Warwick Mathematics Institute Events

Seminar List Entry | Seminars by subject

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Upcoming Seminars

Algebra on 21 November 2024 at 12:00 in B3.02

Speaker: Luca Sabatini (Warwick)
Title: Abelian subgroups and sections of finite groups

Abstract: Let G be a group of order n, where n is a large integer. In 1976, Erdős and Straus used a simple argument to show that G contains an abelian subgroup of order roughly log n. Twenty years later, Pyber used the classification of the finite simple groups to improve this result up to 2^{\sqrt{\log n}}. This is best possible, because of certain wild p-groups of class 2 that were obtained with probabilistic methods by Ol'shanskii. On the other hand, it can be seen that G always contains an abelian section of size at least n^{1/ log log n}, which is much bigger. In this talk, we present these questions and some of the methods used in the proofs. We also introduce new probabilistic constructions of wild p-groups, which is joint work with S. Eberhard.

Geometry and Topology on 21 November 2024 at 13:30 in B3.02

Speaker: Harry Petyt (University of Oxford)
Title: Obstructions to cubulation

Abstract: One can get a lot of information about a group by getting it to act geometrically on a CAT(0) cube complex. When this is possible there is a standard framework for trying to find the action, known as Sageev's construction. On the other hand, whilst most groups will not admit such actions, there is a real lack of ways to actually rule out the possibility that they exist. This talk will discuss joint work with Zach Munro, where we give a geometric obstruction to the possibility of cubulating groups.

Statistical Mechanics on 21 November 2024 at 14:00 in MS.04

Speaker: Harini Desiraju (University of Sydney)
Title: Probabilistic conformal blocks on the torus and the Lamé equation

Abstract: Conformal blocks are the fundamental building blocks of Conformal Field Theories and play an important role in several areas of mathematical physics from random geometry to black hole physics. Starting from their probabilistic formulation in terms of the Gaussian Multiplicative Chaos (GMC) measure by Promit Ghosal, Guillaume Remy, Xin Sun, Yi Sun, I will prove certain conjectures posed by Zamolodchikov regarding the semiclassical behaviour of conformal blocks and show their relation to the Lamé equation, and other associated integrable structures. This talk is based on a joint work with Promit Ghosal and Andrei Prokhorov (arXiv: 2407.05839).

Analysis on 21 November 2024 at 16:00 in B3.02

Speaker: Guido De Philippis (NYU)
Title: Decay of excess for the abelian Higgs model

Abstract: Entire critical points of the Yang–Mills–Higgs functional are known to blow down to (generalized) minimal surfaces. Goal of the talk is to prove an Allard's type large scale regularity result for the zero set of the solution. In particular, in the "multiplicity one" energy regime, we show uniqueness blow-downs and we classify entire solutions in small dimensions and of entire minimizers in any dimension. This is based on a joint work with Aria Halavati and Alessandro Pigati.

Combinatorics on 22 November 2024 at 14:00 in B3.02

Speaker: Maria Ivan (University of Cambridge)
Title: Euclidean Ramsey sets and the block sets conjecture

Abstract: A set $X$ is called Euclidean Ramsey if, for any $k$ and sufficiently large $m$, any $k$-colouring of $\mathbb{R}^m$ contains a monochromatic congruent copy of $X$. This notion was introduced by Erd\H{o}s, Graham, Montgomery, Rothschild, Spencer and Straus. They asked if a set is Ramsey if and only if it is spherical, meaning that it lies on the surface of a sphere. It is not too difficult to show that if a set is not spherical, then it is not Euclidean Ramsey either, but the converse is very much open despite extensive research over the years. On the other hand, the block sets conjecture is a purely combinatorial, Hales-Jewett type of statement. It was introduced in 2010 by Leader, Russell and Walters. If true, the block sets conjecture would imply that every transitive set (a set whose symmetry group acts transitively) is Euclidean Ramsey. Similarly to the first question, the block sets conjecture remains very elusive. In this talk we discuss recent developments on the block sets conjecture and their implications to Euclidean Ramsey sets. Joint work with Imre Leader and Mark Walters.

SBIDER on 25 November 2024 at 14:00 in OC0.01

Speaker: Caroline Trotter (University of Cambridge)
Title: Defeating Meningitis by 2030: how can modelling help?

Abstract: The World Health Organisation has developed an ambitious roadmap to Defeat Meningitis by 2030. The use of vaccines is key to reducing the burden of disease due to meningitis, particularly the most common forms of bacterial meningitis. A key goal is to eliminate epidemics of meningitis, which occur periodically but irregularly, in the African meningitis belt. I will describe how modelling can be used in support of the roadmap, particularly in providing evidence for vaccine policy and epidemic response.

Number Theory on 25 November 2024 at 15:00 in B.302, Zeeman building

Speaker: Elvira Lupoian (Imperial College London)
Title: TBA

Abstract: TBA

Partial Differential Equations and their Applications on 26 November 2024 at 12:00 in B3.02

Speaker: Noemi David (Rouen)
Title: TBA

Abstract: TBA

Algebraic Topology on 26 November 2024 at 17:00 in B3.03

Speaker: Markus Hausmann (University of Bonn)
Title: The universal property of bordism rings of manifolds with commuting involutions

Abstract: My talk concerns bordism rings of compact smooth manifolds equipped with a smooth action by a finite group. I will start by recalling classical results on the subject from the 60's and 70's, mostly due to Conner-Floyd, Boardman, Stong and Alexander. Afterwards I will discuss joint work with Stefan Schwede in which we prove an algebraic universal property for the collection of bordism rings of manifolds with commuting involutions, in the representation-graded sense. This universal property can be viewed as a delocalization of the corresponding one for homotopical equivariant bordism rings.

Algebraic Geometry on 27 November 2024 at 13:00 in MB0.07

Speaker: Elena Denisova (University of Edinburgh)
Title: Delta-invariants of Du Val del Pezzo surfaces

Abstract: It is known that a Fano variety with “mild” singularities admits a Kahler Einstein metric if and only if it is K-polystable. For two-dimensional Fano varieties (del Pezzo surfaces) Tian and Yau proved that a smooth del Pezzo surface is K-polystable if and only if it is not a blow up of P2 in one or two points. A lot of research was done for threefolds however, not everything is known and often the problem can be reduced to computing δ-invariant of (possibly singular) del Pezzo surfaces. In my talk I will describe the status of the problem, present an example of computation of δ-invariant, show the example of application of this result for a singular Fano threefold and explain a possible direction for future research.

Geometry and Topology on 28 November 2024 at 13:30 in B3.02

Speaker: Simon Machado (ETH Zurich)
Title: Approximate lattices: structure and beyond

Abstract: Approximate lattices are aperiodic generalisations of lattices in locally compact groups. Yves Meyer first introduced them in abelian groups before studying them as mathematical models for quasi-crystals. Since then, their structure has been thoroughly investigated in both abelian and non-abelian settings. The primary motivation behind this research was to extend Meyer’s foundational theorem to non-abelian locally compact groups. This generalisation has now been established, and I will discuss the resulting structure theory. I will highlight certain concepts, including a notion of cohomology that lies between group cohomology and bounded cohomology, which plays a significant role in their study. Additionally, I will formulate open problems and conjectures related to approximate lattices.

Combinatorics on 29 November 2024 at 14:00 in B3.02

Speaker: Peleg Michaeli (University of Oxford)
Title: Extremal and probabilistic aspects of graph rigidity

Abstract: Combinatorial rigidity theory addresses questions such as: given a structure defined by geometric constraints, what can be inferred about its geometric behaviour based solely on its underlying combinatorial data? Such structures are often modelled as assemblies of rigid rods connected by rotational joints, in which case the underlying combinatorial data is a graph. A typical question in this context is: given such a framework in generic position in R^d, is it rigid? That is, does every continuous motion of the vertices (joints) that preserves the lengths of all edges (rods) necessarily preserve the distances between all pairs of vertices? In this talk, I will present new sufficient conditions for the rigidity of a framework in R^d based on the notion of rigid partitions - partitions of the underlying graph that satisfy certain connectivity properties. I will outline several broadly applicable conditions for the existence of such partitions and discuss a few applications, among which are new results on the rigidity of highly connected and (pseudo)random graphs. If time allows, I will also discuss new - often sharp - sufficient minimum degree conditions for d-dimensional rigidity and mention a related novel result on the pseudoachromatic number of graphs. The talk is based on joint works with Michael Krivelevich and Alan Lew.

