2019-09-17 | Tue | Francesco Sala (IPMU Tokyo) | Algebra / Algebraic Geometry seminar |
14:00 | J11 | Categorification of 2d cohomological Hall algebras | |
Abstract: Let $\mathcal{M}$ denote the moduli stack of either coherent sheaves on a smooth projective surface or Higgs sheaves on a smooth projective curve $X$. The convolution algebra structure on the Borel-Moore homology of $\mathcal{M}$ is an instance of two-dimensional cohomological Hall algebras. These examples were defined by Kapranov-Vasserot and by Schiffmann and me, respectively. In the present talk, I will describe a full categorification of the cohomological Hall algebra of $\mathcal{M}$. This is achieved by exhibiting a derived enhancement of $\mathcal{M}$. Furthermore, this method applies also to several other moduli stacks, such as the moduli stack of vector bundles with flat connections on $X$ and the moduli stack of finite-dimensional representations of the fundamental group of $X$. In the curve case, we call the corresponding categorified algebras the Betti, de Rham, and Dolbeaut categorified Hall algebras of the curve $X$, respectively. In the second part of the talk, I will discuss some relations between these categorified Hall algebras. This is based on a joint work with Mauro Porta. |
|||
2019-09-11 | Wed | Baofang Song (Center for Applied Mathematics, Tianjin University) | Applied Mathematics Colloquium |
14:00 | Hicks, K14 | The transition to turbulence and turbulence control in pipe flow | |
Abstract: The transition to turbulence in wall-bounded shear flows, such as pipe, channel, and Couette flows, is a fundamental problem of fluid dynamics. The questions of when and how turbulence rises in these flows, as Reynolds number increases, have challenged scientists and engineers for over a century and have not been fully understood till today. The complexity lies in the subcritical nature of the transition in these flows and the coexistence of various turbulent states and the quiescent laminar state during the transition process. Nevertheless, in recent years, significant advancements in this research area have been made. In this talk, I will present some results of our team on the transition to turbulence as well as turbulence control in pipe flow. |
|||
2019-08-14 | Wed | Ben Evans (Bristol) | Mathematical Biology Seminar Series |
16:00 | Alfred Denny Conference Room | Building biological constraints into convolutional neural networks for classification overcomes biases within datasets | |
Abstract: Part of the appeal of deep convolutional networks is their ability to learn on raw data, obviating the need to hand-code the feature space. It has been demonstrated that when networks perform "end- to-end" learning, they develop features in early layers that not only lead to a good classification performance but also resemble the representations found in biological vision systems. These results have been used to draw various parallels between deep learning systems and human visual perception. In this study, we show that end-to-end learning in standard convolutional neural networks (CNNs) trained on a modified CIFAR-10 dataset are found to rely upon idiosyncratic features within the dataset. Instead of relying on abstract features such as object shape, end-to-end learning can pick up on low-level and spatially high-frequency features, such as noise-like masks. Such features are extremely unlikely to play any role in human object recognition, where instead a strong preference for shape is observed. Through a series of empirical studies, we show that these CNNs cannot overcome such problems merely through regularisation methods or more ecologically plausible training regimes. However, we show that these problems can be ameliorated by forgoing end-to-end learning and processing images with Gabor filters in a manner that more closely resembles biological vision systems. These results raise doubts over the assumption that simply learning end-to-end in "vanilla" CNNs leads to the emergence of similar representations to those observed in biological vision systems. By adding more biological input constraints, we show that deep learning models can not only capture more aspects of human visual perception, but also become more robust to idiosyncratic biases within training sets. |
|||
2019-07-02 | Tue | Evgeny Shinder (Sheffield) | |
15:00 | Hicks LT 2 | Geometry of Singularities | |
2019-07-02 | Tue | Caitlin Buck (Sheffield) | |
15:30 | Hicks, LT2 | Maintaining Impact in Interdisciplinary Research | |
2019-07-02 | Tue | Neil Dummigan (Sheffield) | |
16:30 | Hicks LT2 | Lattices and theta series | |
2019-06-20 | Thu | Ioannis Kontogiannis (Leibniz-Institut für Astrophysik Potsdam, AIP) | |
10:00 | LT11 | Emergence of small-scale magnetic flux in the quiet Sun, observed from the photosphere to the corona | |
Abstract: We study the emergence and evolution of new magnetic flux in the vicinity of a quiet Sun network. We employ high-resolution spectropolarimetric, spectroscopic and spectral imaging observations from ground-based (Dutch Open Telescope) and space-born instruments (TRACE, Hinode, SoHO), which provided a multi-wavelength, tomographic view of the region from the photosphere up to the corona. Throughout its evolution, the region exhibited many of the phenomena revealed by recent simulations. The event starts with a series of granular-scale events, which follow the photospheric flow field and merge to form a small-scale magnetic flux system of the order of 1018 Mx. Spectropolarimetric inversions reveal an evolving, complicated pattern of horizontal and vertical magnetic field patches at the region between the main polarities. As the magnetic flux accumulates and the region expands, Doppler-shifted H-alpha absorption features appear above and at the crests of the structure, indicating an immediate interaction with the pre-existing, overlying magnetic field. Roughly 60 min after the region first emerged at the photosphere, a jet-like feature appeared in the chromosphere and a small soft X-ray bright point formed in the corona. The coronal brightening exhibited intense spatial and temporal variations and had a lifetime that exceeded one hour. EUV spectroscopy and DEM analysis revealed temperatures up to 106 K and densities up to 1010 cm-3. Even in the absence of a strong ambient magnetic field, small-scale magnetic flux emergence affects dramatically the dynamics and shape of the quiet Sun. |
|||
2019-06-20 | Thu | Ioannis Kontogiannis (Leibniz-Institut für Astrophysik Potsdam, AIP) | SP2RC seminar |
10:00 | LT11 | Emergence of small-scale magnetic flux in the quiet Sun, observed from the photosphere to the corona | |
Abstract: We study the emergence and evolution of new magnetic flux in the vicinity of a quiet Sun network. We employ high-resolution spectropolarimetric, spectroscopic and spectral imaging observations from ground-based (Dutch Open Telescope) and space-born instruments (TRACE, Hinode, SoHO), which provided a multi-wavelength, tomographic view of the region from the photosphere up to the corona. Throughout its evolution, the region exhibited many of the phenomena revealed by recent simulations. The event starts with a series of granular-scale events, which follow the photospheric flow field and merge to form a small-scale magnetic flux system of the order of 1018 Mx. Spectropolarimetric inversions reveal an evolving, complicated pattern of horizontal and vertical magnetic field patches at the region between the main polarities. As the magnetic flux accumulates and the region expands, Doppler-shifted H-alpha absorption features appear above and at the crests of the structure, indicating an immediate interaction with the pre-existing, overlying magnetic field. Roughly 60 min after the region first emerged at the photosphere, a jet-like feature appeared in the chromosphere and a small soft X-ray bright point formed in the corona. The coronal brightening exhibited intense spatial and temporal variations and had a lifetime that exceeded one hour. EUV spectroscopy and DEM analysis revealed temperatures up to 106 K and densities up to 1010 cm-3. Even in the absence of a strong ambient magnetic field, small-scale magnetic flux emergence affects dramatically the dynamics and shape of the quiet Sun. |
|||
2019-06-18 | Tue | Tong Liu (Purdue) | Number Theory seminar |
14:00 | J11 | p-divisible groups and crystalline representations over relative base | |
Abstract: Let K be a p-adic field, it is known that p-adic Tate module of p-divisible group over O_K is crystalline representation with Hodge-Tate weights in [0, 1]. And conversely any such crystalline representation arise from a p-divisible group over O_K. In this talk, we discuss how to generalize this result to relative bases when O_K is replaced by more general rings, like, Z_p[[t]]. This is a joint work with Yong Suk Moon. |
|||
2019-06-11 | Tue | Andreas Krug (Magburg) | Algebra / Algebraic Geometry seminar |
14:00 | J11 | Stability of Tautological Bundles on Symmetric Products of Curves | |
Abstract: Given a vector bundle E over smooth variety X, there is a natural way to associate a vector bundle, called tautological bundle, on the Hilbert scheme of points on X. In this talk, we will discuss stability of tautological bundles in the case that X is a curve. |
|||
2019-05-31 | Fri | Prof Yuanyong Deng (Director of Huairoi Observatory) (NAOC, CAS, China) | SP2RC seminar |
13:00 | LT09 | The observational research and projects of solar physics in China | |
Abstract: In this presentation I will briefly introduce recent solar observation and related research in China. Up to now all these observations come from ground-based telescopes. In the near future, Chinese will have our first space solar observatory by the Advanced Space solar telescope (ASO-S). In addition to ASO-S, some other projects under development or proposed will also be introduced and discussed. |
|||
2019-05-30 | Thu | David Jess (Queen's University (Belfast)) | Plasma Dynamics Group |
14:00 | Room LT10 (Hicks Building) | Resonance Cavities: A wave amplification mechanism above highly magnetic sunspots | |
Abstract: The solar atmosphere provides a unique astrophysical laboratory to study the formation, propagation, and subsequent dissipation of magnetohydrodynamic (MHD) waves across a diverse range of spatial scales. The concentrated magnetic fields synonymous with sunspots allow the examination of guided magnetoacoustic modes as they propagate upwards into the solar corona, where they exist as ubiquitous 3-minute waves readily observed along loops, plumes and fan structures. While cutting-edge observations and simulations are providing insights into the underlying wave generation and damping mechanisms, the in-situ ampliﬁcation of magnetoacoustic waves as they propagate through the solar chromosphere has proved difﬁcult to explain. Here we provide observational evidence of a resonance cavity existing above a magnetic sunspot, where the intrinsic temperature stratification provides the necessary atmospheric boundaries responsible for the resonant amplification of these waves. Through comparisons with high-resolution numerical MHD simulations, the geometry of the resonance cavity is mapped across the diameter of the underlying sunspot, with the upper boundaries of the chromosphere ranging between 1300–2300 km. This brings forth important implications for next-generation ground-based observing facilities, and provides an unprecedented insight into the MHD wave modelling requirements for laboratory and astrophysical plasmas. |
