# Seminars this semester

Jan 16 | Mon | John Greenlees (Sheffield) | |

11:00 | J11 | What we want from G spectra | |

Jan 17 | Tue | Jordan Williamson (Sheffield) | |

14:00 | J11 | Orthogonal G-spectra | |

Jan 17 | Tue |
Sara Kalisnik (Brown) | Topology seminar |

16:00 | J11 | A short introduction to applied topology | |

Abstract:In the last two decades applied topologists have developed numerous methods for ‘measuring’ and building combinatorial representations of the shape of the data. The most famous example of the former is persistent homology and of the latter, mapper. I will briefly talk about both of these methods and show several successful applications. Time permitting I will talk about my work on making persistent homology easier to combine with standard machine learning tools. |
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Jan 19 | Thu | Jordan Williamson (Sheffield) | |

15:00 | J11 | Orthogonal G-spectra II | |

Jan 20 | Fri | Dr Jiajia Liu (University of Science and Technology of China) | SP2RC seminar |

13:00 | LT 11 |
Energy Rules of Solar Jets from Observational Perspectives | |

Abstract:Solar jets are bulks of plasma materials ejected along elongated trajectories from the solar surface into the atmosphere of the Sun, often leaving the inner corona and determining the physical conditions far outwards in the interplanetary space. These impulsive and energetic ejecta are one of the most common dynamic phenomena occurring within the solar atmosphere. They are often accompanied by (nano-)flares, and some times by Coronal Mass Ejections (CMEs) and radio bursts, which could lead to significant changes of the space weather and terrestrial magnetic fields. After the nearly one-century efforts studying solar jets, we now have mature models for solar jets explaining the process of how magnetic reconnection triggers jets. However, due to the limits of the observational technology, many issues such as the detailed dynamics of, the energy transport during and the interaction with waves of solar jets are not well addressed before. In this talk, I will introduce part of my work during the past few years on the topic of "Energy Rules of Solar Jets from Observational Perspectives". Via high-resolution observations from the SDO and STEREO, I try to address the following questions of solar jets: (1) how the free magnetic energy is distributed between the thermal and kinetic energy during magnetic reconnection, (2) how the kinetic energy of solar jets is gained during and after the magnetic reconnection, and (3) how further release of the free magnetic reconnection proceeds after solar jets. |
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Jan 24 | Tue | Luca Pol (Sheffield) | |

14:00 | Complex cobordism and K theory with reality | ||

Jan 24 | Tue |
Jonathan Sykes | Uncertainty Quantification reading group |

15:00 |
J11 |
Discussion of "Bayesian History Matching of Complex Infectious Disease Models Using Emulation: A Tutorial and a Case Study on HIV in Uganda", by Andrianikis et al. | |

Jan 26 | Thu | Luca Pol (Sheffield) | |

15:00 | Change of groups | ||

Jan 27 | Fri | Prof. B. Hindman (University of Colorado, Boulder) | SP2RC seminar |

13:00 | LT 11 |
Solar convection in the rotationally constrained regime | |

Abstract:Despite knowing that convection and rotation are indispensable components of the solar dynamo, we know vexingly little about how the influence of rotation manifests across the broad range of convective scales present in the Sun. We do know that the structure of deep meridional circulation, which may bear on the timing of the solar cycle, is sensitive to the degree of rotational constraint felt by its underlying convective motions. Similarly, the solar differential rotation, a vital source of large-scale shear in some dynamo models, results from convective motions that transport not just heat, but angular momentum. Rotation imbues convection with a sense of helicity, supplying a source of turbulent EMF to the dynamo, and it is only in regimes of strong rotational constraint that fully nonlinear models of stellar convection have evinced cyclic dynamo behavior. Current helioseismic measurements of the convective flows suggest that rotational influence is strong within the deep convection zone, but are inconsistent in how strong. Therefore, it is prudent to ask ourselves how rotation shapes the spectral distribution of convective power. I will present numerical results from a series of nonrotating and rotating convection simulations conducted in full spherical geometry. This presentation will focus on how convective spectra differ between the rotating and non-rotating models and how that behavior changes as simulations are pushed toward more turbulent and/or more rotationally-constrained regimes. I will conclude with a discussion of the implications that strong rotational constraint in the deep convection zone should have on the surface convective and how decades of surface observations may need re-interpretation. |
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Jan 31 | Tue | Dimitar Kodjabachev (Sheffield) | |