Number Theory on 02 December 2024 at 15:00 in B3.02

Speaker: Nikolaos Diamantis (Nottingham)
Title: TBA

Abstract: TBA

Partial Differential Equations and their Applications on 03 December 2024 at 12:00 in B3.02

Speaker: Amirali Hannani (KU Leuven)
Title: TBA

Abstract: TBA

Algebra on 05 December 2024 at 12:00 in B3.02

Speaker: Patricia Medina Capilla (Warwick)
Title: The second maximal subgroups of the almost simple groups

Abstract: In 2018, Lucchini, Marion, and Tracey showed that every maximal subgroup of an almost simple group is 5-generated, lowering the previously known bound of 6. Naturally, one can ask the same about second maximal subgroups of almost simple groups. Burness, Liebeck and Shalev looked into this question in 2016, determining that almost all such subgroups were 70-generated. In this talk, we will present recent work aiming to lower this bound, and survey some of the key techniques involved. In particular, we will discuss the method of crowns, developed by Lucchini and Dalla Volta and used by Lucchini, Marion and Tracey to great effect, as well as an improved classification of the second maximal subgroups of almost simple groups with alternating or classical socle.

Geometry and Topology on 05 December 2024 at 13:30 in B3.02

Speaker: Peter Patzt (University of Oklahoma)
Title: TBA

Abstract: TBA

Combinatorics on 06 December 2024 at 14:00 in B3.02

Speaker: Debmalya Bandyopadhyay (University of Birmingham)
Title: Monochromatic tight cycle partitions in edge-coloured complete $k$-graphs

Abstract: Let $K_n^{(k)}$ be the complete $k$-uniform hypergraph on $n$ vertices. A tight cycle is a $k$-uniform graph with its vertices cyclically ordered so that every~$k$ consecutive vertices form an edge, and any two consecutive edges share exactly~$k-1$ vertices. A result by Bustamante, Corsten, Frankl, Pokrovskiy and Skokan shows that all $r$-edge coloured $K_{n}^{(k)}$ can be partition into $c_{r,k}$ vertex disjoint monochromatic tight cycles. However, the constant $c_{r,k}$ is of tower-type. In this work, we show that $c_{r,k}$ is a polynomial in~$r$.

Number Theory on 24 February 2025 at 15:00 in B3.02

Speaker: Shin-ya Koyama (Toyo University)
Title: TBA

Abstract: TBA

Past Seminars

Algebraic Geometry on 20 November 2024 at 15:00

Speaker: Alapan Muckhopadyay (EPFL)
Title: Generators of bounded derived categories using the Frobenius map.

Abstract: Since the appearance of Bondal- van den Bergh’s work on the representability of functors, proving existence of strong generators of the bounded derived category of coherent sheaves on a scheme has been a central problem. While for a quasi-excellent, separated scheme the existence of strong generators is established, explicit examples of such generators are not common. In this talk, we show that explicit generators can be produced in prime characteristics using the Frobenius pushforward functor. As a consequence, for affine schemes, we show that the pushforward of the structure sheaf via a high enough iterate of the Frobenius is a generator. This recovers Kunz’s characterization of regularity using Frobenius. We will discuss examples indicating that in contrast to the affine situation, for a smooth projective scheme whether some Frobenius pushforward of the structure sheaf is a generator, depends on the geometry of the underlying scheme. Part of the talk is based on a joint work with Matthew Ballard, Srikanth Iyengar, Patrick Lank and Josh Pollitz.

Soft Matter Lunches on 20 November 2024 at 13:00

Speaker: Thomasina Ball, Danny Booth, Ellen Jolley, Ellen Luckins, Joe Webber (University of Warwick)
Title: APS DFD practice talks

Abstract: Thomasina Ball: Unravelling wrinkle formation in a lubricated viscoplastic beam (60 second flashtalk) Danny Booth: Bubble racing in a Hele-Shaw cell (60 second flashtalk) Ellen Jolley: Translation of a viscoelastic cell surrounded by a Newtonian fluid (10 minute talk) Ellen Luckins: Modelling evaporation-driven flows in capillary porous media (60 second flashtalk) Joe Webber: XOXO, Gossip Gel: oscillating chemical reactions facilitate communication between responsive hydrogels (10 minute talk)

Algebraic Topology on 19 November 2024 at 17:00

Speaker: Martin Gallauer (Warwick)
Title: Derived commutative algebraic geometry

Abstract: Venerable algebraic geometry (AG) has many descendants, including relatively recently "derived noncommutative AG" (Kapranov, Bondal, Orlov, Kontsevich,...). In this expository talk I will discuss an even younger one that one might call "derived commutative AG" and which Balmer introduced as "tensor-triangular geometry". Comparing the main features I'll try to pitch it as an attractive addition to the family.

Partial Differential Equations and their Applications on 19 November 2024 at 12:00

Speaker: Jakub Skrzeczkowski (Oxford)
Title: TBA

Abstract: TBA

Number Theory on 18 November 2024 at 15:00

Speaker: Emma Bailey (Bristol)
Title: Large deviations of Selberg’s CLT: upper and lower bounds

Abstract: Selberg’s CLT informs us that the logarithm of the Riemann zeta function evaluated on the critical line behaves as a complex Gaussian. It is natural, therefore, to study how far this Gaussianity persists. This talk will present conditional and unconditional results on atypically large values, and is joint with Louis-Pierre Arguin and Asher Roberts.

DAGGER on 18 November 2024 at 14:00

Speaker: Anna Jové (Universitat de Barcelona)
Title: What is a Baker domain? Introduction to transcendental dynamics

Abstract: Transcendental dynamics, in contrast with rational dynamics, present richer structures, even in the Fatou set, motivated by the presence of an essential singularity and the infinite degree of the iterated map. Essentially, such structures are wandering domains and Baker domains. In this talk, first I will present the definition of Baker domain and their classification, and then we will shift to their boundary dynamics, which is essentially chaotic, and a great source of open questions. Some conjectures and open problems will be presented, together with some new results, which are joint work with N. Fagella.

SBIDER on 18 November 2024 at 14:00

Speaker: Weini Huang (Queen Mary University)
Title: Mathematical models of extra-chromosomal DNA and their applications in cancer

Abstract: Many diseases in human including cancers are caused by genetic alternations/errors starting from a single cell. The origin of these genetic errors and the expansion of the abnormal cells carrying these genetic errors are often stochastic processes. Here we develop a general framework to model the dynamics of cancer cells carrying extra-chromosomal DNA (ecDNA), a genetic error found in more than 30% of tumour samples across various cancer types and correlated to the worse clinical outcomes. Different from chromosomal DNA where genetic materials are on average equally divided to daughter cells controlled by centromeres during mitosis, the segregation of ecDNA copies is random partition and leads to a fast accumulation of cell-to-cell heterogeneity in copy numbers. We use deterministic and stochastic approaches to analyse the fraction of cells carrying ecDNA and copy number distributions, and use those distributions observed in experimental and clinical data to infer the selection strength of ecDNA positive cells. We further extend our model of a single ecDNA species where all ecDNA copies are identical in genetic structure and function, to multiple ecDNA types where ecDNA copies can have different genes (species), mutations (genotypes), or have different functions without genetic changes (phenotypes). All these variations of our basic model can be applied to relevant biological context and provide insight to understand ecDNA dynamics observed in clinic or experiments and improve treatment strategies.

Combinatorics on 15 November 2024 at 14:00

Speaker: Brett Kolesnik (University of Warwick)
Title: Graphical sequences and plane trees

Abstract: We show that the asymptotic number of graphical sequences can be expressed in terms of Walkup’s formula for the number of plane trees. This yields a more detailed description of the asymptotics by Balister, Donderwinkel, Groenland, Johnston and Scott. Our proof is probabilistic, using what we call the Lévy–Khintchine method. We will discuss other applications of this method, and connections with additive number theory (subset counting formulas by von Sterneck and the Erdös–Ginzburg–Ziv theorem). Joint work with Michal Bassan (Oxford) and Serte Donderwinkel (Groningen).