|||
2019-05-17 | Fri | Gong Show | Topology seminar |
16:00 | J11 | ||
2019-05-16 | Thu | Christopher Fallaize (Nottingham) | Statistics Seminar |
14:00 | LT E | Unlabelled Shape Analysis with Applications in Bioinformatics | |
Abstract: In shape analysis, objects are often represented as configurations of points, known as landmarks. The case where the correspondence between landmarks on different objects is unknown is called unlabelled shape analysis. The alignment task is then to simultaneously identify the correspondence between landmarks and the transformation aligning the objects. In this talk, I will discuss the alignment of unlabelled shapes, and discuss two applications to problems in structural bioinformatics. The first is a problem in drug discovery, where the main objective is to find the shape information common to all, or subsets of, a set of active compounds. The approach taken resembles a form of clustering, which also gives estimates of the mean shapes of each cluster. The second application is the alignment of protein structures, which will also serve to illustrate how the modelling framework can incorporate very general information regarding the properties we would like alignments to have; in this case, expressed through the sequence order of the points (amino acids) of the proteins. |
|||
2019-05-16 | Thu | Peter Wyper (University of Durham) | Plasma Dynamics Group |
16:00 | Room K14 (Hicks Building) | Reconnection, Topology and Solar Eruptions | |
Abstract: The majority of free energy in the solar corona is stored within sheared magnetic field structures known as filament channels. Filament channels spend most of their life in force balance before violently erupting. The largest produce powerful solar flares and coronal mass ejections (CMEs), whereby the filament channel is bodily ejected from the Sun. However, a whole range of smaller eruptions and flares also occur throughout the corona. Some are ejective, whilst others are confined. Recently it has been established that coronal jets are also typically the result of a filament channel eruption. The filament channels involved in jets are orders of magnitude smaller than the ones which produce CMEs. In this talk I will start by considering these tiny, jet producing eruptions. I will introduce our MHD simulation model that well describes them and then discuss what jets can tell us about solar eruptions in general. Specifically, I will argue that many different types of eruption can be understood by considering two defining features: the scale of the filament channel and its interaction via reconnection with its surrounding magnetic topology. |
|||
2019-05-15 | Wed | Martina Balagovic (Newcastle) | Pure Maths Colloquium |
14:00 | J11 | Quantum Yang Baxter equation, the reflection equation, and their universal solutions | |
Abstract: The quantum Yang Baxter equation arose in statistical mechanics around 1970 as the consistency condition for an interaction of two particles on a line. In the 1980s, Drinfled and Jimbo introduced quantum groups (deformations of universal enveloping algebras of Lie algebras), and showed that they allow a universal R matrix - an element constructed from the algebra, which systematically produces a solution of the quantum Yang Baxter equation in every representation of this algebra. In turn, this imposes a structure of a braided tensor category on representations of the quantum group (i.e. gives an action of the braid group of type A) and leads to the Reshetikhin-Turaev construction of invariants of knots, braids, and ribbons. Considering the same problem with a boundary (on a half line instead of a line) leads to the consistency condition called the (quantum) reflection equation, introduced by Cherednik and Sklyanin in the 1980s. I will explain how, in the joint work with S. Kolb, we use quantum symmetric pairs (Noumi, Sugitani, and Dijkhuizen; Letzter 1990s) to construct a universal K-matrix - an element which systematically produces solutions of the reflection equation. This gives an action of the braid group of type B, endowing the corresponding category of representations with a structure of a braided tensor category with a cylinder twist (as defined by T. tom Dieck, R. Haring-Oldenburg 1990s). |
|||
2019-05-14 | Tue | Anna Krystalli / Alison Parton / Lyn Taylor (Sheffield / Sheffield / Phastar) | RSS Seminar Series |
16:00 | LT 5 | Putting the R in Reproducible Research / Cloud Computing with R / R Validation Hub Project | |
Abstract: R and its ecosystem of packages offers a wide variety of statistical and graphical techniques and is increasing in popularity as the tool of choice for data analysis in academia. In addition to its powerful analytical features, the R ecosystem provides a large number of tools and conventions to help support more open, robust and reproducible research. This includes tools for managing research projects, building robust analysis workflows, documenting data and code, testing code and disseminating and sharing analyses. In this talk we’ll take a whistle-stop tour of the breadth of available tools, demonstrating the ways R and the Rstudio integrated development environment can be used to underpin more open reproducible research and facilitate best practice. R has cemented itself as the language of choice for many a statistician and data scientist, but is often heckled as a sluggish competitor to the likes of python. This talk will discuss one avenue for maintaining the comfort and power of R (see Anna’s talk!) without having to wait days for your desktop analysis to complete. The foreach package is a set of functions that allow virtually anything that can be expressed as a for-loop as a set of parallel tasks. By registering a parallel backend through the doParallel package, you can speed up the run-time of your work by utilising the full capacity of your machine. I’ll introduce how to rewrite workflows to utilise the foreach approach and show how you can implement a parallel workflow on your own machine with doParallel. For a low-range machine, this will reduce your run-time by 4-fold and for those lucky few with high-range budgets you’ll receive something around 16-fold. So how about going one step further, and increasing to hundreds-fold? We can achieve this by using cloud computing services, taking the load away from your own machine. Cloud computing services have been seen to have a steep learning curve and this has led to many shying away from using such a useful resource. I’ll introduce you to the doAzureParallel package for R, create by Microsoft to bypass this learning curve and allow you to implement the foreach package in parallel in the cloud with only minor amendments to the R code that has been blighting you for months. To date, the use of R Software in the pharmaceutical industry has been relatively limited to exploratory work and not routinely used in regulatory submissions where SAS® Software is still favored. One of the difficulties in using R for submissions is being able to provide the regulators with appropriate documentation of testing and validation for the packages used. In June 2018 the R consortium granted funding for a PSI AIMS SIG initiative to create an online ‘R package validation repository’. With representatives from Abbvie, Amgen, Biogen, Eli Lilly, FDA, GSK, J&J, Merck, Merck KGaA, Novartis, PPD, PRA, Pfizer, Roche / Genentech, Syne qua non and the Transcelerate project, the ‘R Validation Hub’ team launched a free to access web site to host validation documentation and metrics for R packages (https://www.pharmar.org/). Although, the project is still in its early stages, we are looking to expand on the website content and encourage contribution of R metrics and tests for packages from all R-users. The talk will discuss what is meant by validation, how R differs to SAS, justify our approach to the validation issue and present the future capabilities of the website and how all R-users are set to benefit from the work. |
|||
2019-05-13 | Mon | Natasha Ellison, Sara Hilditch, Elena Marensi, Alison Parton, Lizzie Sheppeck, Sarah Whitehouse, Sadiah Zahoor (University of Sheffield) | International Women in Mathematics Day |
10:00 | LT-5 | ||
2019-05-13 | Mon | Suzana de Souza e Almeida Silva (Technological Institute of Aeronautics, Sao Paulo) | Plasma Dynamics Group |
13:00 | LT9 (Hicks Building) | Lagrangian Coherent Structures: Overview and applications in solar physics | |
Abstract: Lagrangian coherent structures (LCS) is a newly developed theory which describes the skeleton of turbulent flows. LCS act as barriers in the flow, separating regions with different dynamics and organizing the flow into coherent patterns. This talk will introduce some concepts of LCT techniques as well as recent application to solar physics problems. |
|||
2019-05-13 | Mon | Nuria Folguera Blasco (Crick Institute) | Mathematical Biology Seminar Series |
14:00 | Hicks F20 | ||
2019-05-09 | Thu | Rebecca Killick (Lancaster) | Statistics Seminar |
14:00 | LT E | Computationally Efficient Multivariate Changepoint Detection with Subsets | |
Abstract: Historically much of the research on changepoint analysis has focused on the univariate setting. Due to the growing number of high dimensional datasets there is an increasing need for methods that can detect changepoints in multivariate time series. In this talk we focus on the problem of detecting changepoints where only a subset of the variables under observation undergo a change, so called subset multivariate changepoints. One approach to locating changepoints is to choose the segmentation that minimises a penalised cost function via a dynamic program. The work in this presentation is the first to create a dynamic program specifically for detecting changes in subset-multivariate time series. The computational complexity of the dynamic program means it is infeasible even for medium datasets. Thus we propose a computationally efficient approximate dynamic program, SPOT. We demonstrate that SPOT always recovers a better segmentation, in terms of penalised cost, then other approaches which assume every variable changes. Furthermore under mild assumptions the computational cost of SPOT is linear in the number of data points. In small simulation studies we demonstrate that SPOT provides a good approximation to exact methods but is feasible for datasets that contain thousands of variables observed at millions of time points. Furthermore we demonstrate that our method compares favourably with other commonly used multivariate changepoint methods and achieves a substantial improvement in performance when compared with fully multivariate methods. |