14:00 | Mackey functors | ||

Feb 2 | Thu | Dimitar Kodjabachev (Sheffield) | |

15:00 | Fixed point functors | ||

Feb 3 | Fri | Dr. Rekha Jain (University of Sheffield) | SP2RC seminar |

13:00 | LT 11 |
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Feb 6 | Mon | Moty Katzman (Sheffield) | Algebraic Geometry Learning Seminar |

02:00 | J11 | Cohen-Macaulay modules | |

Feb 6 | Mon |
Torbjorn Lundh (Chalmers) | Mathematical Biology Seminar Series |

14:00 |
Hicks LT9 |
Four surgery problems "solved" by a "mathematical" approach | |

Feb 7 | Tue |
Jeremy Oakley (Sheffield) | Uncertainty Quantification reading group |

15:00 |
J11 |
Discussion of Wu, H., & Browne, M. W. (2015). Quantifying adventitious error in a covariance structure as a random effect. Psychometrika. | |

Feb 7 | Tue |
Jeff Giansiracusa (Swansea) | Topology seminar |

16:00 | J11 | ||

Feb 8 | Wed | John Coates (University of Cambridge) | Pure Maths Colloquium |

14:00 | J11 | The conjecture of Birch and Swinnerton-Dyer | |

Abstract:The conjecture of Birch and Swinnerton-Dyer is one of the principal open problems in number theory today. In my lecture, I shall give a brief account of the history of the conjecture, its precise formulation, and the partial results obtained so far in support of it. |
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Feb 8 | Wed | George Papadakis (Imperial) | Applied Mathematics Colloquium |

14:00 | Hicks, LT 10 |
Nonlinear optimal control of bypass transition in a boundary layer flow | |

Abstract:We apply and assess a nonlinear optimal control strategy to suppress bypass transition in a zero-pressure-gradient boundary layer. To this end, a Lagrange variational formulation is employed that results in a set of adjoint equations. The optimal wall actuation (blowing and suction from a control slot) is found by solving iteratively the nonlinear Navier-Stokes and the adjoint equations in a forward/backward loop using DNS. The optimization is performed in a finite time horizon. Large values of optimization horizon result in instability of the adjoint equations. The control slot is located exactly in the region of transition. The results show that the control is able to significantly reduce the objective function, which is defined as the spatial and temporal integral of the quadratic deviation from the Blasius profile plus a term that quantifies the control cost. The physical mechanism with which the actuation interacts with the flow field is investigated and analysed in relation to the objective function employed. The spanwise averaged velocity is distorted by the control action, resulting in a significant reduction of the skin friction coefficient. We performed simulations with and without zero-net mass flow constraint of the actuation velocity. Results are also compared with uniform blowing using the same time-average velocity obtained from the non-linear optimal algorithm. |
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Feb 10 | Fri | Dr Andrew Leonard (University of Sheffield) | SP2RC seminar |

13:00 | F41 |
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Feb 13 | Mon | Moty Katzman (Sheffield) | Algebraic Geometry Learning Seminar |

02:00 | J11 | Maximal Cohen Macaulay modules over hypersurfaces: matrix factorizations and periodic resolutions | |

Feb 14 | Tue |
Nick Kuhn (Virginia) | Topology seminar |

16:00 | J11 | The circle product of O-bimodules with O-algebras, with applications. | |

Abstract:If O is an operad (in a friendly category, e.g. the category of S-modules of stable homotopy theory), M is an O-bimodule, A is an O-algebra, then the circle product over O of M with A is again an O-algebra. A useful derived version is the bar construction B(M,O,A). We survey many interesting constructions on O-algebras that have this form. These include an augmentation ideal filtration of an augmented O-algebra A, the topological Andre-Quillen homology of A, the topological Hochschild homology of A, and the tensor product of A with a space. Right O-modules come with canonical increasing filtrations, and this leads to filtrations of all of the above. In particular, I can show that a filtration on TAQ(A) defined recently by Behrens and Rezk agrees with one I defined about a decade ago, as was suspected. This is joint work with Luis Pereira. |
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Feb 15 | Wed | Nicola Gambino (University of Leeds) | Pure Maths Colloquium |