Analysis on 14 November 2024 at 16:00

Speaker: Annalisa Massaccesi (University of Padova)
Title: Constructions for a C^1 function with prescribed gradient on a Cantor-type set

Abstract: In this talk I will outline the iterative construction of a C^1 function u, with \|u\|_\infty \leq \eta and Du(x)=F(x,u(x)) on a Cantor-type set C. It is transparent from the construction the presence of a trade off between the size of C and the Hölder regularity of Du. This type of construction is the building block for counterexamples to Frobenius theorem when the tangency set is not regular enough.

Statistical Mechanics on 14 November 2024 at 14:00

Speaker: Jakob Björnberg (University of Gothenburg/Chalmers)
Title: Dimerisation in mirror models and quantum spin chains

Abstract: We consider two models of random loops where we prove breaking of translational symmetry. The first is a mirror model, where the loops are formed by light rays bouncing in a labyrinth of randomly oriented mirrors. The second is a probabilistic representation of a quantum spin chain, and can be obtained as a limit of the first, for inhomogeneous mirror weights. In the terminology of quantum spins, this symmetry-breaking is called “dimerisation”. Based on joint works with K. Ryan as well as with P. Muehlbacher, B. Nachtergaele and D. Ueltschi.

Geometry and Topology on 14 November 2024 at 13:30

Speaker: Mikhail Hlushchanka (University of Amsterdam)
Title: Canonical decomposition of rational maps

Abstract: There are various classical and more recent decomposition results in mapping class group theory, geometric group theory, and complex dynamics (which include celebrated results by Bill Thurston). The goal of this talk is to introduce a powerful decomposition of rational maps based on the topological structure of their Julia sets. Namely, we will discuss the following result: every postcritically-finite rational map with non-empty Fatou set can be canonically decomposed into crochet maps (these have very "thinly connected" Julia sets”) and Sierpinski carpet maps (these have very "heavily connected" Julia sets). If time permits, I will discuss applications of this result in various aspects of geometric group theory. Based on a joint work with Dima Dudko and Dierk Schleicher.

Algebra on 14 November 2024 at 12:00

Speaker: Charley Cummings (Aarhus)
Title: Metric completions of cluster categories

Abstract: The completion of a metric space is a classical method for generating new mathematical structures from old. Recently, Neeman emulated this idea to define a metric completion of a triangulated category, thereby providing a novel way to construct new triangulated categories. However, computing these completions often requires using the properties of an already completed ambient category, like the derived category. In this talk, based on joint work with Sira Gratz, we present an example from cluster theory that avoids this requirement by focusing on categories that have combinatorial models, and show that their categorial completions can be viewed as topological completions of the associated models.

Algebraic Geometry on 13 November 2024 at 15:00

Speaker: Tiago Duarte Guerreiro (Paris-Saclay)
Title: On hypersurfaces in projective bundles

Abstract: Mori dream spaces are a special kind of varieties introduced by Hu and Keel in 2000 that enjoy very good properties with respect to the minimal model program. On the other hand, not many classes of examples of these are known. In this talk we introduce general hypersurfaces in certain projective bundles of Picard rank 2 and show that (some of) these are Mori dream spaces, partially generalising Ottem's result on hypersurfaces in products of projective spaces.

Algebraic Topology on 12 November 2024 at 17:00

Speaker: John Greenlees (University of Warwick)
Title: An algebraic model for rational SU(3)-spectra in 18 blocks

Abstract: For each compact Lie group G, one may hope to construct an algebraic category A(G) which is Quillen equivalent to the category of rational G-equivariant cohomology theories. A(G) takes the form of a category of sheaves over a space X_G of conjugacy classes of subgroups of G. When G is SU(3) there is a partition of X_G into 18 blocks, over each of which one may make A(G) explicit. This example is small enough to be explicit and large enough to illustrate some general techniques.

Number Theory on 11 November 2024 at 15:00

Speaker: Chris Daw (University of Reading)
Title: Large Galois orbits under multiplicative degeneration

Abstract: The Pila-Zannier strategy is a powerful technique for proving results in unlikely intersections. In this talk, I will recall the Zilber-Pink conjecture for Shimura varieties and describe how Pila-Zannier works in this setting. I will highlight the most difficult outstanding obstacle to implementing the strategy — the so-called Large Galois Orbits conjecture — and I will explain recent progress towards this conjecture, building on the works of André and Bombieri. This is joint with Martin Orr (Manchester).

SBIDER on 11 November 2024 at 14:00

Speaker: Laura Wadkin (Newcastle University)
Title: Modelling the spread of tree diseases and invasive pests through UK treescapes

Abstract: The loss of biodiversity due to the spread of destructive tree diseases and invasive pests within our native forests is having an enormous environmental, economic, and social impact. In the ‘25 Year Environment Plan’ the UK government highlights enhancing biosecurity as a key priority, through the control of existing diseases and pests, and by building forest resilience against new ones. We are working in collaboration with Defra to develop mathematical models to deepen our understanding of the fundamental behaviours of key pests and pathogens, act as predictive tools for forecasting, and to explore different control strategies. Broadly, we use a combination of partial differential equations, agent-based modelling, and statistical inference techniques. In this talk I will give an overview of the collaborative work to date and present a case study example of the oak processionary moth epidemic in London parks to show how the parameters for a compartmental SIR model with a time varying infection rate can be inferred.

Colloquium on 08 November 2024 at 16:00

Speaker: Stefan Güttel (Manchester)
Title: Randomized algorithms in numerical linear algebra

Abstract: Randomization is an established technique to speed up the numerical solution of very large-scale linear algebra problems that have some form of redundancy, with overdetermined least-squares problems and low-rank matrix approximation being the most prominent examples. Until recently, it has been less clear how to apply randomization to problems that do not have inherent redundancy, including linear systems of equations, matrix functions, and (non)linear eigenvalue problems. I will discuss some new ideas to speed up computational methods for these problems.

Combinatorics on 08 November 2024 at 14:00

Speaker: Matías Pavez-Signé (University of Chile)
Title: Ramsey numbers of cycles in random graphs

Abstract: Let C_n denote the cycle on n vertices. We say a graph G is C_n-Ramsey if every 2-colouring of the edges of G contains a monochromatic copy of C_n. The classical Ramsey problem for cycles asks for determining the minimum number R(C_n) so that the complete graph on R(C_n) vertices is C_n-Ramsey. This talk will study when a random graph G(N,p) is C_n-Ramsey with high probability. In particular, we will show that even for very sparse edge probability p and N quite close to R(C_n), G(N,p) remains C_n-Ramsey.

Analysis on 07 November 2024 at 16:00

Speaker: Marco Pozzetta (Polytechnic University of Milan)
Title: On the uniqueness of isoperimetric sets on manifolds with nonnegative curvature

Abstract: We consider the isoperimetric problem, that is the minimization of the measure of the boundary among subsets having a given volume, on noncompact manifolds with nonnegative Ricci curvature, Euclidean volume growth and with quadratic Riemann curvature decay. The aim of the talk is to discuss uniqueness and stability properties of minimizers, called isoperimetric sets. Assuming the manifold is not the Euclidean space, we show that for most large volumes (in a quantified way) there exists a unique isoperimetric set, and its boundary is strictly volume preserving stable. Uniqueness here is meant in the set theoretical sense and it is not understood up to isometry of the ambient. We show with a counterexample that the result cannot be improved to uniqueness or strict stability for every large volume. The lack of higher regularity at infinity prevents the application of classical methods based on the implicit function theorem. A key tool for deriving the needed effective estimates on the Jacobi operator of the boundary of large isoperimetric sets is provided here by the sharp concavity property of the isoperimetric profile function, which will be briefly reviewed. The talk is based on a joint work with Gioacchino Antonelli and Daniele Semola.

Statistical Mechanics on 07 November 2024 at 14:00

Speaker: Barbara Roos (University of Tübingen)
Title: Macroscopic Thermalization for Highly Degenerate Hamiltonians

Abstract: A closed quantum system thermalizes in the sense of typicality, if any initial state will reach a suitable equilibrium subspace and stay there most of the time. For non-degenerate Hamiltonians, a sufficient condition for thermalization is the eigenstate thermalization hypothesis (ETH). Shiraishi and Tasaki recently proved the ETH for a perturbation of the Hamiltonian of free fermions on a one-dimensional lattice. The perturbation is needed to remove the high degeneracies of the Hamiltonian. We point out that also for degenerate Hamiltonians ETH implies thermalization. Additionally, we develop another strategy for proving thermalization by adding small generic perturbations. This is joint work with Stefan Teufel, Roderich Tumulka, and Cornelia Vogel.