|||
2019-05-08 | Wed | Aditi Kar (Royal Holloway) | Pure Maths Colloquium |
14:00 | J11 | 2D Problems in Groups | |
Abstract: I will discuss a conjecture about stabilisation of deficiency in finite index subgroups of a finitely presented group and relate it to the D2 Problem of C.T.C. Wall and the Relation Gap problem. I will explain a pro-p version of the conjecture, as well as its higher dimensional abstract analogues and why we can verify the conjecture in these cases. |
|||
2019-05-08 | Wed | Kasia Rejzner (York) | Applied Mathematics Colloquium |
14:00 | Hicks, LT 9 | Perturbative algebraic QFT - Example of the sine-Gordon model | |
Abstract: In this talk I will present recent results on the construction of the net of local algebras for the sine-Gordon model. The approach I will present is that of perturbative algebraic QFT, in which the interacting fields are constructed using formal S-matrices. It has been shown that in sine-Gordon model these formal S-matrices can be realized as unitary operators in certain Hilbert space representation, appropriate for massless scalar field in 2 dimensions. |
|||
2019-05-03 | Fri | James Brotherston | THH reading group |
14:00 | J11 | Loop Spaces | |
2019-05-02 | Thu | Celeste Damiani (Leeds) | Topology seminar |
16:00 | J11 | TBA | |
2019-05-02 | Thu | Youra Taroyan (Aberystwyth University) | Plasma Dynamics Group |
16:00 | Room K14 (Hicks Building) | Amplification of magnetic twists during prominence formation | |
Abstract: Solar prominences are dense magnetic structures that are anchored to the visible surface known as the photosphere. They extend outwards into the Sun’s upper atmosphere known as the corona. Twists in prominence field lines are believed to play an important role in supporting the dense plasma against gravity as well as in prominence eruptions and coronal mass ejections (CMEs), which may have severe impact on the Earth and its near environment. We will use a simple model to mimic the formation of a prominence thread by plasma condensation. The process of coupling between the inflows and the twists will be discussed. We show that arbitrarily small magnetic twists should be amplified in time during the mass accumulation process. The growth rate of the twists is proportional to the mass condensation rate. |
|||
2019-05-01 | Wed | Sam Falle (Leeds) | Applied Mathematics Colloquium |
14:00 | Hicks, LT 9 | Shock structures described by hyperbolic balance laws | |
Abstract: In this talk I will consider shock structures that arise in systems of hyperbolic balance laws, i.e. hyperbolic systems of conservation laws with source terms. I show how the Whitham criterion for the existence of such shock structures can be extended to systems with more than one relaxation variable. In addition, I descibe a method based on the Hermite-Biehler theorem that is useful for determining the stability of the equilibrium states of such systems. The utility of this method is illustrated by a number of examples: ideal gas with two internal degrees of freedom, two fluid magnetohydrodynamics and magnetohydrodynamics with tensor resistivity. |
|||
2019-05-01 | Wed | Esmee te Winkel (Warwick) | ShEAF: postgraduate pure maths seminar |
16:00 | J11 Hicks | A combinatorial approach to surface homeomorphisms | |
Abstract: In geometric group theory, it is common to try to study a group by finding a meaningful action on a metric space. This talk is about the mapping class group of a compact surface and its action on various graphs. The mapping class group is the group of homeomorphisms up to isotopy. I will define this group and state some of its properties and open questions. After this motivation, I will introduce the curve graph and the pants graph associated to a surface and explain how the mapping class group acts on them. |
|||
2019-04-18 | Thu | Philippa Browning (University of Manchester) | Plasma Dynamics Group |
16:00 | Room K14 (Hicks Building) | Plasma heating and particle acceleration by magnetic reconnection in solar and stellar flares | |
Abstract: In this talk, I will describe recent models of plasma heating and non-thermal particle acceleration in flares, focussing on the role of twisted magnetic flux ropes as reservoirs of free magnetic energy. First, using 2D magnetohydrodynamic simulations coupled with a guiding-centre test-particle code, I will describe magnetic reconnection and particle acceleration in a large-scale flaring current sheet, triggered by an external perturbation – the “forced reconnection” scenario. I will show how reconnection is involved both in creating twisted flux ropes, and in their merger, how this depends on the nature of the driving disturbance, and how particles are accelerated by the different modes of reconnection. Moving to 3D models, showing how fragmented current structures in kink-unstable twisted loops can both heat plasma and accelerate charged particles. Forward modelling of the observational signatures of this process in EUV, hard X-rays and microwaves will be described, and the potential for observational identification of twisted magnetic fields in the solar corona discussed. Then, coronal structure with multiple twisted threads will be considered, showing how instability in a single unstable twisted thread may trigger reconnection with stable neighbours, releasing their stored energy and causing an "avalanche" of heating events, with important implications for solar coronal heating. This avalanche can also accelerate electrons and ions throughout the structure. Many other stars exhibit flares, and I will briefly discuss recent work on modelling radio emission in flares in young stars (T Tauri stars). In particular, the enhanced radio luminosity of these stars relative to scaling laws for the Sun and other Main Sequence stars will be discussed. |
|||
2019-04-05 | Fri | Luca Pol | THH reading group |
14:00 | J11 | Topological Cyclic Homology | |
2019-04-04 | Thu | Chris Birkbeck (UCL) | Number Theory seminar |
14:00 | J11 | Overconvergent Hilbert modular forms via perfectoid methods | |
Abstract: Following a construction of Chojecki-Hansen-Johansson, we use Scholze's infinite level modular varieties and the Hodge-Tate period map to give a new definition of overconvergent elliptic and Hilbert modular forms which is analogous to the standard construction of modular forms as functions on the upper half plane. This has applications to constructing overconvergent Eichler-Shimura maps in these settings. This is all work in progress joint with Ben Heuer and Chris Williams. |
|||
2019-04-04 | Thu | Richard Hepworth (Aberdeen) | Topology seminar |
16:00 | J11 | CANCELLED | |
2019-04-03 | Wed | Mahesh Kakde (King's College London) | Pure Maths Colloquium |
14:00 | J11 | Explicit formulae for Gross-Stark units and Hilbert’s 12 problem | |
Abstract: I will introduce the Gross-Stark units and present their application to Hilbert’s 12th problem. Following an earlier work in special case with Darmon, Dasgupta gave precise conjectural p-adic analytic formula for these units. After giving a formulation of this conjecture, I will sketch a proof of this conjecture. This is a joint work in progress with Samit Dasgupta. |
|||
2019-04-03 | Wed | Matt Turner (Surrey) | Applied Mathematics Colloquium |
14:00 | Hicks, LT 9 | Dynamic sloshing via time-dependent conformal mappings | |
Abstract: In this talk we examine two-dimensional, inviscid, irrotational fluid sloshing in both fixed and moving vessels. In particular we focus on a numerical scheme which utilizes time-dependent conformal mappings of doubly-connected domains to produce a scheme which is fast and efficient. Results are presented for flows in a fixed vessel, a moving vessel with bottom topography, a coupled pendulum slosh problem and a fixed vessel with multiple horizontal baffles. The application of this work is to the modelling of offshore wave energy converters. |
|||
2019-04-02 | Tue | Matthew Allcock, Farhad Allian, Callum Reader (Sheffield) | Postgraduate seminars |
13:00 | J11 | Postgraduate Student Seminar | |
Abstract: Matthew Allcock - The mathematics of making the right decisions In philosophy, there are two types of uncertainty: empirical uncertainty - uncertainty about what “is” - and normative uncertainty - uncertainty about what “should be”. We know how to deal with empirical uncertainty - we use expected value theory. But how should we deal with normative uncertainty? It turns out that we can define a mathematical framework analogous to expected value theory that deals with normative uncertainty. This framework works… sometimes… until it breaks. Let’s try to fix those breaks using mathematics. Farhad Allian - Observations of solar coronal loop oscillations Imagine you're hiking up the highest hill in the Peak District. But to your disbelief, you notice that your surrounding air becomes hotter as you approach the summit. This is exactly what happens on our Sun: The solar atmosphere is around 200 times hotter than its surface, and this seemingly paradoxical statement has left solar physicists puzzled for decades. In this talk, I will present my research on how I'm combining high-resolution images with mathematics to understand how the Sun's atmosphere can be heated to 2,000,000 Kelvin. Callum Reader - Biodiversity metrics from category theory In 1973 philosopher and mathematician Lawvere published his paper “Metric Spaces, Generalised Logic, and Closed Categories”, outlining the theory of enriched categories: a generalisation common of both regular categories and metric spaces. Around forty years later, Leinster introduced the concept of magnitude (a generalisation of Euler characteristic) for an enriched category, distilling all its information to a single value in some ring. Interestingly, when specifically applied to a metric space this seems to give some information about the “effective number of points” of the space, providing a better means of measuring biodiversity and answering the age old question: “yeah but what are the applications?” |
|||
2019-04-02 | Tue | Arne Grauer, Lukas Lüchtrath (Cologne) | Statistics Seminar |
16:00 | F28 | The age-dependent random connection model | |
Abstract: We consider a class of growing graphs embedded into the $d$-dimensional torus where new vertices arrive according to a Poisson process in time, are randomly placed in space and connect to existing vertices with a probability depending on time, their spatial distance and their relative ages. This simple model for a scale-free network is called the age-based spatial preferential attachment network and is based on the idea of preferential attachment with spatially induced clustering. The graphs converge weakly locally to a variant of the random connection model, which we call the age-dependent random connection model. This is a natural infinite graph on a Poisson point process where points are marked by a uniformly distributed age and connected with a probability depending on their spatial distance and both ages. We use the limiting structure to investigate asymptotic degree distribution, clustering coefficients and typical edge lengths in the age-based spatial preferential attachment network. |