14:00 | J11 | Commutative 2-algebra, operads, and analytic functors | |

Abstract:Standard commutative algebra is based on commutative monoids, Abelian groups and commutative rings. In recent years, there has been some progress in developing an area that may be referred to as commutative 2-algebra, in which the familiar notions used in commutative algebra are replaced by their categorified counterparts (for example, commutative monoids are replaced by symmetric monoidal categories). The aim of this talk is to explain the analogy between standard commutative algebra and commutative 2-algebra, and to outline how this analogy suggests analogues of basic aspects of algebraic geometry. In particular, I will describe how some joint work with Andre’ Joyal on operads and analytic functors fits in this context. |
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Feb 15 | Wed | Felix Ng (Department of Geography, Sheffiled) | Applied Mathematics Colloquium |

15:00 |
Hicks, LT 10 |
Grain-scale processes in the Earth's polar ice sheets | |

Abstract:The spreading of the Antarctic and Greenland Ice Sheets is a slow viscous flow with nonlinear rheology. Besides temperature, grain sizes and crystal orientation within the polycrystalline ice are important factors behind the rheology. After giving this glaciological background, I will describe two mathematical models recently built to understand grain-size evolution. The first model is formulated to capture the observed grain-size profiles in ice cores. The second model tackles the fundamental process of normal grain growth (NGG), a coarsening process that occurs in metals as well as ice. |
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Feb 20 | Mon | Nebojsa Pavic (Sheffield) | Algebraic Geometry Learning Seminar |

02:00 | J11 | Singularity category and MCM sheaves | |

Feb 20 | Mon |
Gary Mirams (Nottingham) | Mathematical Biology Seminar Series |

14:00 |
Alfred Denny Conference Room |
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Feb 21 | Tue |
Paul Gardner (Sheffield) | Uncertainty Quantification reading group |

15:00 |
J11 |
The use of Bayesian calibration in the prediction of damage in structures | |

Abstract:This talk will include an overview of the field of structural health monitoring and damage identification, where the use of Bayesian calibrated models fit in and the aims of using this technique. It will conclude with challenges and future aims of using Bayesian calibrated subsystem models to make full system predictions of damage. |
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Feb 21 | Tue |
Angelica Osorno (Reed College) | Topology seminar |

16:00 | J11 | On equivariant infinite loop space machines | |

Abstract:An equivariant infinite loop space machine is a functor that constructs genuine equivariant spectra out of simpler categorical or space level data. In the late 80's Lewis-May-Steinberger and Shimakawa developed generalizations of the operadic approach and the Gamma-space approach respectively. In this talk I will describe work in progress that aims to understand these machines conceptually, relate them to each other, and develop new machines that are more suitable for certain kinds of input. This work is joint with Anna Marie Bohmann, Bert Guillou, Peter May and Mona Merling. |
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Feb 23 | Thu |
Number Theory Learning Seminar | |

13:00 | J-11 | Organizational First Seminar | |

Abstract:Organizational meeting: all interested parties are invited. |
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Feb 24 | Fri | Alex Shukhobodskiy (University of Sheffield) | SP2RC seminar |

13:00 | F41 |
Kink oscillations of expanding coronal loops | |

Abstract:Kink waves and oscillations in a thin expanding magnetic tube in the presence of flow are studied. The tube consists of a core region and a thin transitional region at the tube boundary. In this region 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 the multiscale expansions the system of two equations describing the kink oscillations is derived. When there is no transitional layer the oscillations are described by the first of these two equations. This equation is used to study the effect of plasma density variation with time on kink oscillations of expanding tube with a sharp boundary. It is assumed that the characteristic time of the density variation is much larger than the characteristic time of kink oscillations. Then the WKB method is used to derive the expression for the aidiabatic invariant, which is the quantity that is coserved when the plasma density varies. The general theoretical results are applied to the kink oscillations of coronal magnetic loops. The expanding loops with the half-circle shape is considered and it is assumed that the plasma temperature inside a loop decays exponentially. The dependencies of the fundamental mode frequency, the ratio of frequencies of the first overtone and fundamental mode, and the oscillation amplitude on time are calculated numerically. |
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Feb 27 | Mon | Evgeny Shinder (Sheffield) | Algebraic Geometry Learning Seminar |

02:00 | J11 | Knorrer periodicity | |

Feb 27 | Mon |
David Grimes (Oxford) | Mathematical Biology Seminar Series |

14:00 |
Alfred Denny Conference Room |
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Feb 28 | Tue |
Sam Morgan (Sheffield) | Differential geometry seminar |