Geometry and Topology on 07 November 2024 at 13:30

Speaker: Sami Douba (IHES)
Title: Zariski closures of linear reflection groups

Abstract: We show that linear reflection groups in the sense of Vinberg are often Zariski dense in PGL(n). Among the applications are examples of low-dimensional closed hyperbolic manifolds whose fundamental groups virtually embed as Zariski-dense subgroups of SL(n,Z), as well as some one-ended Zariski-dense subgroups of SL(n,Z) that are finitely generated but infinitely presented, for all sufficiently large n. This is joint work with Jacques Audibert, Gye-Seon Lee, and Ludovic Marquis.

Algebra on 07 November 2024 at 12:00

Speaker: Chris Bowman (University of York)
Title: A combinatorial introduction to Hecke Categories

Abstract: Hecke categories control the representation theory of symmetric and algebraic groups, and generalise this theory from Weyl groups to all parabolic Coxeter systems. We give an introductory survey of some of the recent results in this area from a concrete combinatorial point of view.

Algebraic Geometry on 06 November 2024 at 15:00

Speaker: Tarig Abdelgadir (Loughborough University)
Title: The McKay correspondence via VGIT (case D4)

Abstract: For a Kleinian singularity, the McKay correspondence famously relates the orbifold cover of the singularity to a crepant resolution. In type A, both are toric and it is easy to write down a GIT problem which produces both the orbifold and the geometric resolution as possible quotients. However, no such construction seems to be known for types D and E. I'll describe how we fill this gap for the simplest non-trivial case D4. The construction is inspired by Tannaka duality and sets out a strategy to tackle general types D and E. This is joint work with Ed Segal.

Mathematics Teaching and Learning on 06 November 2024 at 14:00

Speaker: Beatriz Navarro Lameda (UCL)
Title: Online Assessment Platforms: Crowdmark Demo

Abstract: Crowdmark is an online assessment platform that helps educators assess student work more effectively. This tool allows you to mark two to three times faster compared to marking paper exams while at the same time leaving richer feedback for students. It supports LaTeX and Markdown formatting, multiple choice questions (marked automatically) and longer questions for which student can submit scanned handwritten answers. Crowdmark supporting both online assessments and in-person, paper-based exams that can be scanned for digital marking. The ability to mark paper-based exams online is especially beneficial for department that frequently administer in-person exams, such as maths and stats departments. In this demo, I’ll walk you through the essentials of using Crowdmark. I’ll cover how to set up assessments and create questions, guide students through the submission process, and leverage Crowdmark’s features to maximise marking efficiency. Additionally, I’ll demonstrate how to create a workflow for delivering and marking in-person exams with Crowdmark.

Algebraic Topology on 05 November 2024 at 17:00

Speaker: Marco La Vecchia (Warwick)
Title: Twisted equivariant Chern Classes and Equivariant Formal Group Laws

Abstract: Chern classes are classical invariants that play a fundamental role in the study of vector bundles. Recently, Schwede introduced U(n)-equivariant Chern classes within the context of equivariant bordism. In this talk, I will extend this framework by defining G-twisted equivariant Chern classes in the setting of G \times U(n)-equivariant bordism. I will then explore the connection between these twisted equivariant Chern classes and the theory of G-equivariant formal group laws, where G is any compact Lie group.

Partial Differential Equations and their Applications on 05 November 2024 at 12:00

Speaker: Lorenzo Pareschi (Heriot Watt)
Title: TBA

Abstract: TBA

Number Theory on 04 November 2024 at 15:00

Speaker: Andrei Yafaev (University College London)
Title: TBA

Abstract: TBA

DAGGER on 04 November 2024 at 14:00

Speaker: Eduardo Silva (University of Münster)
Title: Harmonic functions on groups and the Poisson boundary

Abstract: The Poisson boundary of a countable group [latex alt='g']$G$[/latex] endowed with a probability measure [latex alt='g']$\mu$[/latex] is a probability space that encodes all bounded [latex alt='g']$\mu$[/latex]-harmonic functions on the group. Alternatively, it captures the asymptotic directions of the [latex alt='g']$\mu$[/latex]-random walk on the group. A natural problem is to identify an explicit model of the associated Poisson boundary, described in terms of the geometry of [latex alt='g']$G$[/latex]. In this talk I will give an introduction to the theory of Poisson boundaries, discuss the identification problem for diverse classes of groups, and explain the connections with entropy. I will concentrate on joint work with Joshua Frisch, where we identify the Poisson boundary of lamplighter groups [latex alt='g']$A \wr \mathbb{Z}^d$[/latex], for [latex alt='g']$d\geq 3$[/latex] and [latex alt='g']$A$[/latex] any countable group, for probability measures with finite entropy and that satisfy a stabilization condition that naturally arises in this context.

SBIDER on 04 November 2024 at 14:00

Speaker: Xander O'Neill (Heriot-Watt University)
Title: Pathogen persistence in wildlife populations

Abstract: How do highly virulent pathogens persist? We start by delving into the dynamics of African swine fever, a highly virulent pathogen, which can be sustained in a wild boar population despite a mortality rate of 90-100%. How does this persist? How could the introduction of this virus impact other, more chronic illnesses, such us tuberculosis? Can the lack of control for one make it easier to control or eradicate the other? This comes full circle when we propose a more general study, asking the question, what key model characteristics lead to slower (or faster) approximate times to extinction?

Soft Matter Lunches on 04 November 2024 at 12:00

Speaker: Michiko Shimokawa (Nara Women's University, Nara, Japan)
Title: Bifurcation of rotational motion of an elliptical camphor-coated disk

Abstract: A camphor boat is one of the famous self-propelled particles, which moves spontaneously on the water due to the difference in surface tension around the boat. The collective motions of the camphor boats resemble traffic jams and the quorum sensing in living things, and the camphor boat has been studied as an example of an active matter in non-biological systems. Furthermore, the camphor boat has been interesting topics as bifurcation phenomena, where the behaviour changes drastically at a certain value of the parameter. When an elliptical camphor disk, with the center of mass fixed at the axis of rotation, is placed on the surface of water, it rotates spontaneously. We found the appearance of the bifurcation from the stable state to the rotational state in the control of the water depth. The bifurcation type, which is subcritical bifurcation or supercritical bifurcation, depended on the aspect ratio of the elliptical camphor disk. We discussed the important factor to determine the bifurcation type through the results of our experimental results and our phenomenological model.

Colloquium on 01 November 2024 at 16:00

Speaker: Harry Schmidt (Warwick)
Title: Canonical heights and equidistribution

Abstract: Algebraic numbers are roots of (non-zero) polynomials with coefficients in the integers. We can measure their size with a height function. There are various height functions associated to geometric and dynamical objects, and I will give some examples coming from dynamical systems induced by polynomial maps. I will present some joint work with Philipp Habegger in which we prove lower bounds for such heights and give some applications of our work. Time permitting, I will also present some joint work with Myrto Mavraki in which we study points that are small with respect to two distinct heights.

Combinatorics on 01 November 2024 at 14:00

Speaker: Marius Tiba (King's College London)
Title: Upper bounds for multicolour Ramsey numbers

Abstract: The $r$-colour Ramsey number $R_r(k)$ is the minimum $n \in \mathbb{N}$ such that every $r$-colouring of the edges of the complete graph $K_n$ on $n$ vertices contains a monochromatic copy of $K_k$. We prove, for each fixed $r \geqslant 2$, that $$R_r(k) \leqslant e^{-\delta k} r^{rk}$$ for some constant $\delta = \delta(r) > 0$ and all sufficiently large $k \in \mathbb{N}$. For each $r \geqslant 3$, this is the first exponential improvement over the upper bound of Erd\H{o}s and Szekeres from 1935. In the case $r = 2$, it gives a different proof of a recent result of Campos, Griffiths, Morris and Sahasrabudhe. This is based on joint work with Paul Balister, B\'ela Bollob\'as, Marcelo Campos, Simon Griffiths, Eoin Hurley, Robert Morris and Julian Sahasrabudhe.