|||
2019-03-29 | Fri | Jordan Williamson | THH reading group |
14:00 | J11 | Tate Diagonal | |
2019-03-28 | Thu | Stanislav Gunár (Astronomical Institute of the Czech Academy of Sciences) | SP2RC seminar |
10:00 | F28 | 3D Whole-Prominence Fine Structure models: the current state of the affairs | |
Abstract: To understand the links between the distribution of the prominence plasma, the configuration of its magnetic field and the observations of prominence/filament fine structures obtained in UV/EUV, optical and radio domains from various vantage points, we need complex 3D prominence models. We have developed two such models which combine 3D magnetic field configurations of an entire prominence with a detailed description of the prominence plasma distributed along hundreds of fine structures. The first 3D Whole-Prominence Fine Structure (WPFS) model, developed by Gunár & Mackay (2015), uses a magnetic field configuration obtained from non-linear force-free field simulations of Mackay & van Ballegooijen (2009). The second WPFS model was developed by Gunár, Dudík, Aulanier, Schmieder & Heinzel (2018). The model employs a magnetic field configuration of a polar crown prominence based on the linear force-free field modelling approach designed by Aulanier & Démoulin (1998) which allows us to calculate linear magneto-hydrostatic extrapolations from photospheric flux distributions. The prominence plasma in both models is located in magnetic dips that occur naturally in the predominantly horizontal prominence magnetic field. This plasma has a realistic distribution of the density and temperature, including the prominence-corona transition region. The models thus provide comprehensive information about the 3D distribution of the prominence plasma and magnetic field which can be consistently studied both as a prominence on the limb and as a filament on the disk. These models can be visualized for example in the H-alpha spectral line. Together with the models, we will present some of their capabilities which allow us to study the evolution of prominences/filaments or to analyze the true and apparent shapes and motions of the prominence fine structures. |
|||
2019-03-28 | Thu | Mark Quinn (Sheffield) | Teaching Lunch |
13:00 | Hicks LT6 | Colcalc and Sagemath | |
2019-03-28 | Thu | Jeremy Oakley (Sheffield) | Statistics Seminar |
14:00 | LT E | Variational inference reading group | |
Abstract: We will be spending two seminar slots on the following: Variational Inference: A Review for Statisticians https://arxiv.org/abs/1601.00670 David M. Blei, Alp Kucukelbir, Jon D. McAuliffe |
|||
2019-03-28 | Thu | Jordan Williamson (Sheffield) | Topology seminar |
16:00 | J11 | A Left Localization Principle and Cofree G-Spectra | |
Abstract: Greenlees-Shipley developed a Cellularization Principle for Quillen adjunctions in order to attack the problem of constructing algebraic models for rational G-spectra. One example of this was the classification of free rational G-spectra as torsion modules over the cohomology ring H*(BG) (for G connected). This has some disadvantages; namely that it is not monoidal and that torsion modules supports only an injective model structure. I will explain a related method called the Left Localization Principle, and how this can be used to construct a monoidal algebraic model for cofree G-spectra. This will require a tour through the different kinds of completions available in homotopy theory. This is joint work with Luca Pol. |
|||
2019-03-27 | Wed | Daniele Avitabile (Nottingham) | Applied Mathematics Colloquium |
14:00 | Hicks, LT 9 | Interfacial dynamics for neurobiological networks: from excitability thresholds to localised spatiotemporal chaos | |
Abstract: We will discuss level-set based approaches to study the existence and bifurcation structure of spatio-temporal patterns in biological neural networks. Using this framework, which extends previous ideas in the study of neural field models, we study the first example of canards in an infinite-dimensional dynamical system, and we give a novel characterisation of localised structures, informally called “bumps”, supported by spiking neural networks. We will initially consider a spatially-extended network with heterogeneous synaptic kernel. Interfacial methods allow for the explicit construction of a bifurcation equation for localised steady states. When the model is subject to slow variations in the control parameters, a new type of coherent structure emerges: the structure displays a spatially-localised pattern, undergoing a slow-fast modulation at the core. Using interfacial dynamics and geometric singular perturbation theory, we show that these patterns follow an invariant repelling slow manifold, hence we name them "spatio-temporal canards". We classify spatio-temporal canards and give conditions for the existence of folded-saddle and folded-node canards. We also find that these structures are robust to changes in the synaptic connectivity and firing rate. The theory correctly predicts the existence of spatio-temporal canards with octahedral symmetries in a neural field model posed on a spherical domain. We will then discuss how the insight gained with interfacial dynamics may be used to perform coarse-grained bifurcation analysis on neural networks, even in models where the network does not evolve according to an integro-differential equation. As an example I will consider a well-known event-driven network of spiking neurons, proposed by Laing and Chow. In this setting, we construct numerically travelling waves whose profiles possess an arbitrary number of spikes. An open question is the origin of the travelling waves, which have been conjectured to form via a destabilisation of a bump solution. We provide numerical evidence that this mechanism is not in place, by showing that disconnected branches of travelling waves with countably many spikes exist, and terminate at grazing points; the grazing points correspond to travelling waves with an increasing number of spikes, a well-defined width, and decreasing propagation speed. We interpret the so called “bumps” and “meandering bumps”, supported by this model as localised states of spatiotemporal chaos, whereby the dynamics visits a large number of unstable localised travelling wave solutions. |
|||
2019-03-27 | Wed | Andreea Mocanu (Nottingham) | ShEAF: postgraduate pure maths seminar |
16:00 | J11 Hicks | On the connection between Jacobi forms and elliptic modular forms | |
Abstract: Jacobi forms arise naturally in number theory, for example as functions of lattices or as Fourier-Jacobi coefficients of other types of modular forms. They have applications in algebraic geometry, string theory and the theory of vertex operator algebras, among other areas. We are interested in establishing a precise connection between Jacobi forms of lattice index and elliptic modular forms, in other to transfer information from one side to the other. In this talk, we illustrate this connection via an example, namely that of Jacobi forms whose indices are the root lattices of type $D_n$. |
|||
2019-03-22 | Fri | Steffen Gielen (Nottingham) | Applied Mathematics Colloquium |
14:00 | TBA | The universe as a condensate of spacetime atoms | |
Abstract: In the standard picture of cosmology, the Universe began at the Big Bang; the Big Bang itself is a singularity where the laws of physics break down. A quantum theory of gravity should resolve this singularity and help in understanding the initial state of the Universe needed to account for present observations. I will present some progress towards this goal in the group field theory approach to quantum gravity, using the idea of a universe formed as a "condensate", i.e. a very homogeneous quantum configuration, from a large number of discrete building blocks of geometry. I will show how this setting produces new cosmological models without an initial singularity; demanding that such models be both theoretically self-consistent and potentially compatible with observation then gives new ways for constraining theories of quantum gravity. |
|||
2019-03-22 | Fri | Nicola Bellumat | THH reading group |
14:00 | J11 | Periodic Theories | |
2019-03-21 | Thu | Theo Kypraios (Nottingham) | Statistics Seminar |
14:00 | LT E | Recent Advances in Identifying Transmission Routes of Healthcare Associated Infections using Whole Genome Sequence Data | |
Abstract: Healthcare-associated infections (HCAIs) remain a problem worldwide, and can cause severe illness and death. It is estimated that 5-10% of acute-care patients are affected by nosocomial infections in developed countries, with higher levels in developing countries. Statistical modelling has played a significant role in increasing understanding of HCAI transmission dynamics. For instance, many studies have investigated the dynamics of MRSA transmission in hospitals, estimating transmission rates and the effectiveness of various infection control measures. However, uncertainty about the true routes of transmission remains and that is reflected on the uncertainty of parameters governing transmission. Until recently, the collection of whole genome sequence (WGS) data for bacterial organisms has been prohibitively complex and expensive. However, technological advances and falling costs mean that DNA sequencing is becoming feasible on a larger scale. In this talk we first describe how to construct statistical models which incorporate WGS data with regular HCAIs surveillance data (admission/discharge dates etc) to describe the pathogen's transmission dynamics in a hospital ward. Then, we show how one can fit such models to data within a Bayesian framework accounting for unobserved colonisation times and imperfect screening sensitivity using efficient Markov Chain Monte Carlo algorithms. Finally, we illustrate the proposed methodology using MRSA surveillance data collected from a hospital in North-East Thailand. |
|||
2019-03-21 | Thu | Tom Fisher (Cambridge) | Number Theory seminar |
14:00 | J11 | The proportion of genus one curves that are everywhere locally soluble | |
Abstract: I will describe joint work with Bhargava and Cremona, and with Ho and Park, on the probability that a randomly chosen genus one curve is soluble over the p-adics. A striking feature of this work is that we obtain exact answers in the form of explicit rational functions of p. I will also discuss what is expected to happen globally. |
|||
2019-03-21 | Thu | Mike Prest (Manchester) | Topology seminar |
16:00 | J11 | Categories of imaginaries for additive categories | |
Abstract: There is a construction of Freyd which associates, to any ring R, the free abelian category on R. That abelian category may be realised as the category of finitely presented functors on finitely presented R-modules. It has an alternative interpretation as the category of (model-theoretic) imaginaries for the category of R-modules. In fact, this extends to additive categories much more general than module categories, in particular to finitely accessible categories with products and to compactly generated triangulated categories. I will describe this and give some examples of its applications. |