11:00 |
J11 | Double Lie groupoids and their double Lie algebroids, I | |

Abstract:The series of talks will consist of a precise formulation of the double Lie algebroid of a double Lie groupoid. We will also discuss some of the examples arising in Poisson geometry. In the first talk we will consider the construction of the double Lie algebroid of an LA-groupoid. This will be a stepping stone in the general construction for a double Lie groupoid. Knowledge of the standard formation of the Lie algebroid of a Lie groupoid will not be assumed, and the notions of a Lie groupoid and a Lie algebroid will be recalled. |
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Feb 28 | Tue | Haluk Sengun (Sheffield) | Number Theory Learning Seminar |

13:00 | J-11 | Automorphic Forms and Representation Theory: An Overview | |

Abstract:We shall sketch the path that goes from modular forms to automorphic representations. |
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Feb 28 | Tue | Operator K-theory and Noncommutative Geometry Seminar | |

14:00 | J11 | Organizational First Meeting | |

Feb 28 | Tue |
Gareth Williams (Open) | Topology seminar |

16:00 | J11 | Weighted projective spaces, equivariant K-theory and piecewise algebra | |

Abstract:Weighted projective spaces are interesting through many lenses: for example, as natural generalisations of ordinary projective spaces, as toric varieties and as orbifolds. From the point of view of algebraic topology, it is natural to study their algebraic topological invariants – notably, their (equivariant) cohomology rings. Recent work has provided satisfying qualitative descriptions for these rings, in terms of piecewise algebra, for various cohomology theories. This talk will introduce weighted projective spaces as toric varieties and survey results on their (equivariant) cohomology rings, with particular focus on equivariant K-theory. It will conclude with recent results of Megumi Harada, Tara Holm, Nige Ray and the speaker, and indicate the flavour of current work of Tara Holm and the speaker. |
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Mar 1 | Wed | Anne Taormina (University of Durham) | Pure Maths Colloquium |

15:00 |
J11 | The riches of Mathieu Moonshine | |

Abstract:In 2009, three Japanese theoretical particle physicists observed that the elliptic genus of a K3 surface, when expressed in terms of mock modular forms, exposes numbers that can be linked to the dimensions of finite dimensional representations of the sporadic group Mathieu 24. Since then, this intriguing connection has been studied from several points of view, other examples of the same type of phenomenon for other finite groups and mock modular forms have been discovered, and the research topic of `New Moonshines’ has slowly caught the attention of researchers across fields. In this talk, I will describe the 2009 observation, now referred to as `Mathieu Moonshine’, and explain the challenges faced by the theoretical physics community in understanding the origin and role of the huge Mathieu 24 finite symmetry in the context of strings compactified on K3 surfaces. In particular, I will discuss how this phenomenon is related to the geometry of K3 surfaces and introduce the concept of symmetry surfing. |
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Mar 2 | Thu |
Andrew Corbett (Bristol) | Number Theory seminar |

13:00 | J11 |
Period integrals and special values of L-functions | |

Abstract:In many ways L-functions have been seen to contain interesting arithmetic information; evaluating at special points can make this connection very explicit. In this talk we shall ask what information is contained in central values of certain automorphic L-functions, in the spirit of the Gan--Gross--Prasad conjectures, and report on recent progress. We also describe some surprising applications in analytic number theory regarding the `size' of a modular form. |
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Mar 7 | Tue |
Sam Morgan (Sheffield) | Differential geometry seminar |

11:00 |
J11 | Double Lie groupoids and their double Lie algebroids, II | |

Abstract:To follow |
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Mar 7 | Tue |
Jeremy Colman (Sheffield) | Uncertainty Quantification reading group |

15:00 |
J11 |
Discussion of "Modelling extremes using approximate Bayesian Computation", by R. Erhardt and S. A. Sisson | |

Mar 7 | Tue |
Will Mycroft | Topology seminar |

16:00 | J11 | ||

Mar 14 | Tue |
Sam Morgan (Sheffield) | Differential geometry seminar |

11:00 |
J11 | Double Lie groupoids and their double Lie algebroids, III | |

Abstract:To follow |
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Mar 14 | Tue |
Dimitar Kodjabachev (Sheffield) | Topology seminar |