Analysis on 31 October 2024 at 16:00

Speaker: Max Goering (Jyvaskyla)
Title: Tangents and rectifiability in a rough Riemannian setting

Abstract: This talk will introduce $\Lambda$-tangents, a recent extension of Preiss' tangent measures which adapt tangent measure techniques to the study of elliptic problems. We then discuss forthcoming results using $\Lambda$-tangents to characterize rectifiable measures in terms of singular integrals. The relationship between (these) singular integrals, PDEs, and the geometry of measures is then developed. Finally, we remark on some new insights and open questions resulting from this joint work with Emily Casey, Tatiana Toro, and Bobby Wilson.

Mathematics Teaching and Learning on 31 October 2024 at 16:00

Speaker: Mojca Premuš (University of Ljubljana)
Title: Bridging the Gap: A Strategic Refresher Course in Mathematics

Abstract: This presentation outlines the structure and objectives of a non-compulsory, intensive one-week refresher course designed to bridge the gap between high school and university-level mathematics for incoming engineering students at the University of Ljubljana. The course includes an entry test. It also offers two levels of difficulty. The aim is to reinforce foundational mathematical concepts and enhance students’ readiness for the rigorous demands of their academic programs. Additionally, the presentation will highlight the innovative use of e-classrooms and STACK questions to facilitate interactive learning and continuous assessment. Note: Prof. Premuš will also be giving a short presentation on the University of Ljubljana.

Geometry and Topology on 31 October 2024 at 13:30

Speaker: Stefanie Zbinden (Heriot-Watt University)
Title: Morse directions in classical small cancellation groups

Abstract: Morse geodesics are geodesics that capture the hyperbolic-like features of not necessarily hyperbolic spaces. They were studied in order to generalize proofs about hyperbolic groups. However, it quickly became clear that having a Morse geodesic is not enough to exclude various types of pathological behaviours, which makes many genearlizations impossible. Luckily, it turns out that having slightly stronger assumptions on the group, such as having a WPD element or being "Morse-local-to-global" makes certain pathologies impossible. In this talk, we explore how those stronger assumptions relate to each other in the case of small cancellation groups.

Algebra on 31 October 2024 at 12:00

Speaker: Evgeny Khukhro (University of Lincoln)
Title: Engel sinks in finite, profinite, and compact groups

Abstract: Using Zelmanov's deep results on Engel Lie algebras, Wilson and Zelmanov proved that any profinite Engel group is locally nilpotent, and Medvedev extended this result to Engel compact groups. We state generalizations of the Engel condition as restrictions on the so-called Engel sinks of group elements. For example, a group can be considered to be `almost Engel' if all Engel sinks are finite. We proved that almost Engel compact groups are almost locally nilpotent (in certain precise terms). Similar results for finite groups have quantitative nature, with almost nilpotency expressed as a function of sizes of Engel sinks. Our most recent results concern imposing restrictions on Engel sinks of commutators (rather than all elements). This is joint work with Pavel Shumyatsky.

Algebraic Topology on 29 October 2024 at 17:00

Speaker: Matt Booth (Imperial College London)
Title: Calabi-Yau structures and Koszul duality

Abstract: I'll give a reminder of Koszul duality, before talking about a generalised notion of Calabi-Yau structure for dg (co)algebras and indicating why it is Koszul dual to a symmetric Frobenius condition. There is also an analogous one-sided version: Gorenstein (co)algebras are Koszul dual to Frobenius (co)algebras. This leads to a surprising example: the ring k[[x]] of formal power series, equipped with its natural topology, is a pseudocompact Frobenius algebra. As an application of the above theory, we obtain a new characterisation of Poincaré duality spaces, which for simply connected spaces recovers Félix-Halperin-Thomas's notion of Gorenstein space. This is joint work with Joe Chuang and Andrey Lazarev, to appear on the ArXiv soon.

Ergodic Theory and Dynamical Systems on 29 October 2024 at 14:15

Speaker: Alexey Korepanov (Loughborough)
Title: Memory loss near the boundary of null recurrence for Harris recurrent Markov chains and infinite measure preserving intermittent dynamical systems

Abstract: I'll talk about our joint work in progress with Ilya Chevyrev. Memory loss is a quantification of how quickly an evolving system forgets its initial state. For example, for a Markov chain with transition operator P, given two probability measures mu and nu, we may want to know how quickly the distance between P^n mu and P^n nu decays in total variation. For Markov chains with slow (polynomial) recurrence, memory loss has been very well understood half a century ago (starting with Orey or Pitman) as long the chain is positive recurrent, yet we could not find any results in the null recurrent case (even though related questions are a subject of well developed Renewal Theory). A similar situation takes place in chaotic dynamical systems. I'll present (first?) results on memory loss that work for positive as well as null recurrent systems, taking a particular interest in proofs that survive the transition between positive and null recurrence.

Partial Differential Equations and their Applications on 29 October 2024 at 12:00

Speaker: Esther Bou-Dagher (Paris Dauphine)
Title: TBA

Abstract: TBA

Number Theory on 28 October 2024 at 15:00

Speaker: Akshat Mudgal (Warwick)
Title: Recent progress towards the sum–product conjecture and related problems

Abstract: An important open problem in combinatorial number theory is the Erdős–Szemerédi sum–product conjecture, which suggests that for any positive integers s, N, and for any set A of N integers, either there are many s-fold sums of the form $a_1 + … + a_s$ or there are many s-fold products of the form $a_1\dots a_s$. While this remains wide open, various generalisations of this problem have been considered more recently, including the question of finding optimal variations of the so-called low energy decompositions. In this talk, I will outline some recent progress towards the above questions, as well as highlight how these connect very naturally to other key conjectures in additive combinatorics.

SBIDER on 28 October 2024 at 14:00

Speaker: Anne Skeldon (University of Surrey)
Title: Mathematical modelling of the sleep-wake cycle: light, clocks and digital-twins

Abstract: We all sleep. But what determines when and for how long? In this talk I’ll describe some of the fundamental mechanisms that regulate sleep. I’ll introduce the nonsmooth coupled oscillator systems that form the basis of current mathematical models of sleep-wake regulation and discuss their dynamical behaviour. I will describe how we are using models to unravel environmental, societal and physiological factors that determine sleep timing and outline how constructing digital-twins could enable us to create personalised light interventions for sleep timing disorders.

Combinatorics on 25 October 2024 at 14:00

Speaker: Sarah Selkirk (University of Warwick)
Title: Directed lattice paths with negative boundary

Abstract: Given a set $\mathcal{S} \subseteq \{1\}\times \mathbb{Z}$, a directed lattice path with stepset $\mathcal{S}$ is a finite sequence whose elements are in $\mathcal{S}$. Visually, the elements of the sequence are drawn as vectors starting at $(0, 0)$. Further restrictions, or a lack thereof, on the height of $y$-coordinates ($y\geq 0$) and end-point ($y=0$) of the sequence result in a classification of paths into the four main varieties of lattice path: walks, bridges, meanders, and excursions. For these families, generating functions have been derived in general in the influential work of Banderier and Flajolet (2002) by means of the kernel method. In recent years, directed lattice paths with height restriction $y \geq -t$ with $t\in \mathbb{N}$ have been connected to a number of other combinatorial objects, but have not yet been studied in general. In this talk, we discuss first enumerative results towards a general Banderier-Flajolet-style result for paths with a negative boundary.

Statistical Mechanics on 24 October 2024 at 14:00

Speaker: Juan Neirotti (Aston University)
Title: Legislative impeachments in a neural network society

Abstract: Inspired by studies of government overthrows in modern South American presidential democracies, we present an agent-based Statistical Mechanics analysis of the coordinated actions of strategic political actors within legislative chambers and the conditions that can lead to premature changes in executive leadership, such as presidential impeachments or motions of no confidence in prime ministers. The legislative actors are modeled as information-processing agents, equipped with neural networks, who express opinions on issues from the presidential agenda. We construct a Hamiltonian representing the collective cost incurred by agents for holding a particular set of opinions from a range of possible stances. Using replica methods, we explore two types of disorder: in the distribution of neural network weights and in the structure of agent interactions. The resulting phase diagram illustrates how control parameters -- loosely interpreted as indices of legislative strategic support, presidential polling popularity, and the volume of issues on the presidential agenda -- govern the system behavior. The model reveals an intermediate phase where strategic behaviors in support of or against the executive coexist, flanked by phases (characterised by a pure state) where the legislative vote aligns fully with either supporting or opposing the executive. Changes in these indices, driven by external factors, can push the system out of the coexistence phase and into the opposing pure phase, triggering a phase transition that leads to the removal of the executive through constitutional means. Using data from Brazil, we analyze presidential trajectories during the democratic period starting in 1989, showing that these trajectories align with the phase diagram in terms of whether the president was removed or remained in office.