|||
2019-03-21 | Thu | Peter Keys (Queen's University (Belfast)) | Plasma Dynamics Group |
16:00 | Room K14 (Hicks Building) | Small-scale magnetic field evolution with high resolution observations. | |
Abstract: Small-scale magnetic fields, ubiquitous across the solar surface, manifest as intensity enhancements in intergranular lanes and, thus, often receive the moniker of magnetic bright point (MBP). MBPs are frequently studied as they are considered as a fundamental building block of magnetism in the solar atmosphere. The theory of convective collapse developed in the late 70’s and early 80’s is often used to explain how kilogauss fields form in MBPs. The dynamic nature of MBPs coupled with these kilogauss fields means that they are frequently posited as a source of wave phenomena in the solar atmosphere. Here, with high resolution observations of the quiet Sun with full Stokes spectropolarimetry, we investigate the magnetic properties of MBPs. By analysing the temporal evolution of various physical properties obtained from inversions, we show that kilogauss fields in MBPs can appear due to a variety of reasons, and is not limited to the process of convective collapse. Analysis of MURaM simulations confirms the processes we observe in our data. Also, magnetic field amplification happens on rapid timescales, which has significant implications for many wave studies. |
|||
2019-03-20 | Wed | Sven Meinhardt (Sheffield) | Pure Maths Colloquium |
14:00 | J11 | New developments in modern moduli theory | |
Abstract: The idea of moduli spaces classifying structures in various fields of mathematics dates back to Riemann who tried to classify complex structures on a compact surface. It took another hundred years and many ingenious ideas of Grothendieck, Mumford and other mathematicians to write down a proper definition of moduli spaces and to construct nontrivial examples including Riemann‘s vague idea of a moduli space of complex structures on a surface. However, it became quite obvious that the concept of moduli spaces/stacks developed in the 60‘s and 70‘s is not sufficient to describe all moduli problems. Another 50 years and a fair amount of homotopy theory was needed to provide a definition of moduli spaces having all required properties. A large class of examples comes from (higher) representation theory. The aim of my talk is to provide a gentle introduction into these new concepts and thereby to show how nicely algebraic geometry, topology and representation theory interact with each other. If time permits, I will also sketch applications in Donaldson-Thomas theory. |
|||
2019-03-20 | Wed | Gianmarco Brocchi (Birmingham) | ShEAF: postgraduate pure maths seminar |
16:00 | J11 Hicks | What does extremise a Strichartz estimate? | |
Abstract: This will be a blunt talk on sharp inequalities. Roughly speaking, these are inequalities which cannot be improved. In particular, I will introduce inequalities for the restriction of the Fourier transform, explaining why I got interested in them and how they are related to other inequalities in PDE, such as Strichartz estimates. These are a key tool in understanding the evolution of waves in dispersive PDE. If time allows, I will discuss a sharp Strichartz estimate for the fourth order Schrödinger equation from a joint work with Diogo Oliveira e Silva and René Quilodrán. |
|||
2019-03-15 | Fri | Jonathan Potts (Sheffield) | Teaching Lunch |
13:00 | Hicks LT10 | MOLE exams are great | |
Abstract: We explain our use of MOLE exams in MAS222 and why it is a win-win tool for students and staff alike. The "win" for staff is particularly wonderful as it removes that most onerous task: exam marking. We'll start with a very brief presentation of how to set a MOLE exam up and how we've used them in MAS222. Then we'll open the floor to discussion about how they might be used more widely in SoMaS. |
|||
2019-03-15 | Fri | Luca Pol | THH reading group |
14:00 | J11 | $E_n$ algebras | |
2019-03-14 | Thu | Luca Giovannelli (University of Rome Tor Vergata) | SP2RC seminar |
10:00 | F28 | Emerging bipolar magnetic pairs in the solar photosphere: diffusion properties and contribution to the coronal heating | |
Abstract: The ubiquitous presence of small magnetic elements in the Quiet Sun represents a prominent coupling between the photosphere and the upper layers of the Sun’s atmosphere. Small magnetic element tracking has been widely used to study the transport and diffusion of the magnetic field on the solar photosphere. From the analysis of the displacement spectrum of these tracers, it has been recently agreed that a regime of super-diffusivity dominates the solar surface. In this talk we will focus on the analysis of the bipolar magnetic pairs in the solar photosphere and their diffusion properties, using a 25-h dataset from the HINODE satellite. Interestingly, the displacement spectrum for bipolar couples behaves similarly to the case where all magnetic pairs are considered. We also measure, from the same dataset, the magnetic emergence rate of the bipolar magnetic pairs and we interpret them as the magnetic footpoints of emerging magnetic loops. The measured magnetic emergence rate is used to constrain a simplified model that mimics the advection on the solar surface and evolves the position of a great number of loops, taking into account emergence, reconnection and cancellation events. In particular we compute the energy released by the reconnection between different magnetic loops in the nano-flares energy range. Our model gives a quantitative estimate of the energy released by the reconfiguration of the magnetic loops in a quiet Sun area as a function of height in the solar atmosphere, from hundreds of Km above the photosphere up to the corona, suggesting that an efficiency of ~10% in the energy deposition might sustain the million degree corona. |
|||
2019-03-14 | Thu | Jeremy Oakley (Sheffield) | Statistics Seminar |
14:00 | LT E | Variational inference reading group | |
Abstract: We will be spending two seminar slots on the following: Variational Inference: A Review for Statisticians https://arxiv.org/abs/1601.00670 David M. Blei, Alp Kucukelbir, Jon D. McAuliffe |
|||
2019-03-14 | Thu | Neil Strickland (Sheffield) | Topology seminar |
16:00 | J11 | Dilation of formal groups, and potential applications | |
Abstract: I will describe an extremely easy construction with formal group laws, and a slightly more subtle argument to show that it can be done in a coordinate-free way with formal groups. I will then describe connections with a range of other phenomena in stable homotopy theory, although I still have many more questions than answers about these. In particular, this should illuminate the relationship between the Lambda algebra and the Dyer-Lashof algebra at the prime 2, and possibly suggest better ways to think about related things at odd primes. The Morava K-theory of symmetric groups is well-understood if we quotient out by transfers, but somewhat mysterious if we do not pass to that quotient; there are some suggestions that dilation will again be a key ingredient in resolving this. The ring $MU_*(\Omega^2S^3)$ is another object for which we have quite a lot of information but it seems likely that important ideas are missing; dilation may also be relevant here. |
|||
2019-03-13 | Wed | Fatemeh Mohammadi (Bristol) | Pure Maths Colloquium |
14:00 | J11 | Chip-firing game and Riemann-Roch theory for graphs | |
Abstract: Theory of divisors on graphs is analogous to the classical theory for algebraic curves. The combinatorial language in this setting is "chip-firing game” which has been independently introduced in other fields. A divisor on a graph is simply a configuration of dollars (integer numbers) on its vertices. In each step of the chip-firing game we are allowed to select a vertex and then lend one dollar to each of its neighbors, or borrow one dollar from each of its neighbors. The goal of the chip-firing game is to get all the vertices out of debt. In this setting, there is a combinatorial analogue of the classical Riemann-Roch theorem. I will explain the mathematical structure arising from this process and how it sits in a more general framework of (graphical) hyperplane arrangements. |
|||
2019-03-13 | Wed | Tobias Grafke (Warwick) | Applied Mathematics Colloquium |
14:00 | Hicks, LT 9 | Hydrodynamic instantons and the universal route to rogue waves | |
Abstract: In stochastic systems, extreme events are known to be described by "instantons", saddle point configurations of the action of the associated stochastic field theory. In this talk, I will present experimental evidence of a hydrodynamic instanton in a real world fluid system: A 270m wave channel experiment in Norway. The experiment attempts to model conditions on the ocean in order to observe so-called rogue waves, realisations of extreme ocean surface elevation out of relatively calm surroundings. These rogue waves are also observed in the ocean, where they are rare and hard to predict but pose significant danger to naval vessels. We show that the instanton approach, which is rigorously grounded in large deviation theory, offers a unified description of rogue waves in the water tank, covering the entire range of parameters for deep water waves in the ocean. In particular, this approach allows for a unified description of both the predominantly linear and the highly nonlinear regimes, and is able to explain the experimental data in the tank regardless of the strength of the nonlinearity. |
|||
2019-03-13 | Wed | Karoline Van Gemst (Birmingham) | ShEAF: postgraduate pure maths seminar |
16:00 | J11 Hicks | Enumerative geometry in projective space and Kontsevich's formula | |
Abstract: Enumerative geometers are interested in counting certain geometric objects given a set of conditions. One example of such a counting problem is how many degree d rational curves pass through 3d-1 generically placed given points in the projective plane. This particular problem proved extremely difficult using classical methods, even for low d. In the 1990s however, a revolution within this area took place, originating in the world of physics. This led to Kontsevich solving the counting problem by proving a recursive formula for calculating this number for any d. Kontsevich’s formula requires a single initial datum, the case of d=1, which translates to the fact that a single line passes through two given points in the plane. In this talk, I will present some of the crucial ingredients in setting up for and proving Kontsevich’s formula, and illustrate how it makes sense through a few examples. If time permits, I will also motivate how the formula can be viewed as expressing the associativity of the quantum product. |
|||