16:00 | J11 | ||

Mar 15 | Wed | Andrei Jaikin (Autonomous University of Madrid) | Pure Maths Colloquium |

14:00 | J11 | ||

Mar 16 | Thu |
Martin Dickson (King's College) | Number Theory seminar |

13:00 | J11 |
TBA | |

Mar 16 | Thu |
Lasse Rempe-Gillen (Liverpool) | SoMaS Colloquium |

16:00 |
LT7 |
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Mar 20 | Mon | Evgeny Shinder (Sheffield) | Algebraic Geometry Learning Seminar |

02:00 | J11 | BGG correspondence | |

Mar 20 | Mon |
Louise Riotte-Lambert (Glasgow) | Mathematical Biology Seminar Series |

14:00 |
Alfred Denny Conference Room |
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Mar 22 | Wed | Martin Lotz (University of Manchester) | Pure Maths Colloquium |

14:00 | J11 | ||

Mar 22 | Wed | Abraham Harte (Dublin) | Applied Mathematics Colloquium |

14:00 | Hicks, LT 9 | ||

Mar 23 | Thu |
Jeroen Sijsling (Ulm) | Number Theory seminar |

13:00 | J11 |
TBA | |

Mar 23 | Thu |
Lasse Rempe-Gillen (Liverpool ) | SoMaS Colloquium |

16:05 |
LT7 |
Metronomes and fireflies: Stability in the Arnold family | |

Abstract:*Phase-locking* (or phase synchronisation) is a phenomenon, first discovered by Huygens in the 17th century, in which two interacting oscillators synchronise their frequencies. It occurs in a plethora of physical and biological systems, from simple interacting pendula (search for “metronomes synchronise” on youtube …) to the synchronised behaviour of fireflies. In the 1960s, Vladimir Arnold proposed a one-dimensional discrete-time model of a periodically forced oscillator as the simplest context in which to study phase-locking phenomena. In this talk, I will describe a long-standing problem concerning the density of stable parameters within this family (arising from phase-locking phenomena), which we were able to resolve in recent work with van Strien (Duke Math. J., 2015). The talk will begin with a gentle introduction to one-dimensional discrete dynamics, including computer experiments of both the Arnold family and the well-known logistic family from population dynamics. These experiments naturally lead to the formulation of the density problem. The talk will hence be accessible to a general mathematical audience, including postgraduate students. Time permitting, I will also discuss how these developments are connected to, and were made possible by, recent progress in the study of the dynamics of transcendental entire functions. |
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Mar 27 | Mon | John Greenlees (Sheffield) | Algebraic Geometry Learning Seminar |

02:00 | J11 | Graded singularity category | |

Mar 29 | Wed | Ulrike Tillmann (University of Oxford) | Pure Maths Colloquium |

14:00 | J11 | ||

Apr 24 | Mon | Sven Meinhardt (Sheffield) | Algebraic Geometry Learning Seminar |

02:00 | J11 | Matrix factorizations and Homological Mirror Symmetry | |

Apr 24 | Mon |
Mirela Domijan (Liverpool) | Mathematical Biology Seminar Series |

14:00 |
Alfred Denny Conference Room |
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Apr 25 | Tue |
Ana Lecuona | Topology seminar |

16:00 | J11 | ||

Apr 26 | Wed | Vidit Nanda (Oxford) | Pure Maths Colloquium |

14:00 | J11 | tba | |

Apr 26 | Wed | Cedric Beaume (Leeds) | Applied Mathematics Colloquium |

14:00 | Hicks, LT 9 | ||

Apr 27 | Thu |
Rachel Newton (Reading) | Number Theory seminar |

13:00 | J11 |
TBA | |

May 8 | Mon |
Steve Webb (Liverpool John Moores) | Mathematical Biology Seminar Series |

14:00 |
Alfred Denny Conference Room |
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May 10 | Wed | Barbara Bolognese (Sheffield) | Pure Maths Colloquium |

14:00 | J11 | tba | |

May 11 | Thu |
Herbert Gangl (Durham) | Number Theory seminar |

13:00 | J11 |
TBA | |

May 17 | Wed | Kasia Rejzner (University of York) | Pure Maths Colloquium |

14:00 | J11 | ||

May 24 | Wed | Alvar Daza (Universidad Rey Juan Carlos) | Applied Mathematics Colloquium |

14:00 | Hicks, LT 9 | Topological Properties of Escape Basins in Open Systems | |