Algebra on 24 October 2024 at 12:00

Speaker: Alice Dell'Arciprete (University of York)
Title: Quiver presentations for Hecke categories and KLR algebras

Abstract: We discuss the algebraic structure of KLR algebras by way of the diagrammatic Hecke categories of maximal parabolics of finite symmetric groups. Combinatorics (in the shape of Dyck tableaux) plays a huge role in understanding the structure of these algebras. Instead of looking only at the sets of Dyck tableaux (which enumerate the q-decomposition numbers) we look at the relationships for passing between these Dyck tableaux. In fact, this “meta-Kazhdan-Lusztig combinatorics” is sufficiently rich as to completely determine the complete Ext-quiver and relations of these algebras.

One day ergodic theory meeting on 23 October 2024 at 16:45

Speaker: Meng Wu (Oulu)
Title: On normal numbers in fractals

Abstract: Given any Bernoulli measure μ that is x3 invariant (such as the Cantor-Lebesgue measure on the ternary Cantor set) and an irrational number t, it holds that for almost all x with respect to μ, the product tx is x3 normal (meaning that the orbit of tx under the x3 map is uniformly distributed on [0,1]). This result was recently proved by Dayan, Ganguly, and Barak Weiss using techniques from random walk theory. We will present a new proof of the Dayan-Ganguly-Weiss result, relying on recent advancements in the study of self-similar measures with overlaps. Our approach extends the result to cases where the measure μ is only required to be invariant, ergodic, and of positive dimension.

One Day Ergodic Theory Meeting on 23 October 2024 at 15:30

Speaker: Tanja Schindler (University of Exeter)
Title: A qualitative central limit theorem for certain unbounded observables over piecewise expanding interval maps

Abstract: Many limit theorems in ergodic theory are proven using the spectral gap method. So one of the main ingredients for this method is to have a space on which the transfer operator has a spectral gap. However, most of the classical spaces, like for example the space of Hölder or quasi-Hölder function or BV functions, don't allow unbounded functions. We will give such a space which allows observables with a pole at the fixed points of a piecewise expanding interval transformation and state a quantitative central limit theorem using Edgeworth expansions. As an application we give a sampling result for the Riemann-zeta function over a Boolean type transformation. This is joint work with Kasun Fernando.

Algebraic Geometry on 23 October 2024 at 15:00

Speaker: Will Donovan (Tsinghua University)
Title: Derived symmetries for crepant resolutions of hypersurfaces

Abstract: Given a singularity with a crepant resolution, a symmetry of the derived category of coherent sheaves on the resolution may often be constructed using the formalism of spherical functors. I will introduce this, and new work (arXiv:2409.19555) on general constructions of such symmetries for hypersurface singularities. This builds on previous results with Segal, and is inspired by work of Bodzenta-Bondal.

One Day Ergodic Theory Meeting on 23 October 2024 at 14:00

Speaker: Mike Hochman (HUJI)
Title: Strongly irreducible subshifts without periodic points

Abstract: A symbolic system is strongly irreducible if there is some g>0 such that any two patterns in the subshift can be glued together as long as they are separated by a gap of size g. This is the strongest mixing condition one can place on a symbolic system, and it implies many good properties, especially in combination with the finite type property: For example, globally supported measures of maximal entropy, a Krieger-type embedding theorem, and more. In my talk I will discuss the question of the existence of periodic points in such systems, and its connection to a question about periodic points in higher-dimensional shifts of finite type.

Soft Matter Lunches on 23 October 2024 at 13:00

Speaker: Matthew Butler (UCL)
Title: Modelling mechanics of material replacement in biological systems

Abstract: Many biological systems contain material components that are repaired and replaced over time by accompanying cells. One common example is the extra-cellular matrix, an interconnected network of proteins that provides chemical and mechanical protection and support in many systems, such as tissue basement membranes and bacterial biofilms. Questions remain as to how the bulk mechanical properties of the material depend on cell maintenance. I will present a spring-based model that aims to capture how the microscale replacement of elastic material can give rise to different observed bulk mechanical behaviours. Despite the model’s simplicity, it has a number of interesting characteristics that are biologically-relevant.

Algebraic Topology on 22 October 2024 at 17:00

Speaker: Ming Ng (Queen Mary University of London)
Title: K1(Var) is generated by Quasi-Automorphisms

Abstract: Our understanding of K-theory is changing. In recent years, much work has been done to extend various tools from algebraic K-theory to various non-additive settings. One particular highlight: in the same way one can define the K-theory spectrum of an exact category, one can construct a K-theory spectrum K(Var) recovering the Grothendieck ring of varieties as ¥pi_0 [Zakharevich, Campbell]. Up until recently, no complete characterisation of K_n(Var) was known except for n=0. This talk will discuss a new result that shows K_1(Var) is generated by an interesting generalisation of automorphisms of varieties, and present its full relations. In our language: given any pCGW category C (a generalisation of exact categories that also includes finite sets, varieties, definable sets, etc.), the group K1(C) is generated by double exact squares (which we also call quasi-automorphisms). Time permitting, we discuss future applications, as well as a technical subtlety regarding how composition of 1-simplices split in K1(Var), and compare this with Zakharevich’s original presentation of K1(Var).

Ergodic Theory and Dynamical Systems on 22 October 2024 at 13:00

Speaker: Thomas Jordan (University of Bristol)
Title: Countable Markov shifts, pressure at infinity and large deviations

Abstract: For an ergodic transformation, large deviations measure the rate of the decay of the measure of the points where the Birkhoff average is away from the expected value. For many hyperbolic dynamical systems with Gibbs measures this rate is exponential and the exponential rate can be determined using a suitable pressure function. We will show how these results can be adjusted to the setting of mixing subshifts where the alphabet is countably infinite. We show how the standard results can be adapted to this setting and related to the concept of pressure at infinity (recently introduced by Anibal Velozo). We show how the pressure at infinity interplays with the standard rate function and also show a version of large deviations where the rate functions come directly from the pressure at infinity. This is joint work with Godofredo Iommi and Anibal Velozo.

Partial Differential Equations and their Applications on 22 October 2024 at 12:00

Speaker: Richard Medina (Paris Dauphine)
Title: TBA

Abstract: TBA

Number Theory on 21 October 2024 at 15:00

Speaker: Tobias Berger (University of Sheffield)
Title: Pseudomodularity of residually reducible Galois representations

Abstract: After a survey of previous work I will present new results on pseudomodularity of residually reducible Galois representations with 3 residual pieces. I will discuss applications to proving modularity of Galois representations arising from abelian surfaces and Picard curves. This is joint work with Krzysztof Klosin (CUNY).

SBIDER on 21 October 2024 at 14:00

Speaker: Denis Patterson (University of Durham)
Title: Spatial models of forest-savanna bistability

Abstract: Empirical studies suggest that for vast tracts of land in the tropics, closed-canopy forests and savannas are alternative stable states, a proposition with far-reaching implications in the context of ongoing climate change. Consequently, numerous spatially implicit and explicit mathematical models have been proposed to capture the mechanistic basis of this bistability and quantify the stability of these ecosystems. We present an analysis of a spatially extended version of the so-called Staver-Levin model of forest-savanna dynamics (a system of nonlinear partial integro-differential equations). On a homogeneous domain, we uncover various types of pattern-forming bifurcations in the presence of resource limitation, which we study as a function of the resource constraints and length scales in the problem. On larger (continental) spatial scales, heterogeneity plays a significant role in determining observed vegetative cover. Incorporating domain heterogeneity leads to interesting phenomena such as front-pinning, complex waves, and extensive multi-stability, which we investigate analytically and numerically.

Colloquium on 18 October 2024 at 16:00

Speaker: Liz Fearon (UCL)
Title: Epidemiological modelling of testing, contact tracing and isolation interventions in epidemic response: experiences from COVID-19

Abstract: This talk will explore mathematical modelling used to support design and deployment decisions for one of our key epidemic control interventions: testing, tracing and isolation or quarantine (TTI). I will review the types of questions that were posed at different stages of the COVID-19 pandemic in the UK and how we responded to them, highlighting the types of modelling tools that were used. Thinking more broadly and looking forward, I will consider new challenges, including for other types of infections and learnings for how we develop models and engage with communities and other disciplines in doing so.