2019-03-12 | Tue | Giovanni Marchetti | Algebraic Geometry Learning Seminar |
13:00 | J11 | Motivic Homotopy Reading Seminar | |
Abstract: Talk 1: Ouverture. |
|||
2019-03-08 | Fri | Hope Thackray, Jake Percival, Bryony Moody (Sheffield) | Postgraduate seminars |
16:00 | J11 | PGR Student Seminar | |
Abstract: Hope Thackray - How do we see inside the Sun? Much like the waves known to exist above the solar surface, the Sun itself exhibits a widespread pulsation, mimicking the beating of a heart. Sound waves resonate inside the Sun, buffeting the surface, and causing light emitted to experience Doppler-shifting. The structures of these resonant cavities may then be deduced from observations of these shifts, allowing us to "see'' the Sun's interior. Here, one such method of deriving the Sun's sub-surface flows is described, in a technique known as Ring Diagram Analysis. Jake Percival - RNG's: How computers handle randomness When we want a “random” number in everyday life, such as when playing a board game, we rely on processes that aren't truly random, such as rolling a die. Perhaps more reliable would be to ask a computer to produce a random number for us. The code used to give these numbers is called a Random Number Generator (RNG) and like rolling a die, they aren't truly random! But if they aren't random, what is actually happening “under the hood”? In this talk we'll look at how RNG's work and how they can go wrong, including a fun example from the world of video games! Bryony Moody - The hidden layer of statistics in archaeology This talk will give a brief overview of Bayesian inference and the concepts of prior and posterior knowledge. Then I will discuss the various forms of prior knowledge available in archaeology, as well as the data that are used in conjunction with the prior knowledge to form a posterior. Finally I will conclude by discussing the priors I am focusing on for my PhD and what my plans are for modelling them. |
|||
2019-03-07 | Thu | Christian Fonseca Mora (Costa Rica) | Statistics Seminar |
14:00 | LT E | Stochastic PDEs in Infinite Dimensional Spaces | |
Abstract: In this talk we will give an introduction to SPDEs in spaces of distributions. In the first part of the talk we consider a model of environmental pollution with Poisson deposits that will help to introduce the basic concepts for the study of SPDEs on infinite dimensional spaces. In the second part of the talk, we introduce a generalized form of SPDEs in spaces of distributions and explain conditions for the existence and uniqueness of its solutions. For this talk we will not assume any previous knowledge on SPDEs. |
|||
2019-03-07 | Thu | Jean-Stefan Koskivirta (Tokyo) | Number Theory seminar |
14:00 | J11 | Ampleness and vanishing results | |
Abstract: We explain an application of the existence of generalized Hasse invariants to show ampleness of certain line bundles on flag spaces of Shimura varieties of Hodge type in positive characteristic. These methods generalize to other types of schemes which carry a universal G-zip. We deduce vanishing results for the cohomology of automorphic vector bundles. We compare them with similar results of Lan-Suh. |
|||
2019-03-07 | Thu | Irakli Patchkoria (Aberdeen) | Topology seminar |
16:00 | J11 | Computations in real topological Hochschild and cyclic homology | |
Abstract: The real topological Hochschild and cyclic homology (THR, TCR) are invariants for rings with anti-involution which approximate the real algebraic K-theory. In this talk we will introduce these objects and report about recent computations. In particular we will dicuss components of THR and TCR and some recent and ongoing computations for finite fields. This is all joint with E. Dotto and K. Moi. |
|||
2019-03-07 | Thu | Patrick Antolin (University of St Andrews) | Plasma Dynamics Group |
16:00 | Room K14 (Hicks Building) | Transverse MHD Waves and associated dynamic instabilities in the solar atmosphere | |
Abstract: A large amount of recent simulations and analytical work indicate that standing transverse MHD waves in loops should easily lead to the generation of dynamic instabilities at their edges, and in particular of the Kelvin-Helmholtz kind. While a direct observation of these transverse wave-induced Kelvin-Helmholtz rolls (or TWIH rolls) is still lacking, the forward modelling of these simulations give us an indication of what to look for in next generation instrumentation, and which currently observed features could actually be the result of TWIKH rolls. In this talk I will go through some of these results, comparing observations with various instruments with simulations of coronal loops, prominences and spicules. |
|||
2019-03-07 | Thu | David Robinson (Capital One) | RSS Seminar Series |
16:30 | F38 | Data Science & Machine Learning | |
Abstract: David will start his talk with a brief history of Statistics and Data Science at Capital One: how we got here, what's changed, and what the current expectations and challenges are in the era of "Big Data" and "Machine Learning". The main technical focus will then be on the use of "Gradient Boosting Machines", which over the last few years have emerged as the modelling method of choice for most classification problems within Financial Services. David will cover what they are, why they have become popular and how many of the practical considerations and pitfalls of traditional statistical techniques still very much apply. Example uses will focus on credit risk and affordability, looking at how we can ensure we make fair lending decisions when faced with unfair and biased data. |
|||
2019-03-06 | Wed | Matt Aldridge / Sarah Penington / Helena Stage / Henning Sulzbach (Leeds / Bath / Manchester / Birmingham) | Probability in the North East |
12:30 | LT C | ||
2019-03-06 | Wed | Gwyneth Stallard (Open University) | Pure Maths Colloquium |
14:00 | J11 | Complex dynamics: the intriguing case of wandering domains | |
Abstract: Complex dynamics concerns the iteration of analytic functions of the complex plane. For each function, the plane is split into two sets: the Fatou set (where the behaviour of the iterates is stable under local variation) and the Julia set (where the behaviour is chaotic). The dynamical behaviour of the iterates inside the periodic components of the Fatou set was classified into four different types by the founders of the subject and this classification has played a key role in the development of the subject. One of the most dramatic breakthroughs was given by Sullivan in the 1980s when he proved that, for rational functions, all components of the Fatou set are eventually periodic and there are no so-called wandering domains. For transcendental functions, however, wandering domains can exist and the rich variety of possible behaviours that can occur is only just becoming apparent. |
|||
2019-03-06 | Wed | Rachael Hardman (SoMaS) | Applied Mathematics Colloquium |
14:00 | Hicks, K14 | Measuring the Ocean Spectrum using HF Radar Data and a Neural Network | |
Abstract: High frequency, or HF, coastal radars can provide continuous high resolution measurements of ocean surface currents, winds and waves. First derived in 1972, the expected radar signal when electromagnetic waves are scattered by the ocean surface can be modelled by the radar cross section, a nonlinear integral equation which enables us to predict the radar output for any ocean state. Methods for inverting the radar cross section - which ultimately permit us to measure ocean parameters from HF radar data - have been developed over the last few decades; however there are times when the measured data cannot be modelled by the mathematical equations and are therefore not suitable for inversion using the existing methods. Using a neural network, trained on simulated radar data, we have successfully inverted HF radar data not modelled by the radar cross section. In this talk, I will give an overview of how HF radar is used in ocean sensing before introducing neural networks. I will finish by presenting the results of a validation experiment, showing how a neural network can learn the complex inverse relationship between HF radar and the ocean surface. |
|||
2019-03-06 | Wed | Paolo Dolce (Nottingham) | ShEAF: postgraduate pure maths seminar |
16:00 | J11 Hicks | Two dimensional adelic geometry | |
Abstract: I will give an overview of a novel approach to the study of two dimensional algebraic and arithmetic geometry by means of adelic and idelic structures. Particular emphasis will be given to the case of arithmetic surfaces since the aim of the theory is to give a two dimensional version of Tate's thesis. |
|||
2019-03-01 | Fri | Luca Pol | THH reading group |
14:00 | J11 | Computations of THH | |
2019-02-28 | Thu | Björn Löptien (Max Planck Institute for Solar System Research ) | |
10:00 | F41 | A new method for measuring the Wilson depression of sunspots | |
Abstract: The Wilson depression is the difference in geometric height of the layer of unit continuum optical depth between the sunspot umbra and the quiet Sun. Measuring the Wilson depression is important for understanding the geometry of sunspots. Current methods suffer from systematic effects or need to make assumptions on the geometry of the magnetic field. This leads to large systematic uncertainties of the derived Wilson depressions. Here we present a method for deriving the Wilson depression that only requires the information about the magnetic field that are accessible by spectropolarimetry and that does not rely on assumptions on the geometry of sunspots or on its magnetic field. Our method is based on minimizing the divergence of the magnetic field vector derived from spectropolarimetric observations. We focus on large spatial scales only in order to reduce the number of free parameters. We test the performance of our method using synthetic Hinode data derived from two sunspot simulations. We find that the maximum and the umbral averaged Wilson depression for both spots determined with our method typically lies within 100 km of the true value obtained from the simulations. In addition, we apply the method to spots from the Hinode sunspot database at MPS. The derived Wilson depressions (500-700 km) are consistent with results typically obtained from the Wilson effect. In our sample, larger spots with a stronger magnetic field exhibit a higher Wilson depression than smaller spots. |
|||
2019-02-28 | Thu | Dan Graves and Sarah Whitehouse | Teaching Lunch |
13:00 | Hicks LT6 | Analysis and AiM | |
Abstract: Sarah will start by talking about various changes that have been made to MAS221 Analysis to address issues of poor student engagement and poor exam performance. This includes use of the AiM online test system for mid-term assessment. Dan will present examples of the type of AiM questions that have been used in MAS221 and in MAS220 Algebra, including proofs and multiple choice questions. |
|||
2019-02-28 | Thu | Wil Ward (Sheffield) | Statistics Seminar |
14:00 | LT E | A Variational Approach to Approximating State Space Gaussian Processes | |
