminar
14:00 J11 Q-stability conditions on Calabi-Yau-X categories of quivers with superpotential
We introduce X-stability conditions on Calabi-Yau-X categories and spaces of their specializations, the q-stability conditions. The motivating example comes from the Calabi-Yau-X category D(S) associated to a graded marked surface S, constructed from quivers with superpotential. We show that the cluster category of D(S) is Haiden-Katzarkov-Kontsevich's topological Fukaya category C(S) and Bridgeland-Smith type Calabi-Yau-N categories are the orbit quotients of D(S). Moreover, we show that stability conditions on C(S) induce q-stability conditions on D(S). Finally, we are constructing moduli space to realize the fiber of the spaces of q-stabilty conditions for given complex number s.

2018-01-26 Fri Mihai Barbulescu (Sheffield) SP2RC seminar
13:00 LT 11 Periodic Counter Streaming Flows as a Model of Transverse Coronal Loop Oscillations
Recent numerical simulations have demonstrated that non-linear transverse coronal loop oscillations are susceptible to the Kelvin-Helmholtz instability (KHI) due to the counter streaming motions at the boundaries of the loop. We present the first study of this mechanism using an analytical model. The region at the loop boundary where the shearing motions are greatest is treated as a straight interface separating periodic counter-streaming flows. We derive the governing equations for both a straight and a twisted flux tube model, and find that the magnetic twist contributes significantly towards stabilising the system. Establishing the necessary conditions for coronal loops to become unstable due to shearing is important since, it has been shown, the turbulent behaviour due to the instability may lead to heating of the exterior via Ohmic dissipation.

2018-01-23 Tue Jordan Williamson Chromatic homotopy theory reading seminar
14:00 J11 The classification of formal groups