Combinatorics on 18 October 2024 at 14:00

Speaker: Camila Zárate-Guerén (University of Birmingham)
Title: Colour-bias perfect matchings in hypergraphs

Abstract: Given a k-uniform hypergraph H on n vertices with an r-colouring of its edges, we look for a minimum l-degree condition that guarantees the existence of a perfect matching in H that has more than n/rk edges in one colour. We call this a colour-bias perfect matching. For 2-coloured graphs, a result of Balogh, Csaba, Jing and Pluhár yields the minimum degree threshold that ensures a perfect matching of significant colour-bias. In this talk, I will present an analogous of this result for k-uniform hypergraphs. More precisely, for each 1<=l<k and r>=2 we determined the minimum l-degree threshold that forces a perfect matching of significant colour-bias in an r-edge-coloured k-uniform hypergraph. The presented result is joint work with J. Balogh, H. Hàn, R. Lang, J. P. Marciano, M. Pavez-Signé, N. Sanhueza-Matamala and A. Treglown.

Analysis on 17 October 2024 at 16:00

Speaker: Emily Casey (University of Washington)
Title: Characterizing rectifiability in terms of principal values

Abstract: Since the work of Mattila and Preiss in 1995, it's been known that for a Radon measure with reasonable density assumptions, the almost everywhere existence of principal values of the Riesz transform is equivalent to the measure being rectifiable. In ongoing work with Goering, Toro, and Wilson, we extend this result of Mattila and Preiss to a rough Riemannian setting. In this talk, we discuss the techniques used in proving the almost everywhere existence of principal values for smooth Calderon Zygmund kernels for rectifiable measures, even when the kernel is not of convolution type.

Geometry and Topology on 17 October 2024 at 13:30

Speaker: Mireille Soergel (MPIM Leipzig)
Title: Dyer groups: Coxeter groups, right-angled Artin groups and more...

Abstract: Dyer groups are a family encompassing both Coxeter groups and right-angled Artin groups. Among many common properties, these two families admit the same solution to the word problem. Each of these two classes of groups also have natural piecewise Euclidean CAT(0) spaces associated to them. In this talk I will introduce Dyer groups, give some of their properties.

Algebra on 17 October 2024 at 12:00

Speaker: Jack Saunders (University of Bristol)
Title: Linear groups acting 4-arc-transitively on cubic graphs

Abstract: In this talk, we give a brief overview of s-arc-transitive graphs and show how their study in the case of cubic (3-regular) graphs reduces to a generation problem for finite almost simple groups. We then discuss current progress towards solving this generation problem for PSL(n,q) when n is sufficiently large and q is coprime to 6.

Ada Lovelace Day on 16 October 2024 at 14:00

Speaker: Various speakers (N/A)
Title: Ada Lovelace Day

Abstract: Ada Lovelace Day is an annual celebration of women's achievements in the fields of STEM. See https://warwick.ac.uk/fac/sci/maths/general/edi/ada_lovelace24/ for the schedule and more information. The event is open to all, irrespective of gender, so please do come along!

Algebraic Topology on 15 October 2024 at 17:00

Speaker: Robin Stoll (University of Cambridge)
Title: The stable cohomology of block diffeomorphisms of connected sums of S^k × S^l

Abstract: I will explain an identification of the stable rational cohomology of the classifying spaces of self-equivalences as well as block diffeomorphisms of connected sums of S^k × S^l (relative to an embedded disk), where 2 < k < l < 2k–1. The result is expressed in terms of versions of Lie graph complex homology, the constructions of which I will recall. This also leads to a computation, in a range of degrees, of the stable rational cohomology of the classifying spaces of diffeomorphisms of these manifolds. In the case l = k+1, this recovers and extends results of Ebert-Reinhold. If time permits, I will explain parts of the proof; this includes in particular work joint with Berglund on a certain type of algebraic models for relative self-equivalences of bundles, inspired by results of Berglund-Zeman.

Partial Differential Equations and their Applications on 15 October 2024 at 12:00

Speaker: Jakub Woznicki (Warsaw)
Title: TBA

Abstract: TBA

Number Theory on 14 October 2024 at 15:00

Speaker: Maleeha Khawaja (Sheffield)
Title: Galois groups of low degree points on curves

Abstract: Whilst the study of low degree algebraic points on curves is an active area of research, there has been little emphasis on the Galois-theoretic description of these points. In this talk, we focus on the behaviour of low degree points whose Galois group is primitive. Furthermore, we shall see that the behaviour changes if the degree is large (with respect to the genus of the curve). This talk is based on joint work with Frazer Jarvis (Sheffield) and Samir Siksek (Warwick).

DAGGER on 14 October 2024 at 14:00

Speaker: Mariam Al-Hawaj (Trinity College Dublin)
Title: Generalized pseudo-Anosov Maps and Hubbard Trees

Abstract: The Nielsen-Thurston classification of the mapping classes proved that every orientation preserving homeomorphism of a closed surface, up to isotopy is either periodic, reducible, or pseudo-Anosov. Pseudo-Anosov maps have particularly nice structure because they expand along one foliation by a factor of λ > 1 and contract along a transversal foliation by a factor of 1/λ. The number λ is called the dilatation of the pseudo-Anosov. Thurston showed that every dilatation λ of a pseudo-Anosov map is an algebraic unit, and conjectured that every algebraic unit λ whose Galois conjugates lie in the annulus A_λ = {z : 1/λ < |z| < λ} is a dilatation of some pseudo-Anosov on some surface S. Pseudo-Anosovs have a huge role in Teichmuller theory and geometric topology. The relation between these and complex dynamics has been well studied inspired by Thurston. In this project, I develop a new connection between the dynamics of quadratic polynomials on the complex plane and the dynamics of homeomorphisms of surfaces. In particular, given a quadratic polynomial, we show that one can construct an extension of it which is generalized pseudo-Anosov homeomorphism. Generalized pseudo-Anosov means the foliations have infinite singularities that accumulate on finitely many points. We determine for which quadratic polynomials such an extension exists. My construction is related to the dynamics on the Hubbard tree which is a forward invariant subset of the filled Julia set that contains the critical orbit.

SBIDER on 14 October 2024 at 14:00

Speaker: Emma Davis (University of Warwick)
Title: Applications of branching processes to disease emergence and elimination

Abstract: Branching processes can provide insights into the stochastic dynamics that can occur at low prevalence. We will discuss applications of these methods at opposite ends of the spectrum: 1) emerging outbreaks, with application to assessing the impact of contract tracing on control of COVID-19 in the UK; and 2) elimination, with application to assessing elimination thresholds for the Neglected Tropical Disease lymphatic filariasis.

Colloquium on 11 October 2024 at 16:00

Speaker: Mark Peletier (Eindhoven)
Title: In search of structure: Gradient flows, GENERIC systems, and the role of noise

Abstract: Many ODEs and PDEs describe real-world phenomena. Often it is useful to know that these equations have, more "structure" than that of the bare PDE. For instance, many ODEs and PDEs are Hamiltonian systems, which provides a wealth of additional information about their solutions. Other ODEs and PDEs are gradient flows, and this is the structure that I will concentrate on. Many evolutionary PDEs are known to be gradient flows in an appropriate sense, and again this property gives deep insight and provides many tools for analysis. Despite the importance of such gradient structures, it is only relatively recently that we have discovered the reason why many evolutionary PDEs are gradient flows, in particular why there are so many gradient flows based on the Wasserstein metric. It turns out that this is intimately connected to randomness. In this talk I will discuss gradient flows and their cousin "GENERIC systems", and show how one can understand how these deterministic, geometric structures have their roots in randomness.

Combinatorics on 11 October 2024 at 14:00

Speaker: Ella Williams (UCL)
Title: Covering vertices with monochromatic paths

Abstract: In 1995, Erd\H{o}s and Gy\'arf\'as proved that in every 2-edge-coloured complete graph on $n$ vertices, there exists a collection of $2\sqrt{n}$ monochromatic paths, all of the same colour, which cover the entire vertex set. They conjectured that it is possible to replace $2\sqrt{n}$ by $\sqrt{n}$. We prove this to be true for all sufficiently large $n$. This is based on joint work with Alexey Pokrovskiy and Leo Versteegen.