Abstract: The state space representation of a Gaussian process (GP) models the dynamics of an unknown (non-linear) function as a white-noise driven Itô differential equation. Representation in this form allows for the construction of joint models that mix known dynamics (e.g. population) with latent unknown input. Where these interactions are non-linear, or observed through non-Gaussian likelihoods, there is no exact solution and approximation techniques are required. This talk introduces an approach using black box variational inference to model surrogate samples and estimate the underlying parameters. The approximations are compared with full batch solutions and demonstrated to be indistinguishable in two-sample tests. Software and implementation challenges will also be addressed. |
|||
2019-02-28 | Thu | Scott Balchin (Warwick) | Topology seminar |
16:00 | J11 | Adelic reconstruction in prismatic chromatic homotopy theory | |
Abstract: Prismatic homotopy theory is the study of stable monoidal homotopy theories through their Balmer spectrum. In this talk, I will discuss how one can use localised p-complete data at each Balmer prime in an adelic fashion to reconstruct the homotopy theory in question. There are two such models, one is done by moving to categories of modules, which, for example, recovers the algebraic models for G-equivariant cohomology theories. The other, newer model, works purely at the categorical level and requires the theory of weighted homotopy limits. This is joint work with J.P.C Greenlees. |
|||
2019-02-28 | Thu | Mark Wrigley (Chair IOP Yorkshire) | Plasma Dynamics Group |
16:00 | Room K14 (Hicks Building) | 1201 Alarm Project | |
Abstract: The 1201 Alarm Project is the restoration, exhibition and sharing of materials recorded in 1969 of the Apollo moon landings from a domestic television. The talk will review the Apollo flight plan, the recording technologies of the day and the impact that it had on the speaker. The materials will form the basis for an exhibition celebrating the 50th anniversary of moon landings to be held at the National Science and Media Museum in Bradford, Yorkshire. |
|||
2019-02-27 | Wed | Raven Waller (Nottingham) | ShEAF: postgraduate pure maths seminar |
16:00 | J11 Hicks | Level structures - a crossroads between topology, representation theory, and number theory | |
Abstract: The arithmetic, algebraic, and topological properties of local fields are intimately related. For higher dimensional local objects these relationships begin to break down, and this may cause considerable difficulty when studying them. The notion of a level structure allows us to work around some of these issues. We will discuss various applications of level structures, including the explicit study of representations of reductive groups over higher dimensional local fields, which is also related to the geometric Langlands program. |
|||
2019-02-22 | Fri | James Cranch | THH reading group |
14:00 | J11 | Topological Hochschild Homology | |
2019-02-21 | Thu | Farrell Brumley (Paris 13) | Pure Maths Colloquium |
14:00 | J11 | Automorphic forms and rational points | |
Abstract: In what sense can automorphic forms or Galois representations be viewed as rational points on an algebraic variety? One way to explore this question is by counting arguments. The first result in this direction dates back to an early theorem of Drinfeld, which computes the number of 2-dimensional Galois representations of a function field in positive characteristic; the resulting expression is reminiscent of a Lefschetz fixed point theorem on a smooth algebraic variety over a finite field. More recently it was observed that in the number field setting there are formal similarities between the asymptotic counting problems for rational points on Fano varieties and for automorphic representations on reductive algebraic groups. Very little is known in the latter context. I’ll discuss joint work on this topic with Djordje Milicevic, in which we (mostly) solve the automorphic counting problem on the general linear group. Our results can be viewed as being analogous to the well-known result of Schanuel on the number of rational points of bounded height on projective spaces. If time permits, I may also present a short argument, using sphere packings in large dimensions, to give upper bounds on such automorphic counts. |
|||
2019-02-21 | Thu | Sophia Wright (Warwick) | Statistics Seminar |
14:00 | LT E | Bayesian Networks, Total Variation and Robustness | |
Abstract: This talk explores the robustness of large Bayesian Networks when applied in decision support systems which have a pre-specified subset of target variables. We develop new methodology, underpinned by the total variation distance, to determine whether simplifications which are currently employed in the practical implementation of such graphical systems are theoretically valid. This same process can identify areas of the system which should be prioritised if elicitation is required. This versatile framework enables us to study the effects of misspecification within a Bayesian network (BN), and also extend the methodology to quantify temporal effects within Dynamic BNs. Unlike current robustness analyses, our new technology can be applied throughout the construction of the BN model; enabling us to create tailored, bespoke models. For illustrative purposes we shall explore the field of Food Security within the UK. |
|||
2019-02-20 | Wed | Farrell Brumley (Paris 13) | Northern Number Theory Seminar |
11:00 | J11 | Concentration properties of theta lifts | |
Abstract: I will present some results on the concentration properties of automorphic forms obtained through the theta correspondence. Among other things, the method relies on a distinction principle for these lifts, which detect their functorial origin via the non vanishing of orthogonal periods. The examples we treat are in higher rank, and shed light on a purity conjecture of Sarnak. This is joint work with Simon Marshall. |
|||
2019-02-20 | Wed | Heather Harrington (Oxford) | Applied Mathematics Colloquium |
14:00 | Hicks, LT 9 | Comparing models and biological data using computational algebra and topology | |
Abstract: Many biological problems, such as tumor-induced angiogenesis (the growth of blood vessels to provide nutrients to a tumor), or signaling pathways involved in the dysfunction of cancer (sets of molecules that interact that turn genes on/off and ultimately determine whether a cell lives or dies), can be modeled using differential equations. There are many challenges with analyzing these types of mathematical models, for example, rate constants, often referred to as parameter values, are difficult to measure or estimate from available data. I will present mathematical methods we have developed to enable us to compare mathematical models with experimental data. Depending on the type of data available, and the type of model constructed, we have combined techniques from computational algebraic geometry and topology, with statistics, networks and optimization to compare and classify models without necessarily estimating parameters. Specifically, I will introduce our methods that use computational algebraic geometry (e.g., Gröbner bases) and computational algebraic topology (e.g., persistent homology). I will present applications of our methodology on datasets involving cancer. Time permitting, I will conclude with our current work for analyzing spatio-temporal datasets with multiple parameters using computational algebraic topology. Mathematically, this is studying a module over a multivariate polynomial ring, and finding discriminating and computable invariants. |
|||
2019-02-20 | Wed | Andreea Mocanu (University of Nottingham) | Northern Number Theory Seminar |
14:00 | J11 | Newform theory for Jacobi forms of lattice index | |
Abstract: I will give a brief introduction to Jacobi forms, including some examples and their relation to other types of modular forms. After that, I will discuss some of the ingredients that go into developing a theory of newforms for Jacobi forms of lattice index, namely Hecke operators, level raising operators and orthogonal groups of discriminant modules. |
|||
2019-02-20 | Wed | Clark Barwick (Edinburgh) | Topology seminar |
16:00 | LT11 | Primes, knots, and exodromy | |
Abstract: Half a century ago, Barry Mazur and David Mumford suggested a remarkable dictionary between prime numbers and knots. I will explain how the story of exodromy permits one to make this dictionary precise, and I will describe some applications. |
|||
2019-02-15 | Fri | Dan Graves | THH reading group |
14:00 | J11 | Hochschild Homology and Cyclic Homology of rings | |
2019-02-14 | Thu | Carolina Robustini (Stockholm University) | |
10:00 | Chromospheric observations and magnetic configuration of a supergranular structure | ||
Abstract: We present high spatial resolution narrow-band images in three different chromospheric spectral lines, including Ca II K with the new CHROMospheric Imaging Spectrometer installed at the Swedish 1-m Solar Telescope. These observations feature a unipolar region enclosed in a supergranular cell, and located 68º off the disk-centre. The observed cell exhibits a radial arrangement of the fibrils which recalls of a chromospheric rosette. However, in this case, the convergence point of the fibrils is located at the very centre of the supergranular cell. Our study aims to show how the chromosphere appears in this peculiar region and retrieve its magnetic field and velocity distribution. In the centre of the cell, we measured a significant blue-shift in the Ca II K nominal line core associated to an intensity enhancement. We interpreted it as the product of a strong velocity gradient along the line of sight. In this talk, we will discuss the techniques employed to obtain magnetic field maps so close to the limb and suggest a possible configuration that takes into account also the measured velocity within the unipolar region. |
|||
2019-02-14 | Thu | Eleanor Stillman (Sheffield) | |
12:00 | Hicks LT5 | An overview of the HEA direct application process. | |
Abstract: This talk will outline the process of directly applying to become a (associate-principal) fellow of the HEA. The talk will help Ph.D. students to Professors understand what is required in the application and how to be successful. We may also discuss the value and implications of receiving professional recognition from the HEA. |
|||
2019-02-14 | Thu | Andrey Lazarev (Lancaster) | Topology seminar |
16:00 | J11 | Homotopy theory of monoids | |
Abstract: I will explain how the category of discrete monoids models the homotopy category of connected spaces. This correspondence is based on derived localization of associative algebras and could be viewed as an algebraization result, somewhat similar to rational homotopy theory (although not as structured). Closely related to this circle of ideas is a generalization of Adams’s cobar construction to general nonsimply connected spaces due to recent works of Rivera-Zeinalian and Hess-Tonks. (joint with J. Chuang and J. Holstein) |