Geometry and Topology on 10 October 2024 at 13:30

Speaker: Shaked Bader (University of Oxford)
Title: Hyperbolic subgroups of type FP_2(Ring)

Abstract: In 1996 Gersten proved that if G is a word hyperbolic group of cohomological dimension 2 and H is a subgroup of type FP_2, then H is hyperbolic as well. In this talk, I will present a joint work with Robert Kropholler and Vlad Vankov generalising this result to show that the same is true if G is only assumed to have cohomological dimension 2 over some ring R and H is of type FP_2(R).

Soft Matter Lunches on 09 October 2024 at 13:00

Speaker: Nathan van der Riet (University of Warwick)
Title: Minimal modelling of vesiculation processes as drivers of topology change in cell membranes

Abstract: TBA

Number Theory on 07 October 2024 at 15:00

Speaker: Han Yu (University of Warwick)
Title: TBA

Abstract: TBA

DAGGER on 07 October 2024 at 14:00

Speaker: Short volunteered talks (University of Warwick)
Title: DAGGER welcome-back session

Abstract: DAGGER provides a space for early-career mathematicians with an interest in dynamics, geometry, topology, and related areas to share their research and connect over maths. Everyone — and in particular new PhD students, new master's students, and third-/fourth-year undergraduates — is invited to this introductory session of the DAGGER seminar, which will feature short (5-10 minute) volunteered talks. Come along to give a talk about something you find interesting, hear some maths, or just introduce yourself!

Partial Differential Equations and their Applications on 07 October 2024 at 14:00

Speaker: Clement Mouhot (Cambridge)
Title: TBA

Abstract: TBA

SBIDER on 07 October 2024 at 14:00

Speaker: Francesca Scarabel (University of Leeds)
Title: Numerical methods for structured population models in ecology and epidemiology

Abstract: In this talk I will consider mathematical models for populations where individual rates are completely determined by a continuous structuring variable that evolves in time (e.g. age or size in ecology, age or age of infection in epidemiology). They can be described as renewal equations or partial differential equations of transport type, and the set of software tools available for these types of equations is much more limited compared to that available for compartmental models formulated as ordinary differential equations (ODEs). In recent years, within a collaboration with the University of Udine (Italy), I have developed a series of user-friendly numerical methods to study the stability and bifurcations of structured population models by means of a convenient approximation with ODEs, which can be studied with well-established software for ODEs. More recently, similar numerical techniques have been used to obtain an efficient method to approximate the reproduction numbers. I will illustrate the methods with applications to mathematical ecology and epidemiology.

Combinatorics on 04 October 2024 at 14:00

Speaker: Akshat Mudgal (University of Warwick)
Title: Approximating sumset estimates via translates

Abstract: A finite, non-empty subset A of Z^d is defined to be d-dimensional if it is not contained in a translate of some hyperplane. Given a d-dimensional set A of cardinality N, a classical result in additive combinatorics known as Freiman’s lemma implies that |A+A| >= (d+1)N - d(d+1)/2. Moreover, this estimate is sharp. In the spirit of some recent work of Bollobas–Leader–Tiba, it is natural to ask whether one can approximate this lower bound by just considering a few translates of A. In joint work with Yifan Jing we prove precisely this, that is, for any d-dimensional set A with N elements, there exists a subset X of A with |X| = O_d(1) such that |A+X| >= (d+1)N - d(d+1)/2.

Geometry and Topology on 03 October 2024 at 13:30

Speaker: Saul Schleimer (University of Warwick)
Title: Solving the word problem in the mapping class group in quasi-linear time

Abstract: Mapping class groups of surfaces are of fundamental importance in dynamics, geometric group theory, and low-dimensional topology. The word problem for groups in general, the definition of the mapping class group, its finite generation by twists, and the solution to its word problem were all set out by Dehn [1911, 1922, 1938]. Some of this material was rediscovered by Lickorish [1960's] and then by Thurston [1970-80's] -- they gave important applications of the mapping class group to the topology and geometry of three-manifolds. In the past fifty years, various mathematicians (including Penner, Mosher, Hamidi-Tehrani, Dylan Thurston, Dynnikov) have given solutions to the word problem in the mapping class group, using a variety of techniques. All of these algorithms are quadratic-time. We give an algorithm requiring only O(n log^3(n)) time. We do this by combining Dynnikov's approach to curves on surfaces, M\"oller's version of the half-GCD algorithm, and a delicate error analysis in interval arithmetic. This is joint work with Mark Bell.

Mathematics Teaching and Learning on 02 October 2024 at 15:00

Speaker: Mani Mahal, Sam Petrie (Warwick)
Title: Exploring the use of AI in mathematics and statistics assessments: project outcome and highlights

Abstract: We will discuss highlights from the summer project studying the current use and future potential of generative AI in maths and stats at Warwick. Key questions include: 1. To what extent do our students currently use AI to help with assignments? 2. How well does AI perform in current assignments? 3. What future educational directions should our departments take given the rapid improvement of genAI?

Analysis on 01 October 2024 at 16:00

Speaker: Anna Skorobogatova (SLMath)
Title: Rectifiability of singularities and uniqueness of tangent cones for semicalibrated currents

Abstract: Semicalibrated currents are a subclass of almost area-minimizing surfaces appearing naturally in various geometric problems, and are a generalization of calibrated submanifolds. Unlike general almost area-minimizers, semicalibrated currents are expected to share the regularity properties of area-minimizing currents, and indeed it was shown by Spolaor in 2015 that the singular set is codimension two with respect to the surface, extending the conclusion of Almgren’s celebrated “big regularity paper” to this setting. I will talk about recent joint work with Paul Minter, Davide Parise and Luca Spolaor, in which we build on this to obtain a sharp structural result for the interior singular set, along with a classification of blow-ups at most singular points.

Junior Analysis and Probability Seminar on 30 September 2024 at 15:00

Speaker: Anna Skorobogatova (SLMath)
Title: Regularity for critical points of semilinear elliptic variational problems with a topological constraint

Abstract: I will discuss the regularity of critical points for a free boundary problem arising from the diffuse interface/Allen-Cahn approximation of the set-theoretic Plateau problem recently introduced by Maggi-Novack-Restrepo. Here, a homotopic spanning constraint, first considered by Harrison-Pugh, forces the surfaces (and also the corresponding interface for the diffuse approximation), to remain attached to the given wire frame. The presence of the spanning condition allows for minimizers of this problem to exhibit codimension 1 singularities such as triple junctions and tetrahedral singularities, in stark contrast to the work of Tonegawa-Wickramasekera which shows that any stable minimal hypersurface arising as a limit of interfaces for stable critical points of classical Allen-Cahn. I will further discuss free boundary regularity for minimizers of this problem. This is a joint work with Mike Novack (Louisiana State University) and Daniel Restrepo (Johns Hopkins University).

Number Theory on 30 September 2024 at 15:00

Speaker: Tim Browning (IST Austria)
Title: Pairs of commuting matrices

Abstract: I'll discuss commuting varieties and a new upper bound for the number of pairs of commuting $n \times n$ matrices with integer entries and height at most $T$, as $T \to \infty$. Our approach uses Fourier analysis and mod $p$ information, together with a result about the flatness of the commutator Lie bracket, which we also solve. This is joint work with Will Sawin and Victor Wang.

Soft Matter Lunches on 30 September 2024 at 12:00

Speaker: Joseph Webber (University of Warwick)
Title: Tubular hydrogel pumps through a responsive LENS

Abstract: Modelling thermo-responsive hydrogels can be a challenging task – the physics that underlies the rapid deswelling that occurs when temperatures cross a critical threshold is hard to characterise, and the transition from a swollen to a deswollen state involves the transport of fluid through the pores and the elastic deformation of the polymer scaffold left behind. In this talk, I will outline a new model for thermo-responsive gels that makes the physical processes behind deswelling straightforward to model and can reproduce all of the phenomenology seen in much more complicated energy-based approaches. This new model allows for the development of single-component displacement pumps that can drive flows of water through their lumen in response to pulses of heat, and we can deduce the optimum geometry for maximum pumping rates using the quantitative predictions that it provides.