|||
2019-02-14 | Thu | Will Hulme / Nick Monk / Rhoda Hawkins (Manchester / Sheffield / Sheffield) | RSS Seminar Series |
16:30 | Hicks Room F38 | Experiences of AIMS | |
Abstract: AIMS is an academic network that enables Africa’s talented students to become innovators who propel scientific, educational and economic self-sufficiency. The RSS Local Group are delighted to welcome Will Hulme (University of Manchester, taught at AIMS Cameroon), Prof Nick Monk (University of Sheffield, SOMAS, taught at AIMS Ghana) and Dr Rhoda Hawkins (University of Sheffield, Department of Physics and Astronomy, taught at AIMS South Africa, Senegal and Ghana) to present on their experiences on the AIMS project. Tutor/lecturer opportunities that may be of interest will be highlighted. |
|||
2019-02-13 | Wed | Ana Caraiani (Imperial) | Pure Maths Colloquium |
14:00 | J11 | On the Ramanujan conjecture and its generalisations | |
Abstract: In 1916, Ramanujan made a conjecture that can be stated in completely elementary terms: he predicted an upper bound on the coefficients of a power series obtained by expanding a certain infinite product. In this talk, I will discuss a more sophisticated interpretation of this conjecture, via the Fourier coefficients of a highly symmetric function known as a modular form. I will give a hint of the idea in Deligne’s proof of the conjecture in the 1970’s, who related it to the arithmetic geometry of smooth projective varieties over finite fields. Finally, I will discuss generalisations of this conjecture and some recent progress on these using the machinery of the Langlands program. The last part is based on joint work with Allen, Calegari, Gee, Helm, Le Hung, Newton, Scholze, Taylor, and Thorne. |
|||
2019-02-07 | Thu | Jeremy Colman (Sheffield) | Statistics Seminar |
14:00 | LT E | Accounting for Uncertainty in Estimates of Extremes | |
Abstract: Devastating consequences can flow from the failure of certain structures, such as coastal flood defences, nuclear installations, and oil rigs. Their design needs to be robust under rare (p < 0.0001) extreme conditions, but how can the designers use data typically from only a few decades to predict the size of an event that might occur once in 10,000 years? Extreme Value Theory claims to provide a sound basis for such far-out-of-sample prediction, and using Bayesian methods a full posterior distribution can be obtained. If the past data are supplemented by priors that take into account expert opinion, seemingly tight estimates result. Are such claims justified? Has all uncertainty been taken into account? My research is addressing these questions. |
|||
2019-02-07 | Thu | Masahiro Nakahara (Manchester) | Number Theory seminar |
14:00 | J11 | Index of Fibrations and Brauer classes that never obstruct the Hasse principle | |
Abstract: Let X be a smooth projective variety over a number field with a fibration into varieties that satisfy a certain condition. We study the classes in the Brauer group of X that never obstruct the Hasse principle for X. We prove that if the generic fiber has a zero-cycle of degree d over the generic point, then the Brauer classes whose orders are prime to d do not play a role in the Brauer-Manin obstruction. As a result we show that the odd torsion Brauer classes never obstruct the Hasse principle for del Pezzo surfaces of degree 2, certain K3 surfaces, and Kummer varieties. |
|||
2019-02-06 | Wed | Viveka Erlandsson (Bristol) | Pure Maths Colloquium |
14:00 | J11 | Determining the shape of a billiard table from its bounces | |
Abstract: Consider a billiard table shaped as a Euclidean polygon with labeled sides. A ball moving around on the table determines a bi-infinite “bounce sequence” by recording the labels of the sides it bounces off. We call the set of all possible bounce sequences the “bounce spectrum” of the table. In this talk I will explain why the bounce spectrum essentially determines the shape of the table: with the exception of a very small family (right-angled tables), if two tables have the same bounce spectrum, then they have to be related by a Euclidean similarity. The main ingredient in proving this fact is a technical result about non-singular geodesics on surfaces equipped with flat cone metrics. This is joint work with Moon Duchin, Chris Leininger, and Chandrika Sadanand. |
|||
2019-01-31 | Thu | Prof. Valery Nakariakov (Centre for Fusion, Space and Astrophysics, University of Warwick) | Plasma Dynamics Group |
15:00 | Lecture theatre 1 (Hicks Building) | The effect of thermal misbalance on compressive oscillations in solar coronal loops | |
Abstract: Fast and slow magnetoacoustic waves are a promising tool for the seismological diagnostics of physical parameters of various plasma structures in the corona of the Sun. In particular, compressive waves can provide us with information about the thermodynamic equilibrium in the coronal plasma, and hence the heating function. Compressive perturbations of the thermodynamic equilibrium by magnetoacoustic waves can cause the misbalance of the radiative cooling and unspecified heating. The effect of the misbalance is determined by the derivatives of the combined heating/cooling function with respect to the plasma density and temperature, and can lead to either enhanced damping of the compressive oscillations or their magnification. Moreover, in the regime of strong misbalance, compressive MHD waves are subject to wave dispersion that can slow down the formation of shocks and can cause the formation of quasi-periodic wave trains. |
|||
2019-01-25 | Fri | Kalevi Mursula (University of Oulu) | SP2RC seminar |
10:30 | LT E | Centennial evolution and terrestrial effects of the global solar magnetic field | |
2019-01-23 | Wed | Richard Webb (Cambridge) | Pure Maths Colloquium |
14:00 | J11 | An interplay between topology, geometry, and the algebra of the mapping class group | |
Abstract: The braid groups were defined by Artin in 1925, and are usually defined in terms of strings in 3-dimensional space. However there is a fruitful 2-dimensional perspective of the braid groups as homeomorphisms (up to some natural equivalence) of a disc with holes, in other words, the braid groups are special cases of mapping class groups of surfaces. Mapping class groups can be viewed in a number of ways, and are of interest in several different fields, such as dynamics, algebraic geometry, surface bundles, hyperbolic geometry, to name a few. A key theorem that demonstrates this intradisciplinary feature is the Nielsen--Thurston classification. I will explain what the Nielsen--Thurston classification is, describe some basic examples and analogies, and highlight its importance. I will then explain how to view this from the geometric group theory perspective, and discuss my work with Mark Bell that uses this point of view to solve the conjugacy problem for mapping class groups in polynomial time. At the end of the talk I will discuss some new ideas that may lead to applications in knot theory via the braid groups. |
|||
2019-01-18 | Fri | Norbert Gyenge (Sheffield) | SP2RC seminar |
13:00 | LT 10 | The Nonaxisymmetric Behaviour Of Solar Eruptive Events | |
Abstract: This thesis investigates new approaches for predicting the occurrence of solar eruptive events based on coronal mass ejection (CME), solar flare and sunspot group observations. The scope of the present work is to study the spatio-temporal properties of the above-mentioned solar features. The analysis may also provide a deeper understanding of the subject of solar magnetic field reorganisation. Furthermore, the applied approaches may open opportunities for connecting these local phenomena with the global physical processes that generate the magnetic field of the Sun, called the solar dynamo. The investigation utilises large solar flare statistical populations and advanced computational tools, such as clustering techniques, wavelet analysis, autoregressive moving average (ARIMA) forecast, kernel density estimations (KDEs) and so on. This work does not attempt to make actual predictions because it is out of the scope of the recent investigation. However, the thesis introduces new possible approaches in the subject of flare and CME forecasting. The future aim is to construct a real-time database with the ability to forecast eruptive events based on the findings of this thesis. This potential forecasting model may be crucial for protecting a wide range of satellite systems around the Earth or predicting space weather based on the obtained results may also assist to plan safe space exploration in the future. |
|||
2019-01-17 | Thu | Ricardo Gaferia (Instituto de Astrofísica de Andalucía - CSIC ) | SP2RC seminar |
10:00 | LT 10 | Machine learning assisted parallel inversions | |
Abstract: With the increase of data volume and the need of more complex inversion codes to interpret and analyze solar observations, it is necessary to develop new tools to boost inversions and reduce computation times and costs. In this presentation, I discuss the possibilities and limitations of using machine learning as a tool to estimate optimum initial physical atmospheric models necessary for initializing spectral line inversions. Tests have been carried out for the SIR and DeSIRer inversion codes. This approach allows firstly to reduce the number of cycles in the inversion and increase the number of nodes and secondly to automatically cluster pixels which is an important step to invert maps where completely different regimes are present. Finally, I also present a warp for SIR and DeSIRer inversion codes that allows the user to easily set up parallel inversions. |
|||
2019-01-16 | Wed | Atsushi Takahashi (Osaka) | Algebra / Algebraic Geometry seminar |
16:00 | J11 | On a full exceptional collection in the category of maximally graded matrix factorizations of an invertible polynomial of chain type | |
Abstract: In ’77 Orlik-Randell asked about the existence of a certain distinguished basis of vanishing cycles in the Milnor fiber associated to an invertible polynomial of chain type. With my student, Daisuke Aramaki we transport their conjecture to the category of matrix factorizations by the (conjectural) homological mirror symmetry equivalence and then prove the resulting statement. |
|||
2019-01-08 | Tue | Josep Alvarez-Montaner (Universitat Politecnica de Catalunya) | Algebra / Algebraic Geometry seminar |
14:00 | J11 | Local cohomology of binomial edge ideals and their generic initial ideals | |
Abstract: The aim of this talk is to give a detailed study of local cohomology modules of binomial edge ideals. Our main result is a Hochster type decomposition formula for these modules. As a consequence, we obtain a simple criterion for the Cohen-Macaulayness of these ideals and we describe their Castelnuovo-Mumford regularity and their Hilbert series. We also prove a conjecture of Conca, De Negri and Gorla relating the graded components of the local cohomology modules of binomial edge ideals and their generic initial ideals. |
|||