Tue, 03 Dec 2024
16:00
C3

The space of traces of certain discrete groups

Raz Slutsky
(University of Oxford)
Abstract

A trace on a group is a positive-definite conjugation-invariant function on it. These traces correspond to tracial states on the group's maximal  C*-algebra. In the past couple of decades, the study of traces has led to exciting connections to the rigidity, stability, and dynamics of groups. In this talk, I will explain these connections and focus on the topological structure of the space of traces of some groups. We will see the different behaviours of these spaces for free groups vs. higher-rank lattices, and how our strategy for the free group can be used to answer a question of Musat and Rørdam regarding free products of matrix algebras. This is based on joint works with Arie Levit, Joav Orovitz, and Itamar Vigdorovich.

Tue, 03 Dec 2024
16:00
L6

Large deviations of Selberg’s CLT: upper and lower bounds

Emma Bailey
(University of Bristol)
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 concerns work joint with Louis-Pierre Arguin and Asher Roberts.

Tue, 03 Dec 2024
15:00
L6

Short loxodromics in graph products

Alice Kerr
(University of Bristol)
Abstract
Let G be a finitely generated group, with finite generating set S. Suppose G contains elements with some property that we’re interested in.  Can we find elements with this property uniformly quickly in G? That is, does S^n contain an element with this property for a bounded n?
 
We will discuss this question for graph products, where the elements we are looking for are ones with nice hyperbolic properties, such as loxodromic and Morse elements. We will also talk about consequences for the growth of these groups. This is joint work with Elia Fioravanti.

 
Tue, 03 Dec 2024
14:00
L5

Gecia Bravo-Hermsdorff: What is the variance (and skew, kurtosis, etc) of a network? Graph cumulants for network analysis

Gecia Bravo-Hermsdorff
(University College London)
Abstract

Topically, my goal is to provide a fun and instructive introduction to graph cumulants: a hierarchical set of subgraph statistics that extend the classical cumulants (mean, (co)variance, skew, kurtosis, etc) to relational data.  

Intuitively, graph cumulants quantify the propensity (if positive) or aversion (if negative) for the appearance of any particular subgraph in a larger network.  

Concretely, they are derived from the “bare” subgraph densities via a Möbius inversion over the poset of edge partitions.  

Practically, they offer a systematic way to measure similarity between graph distributions, with a notable increase in statistical power compared to subgraph densities.  

Algebraically, they share the defining properties of cumulants, providing clever shortcuts for certain computations.  

Generally, their definition extends naturally to networks with additional features, such as edge weights, directed edges, and node attributes.  

Finally, I will discuss how this entire procedure of “cumulantification” suggests a promising framework for a motif-centric statistical analysis of general structured data, including temporal and higher-order networks, leaving ample room for exploration. 

Tue, 03 Dec 2024

14:00 - 15:00
L4

A Zarankiewicz problem in tripartite graphs

Freddie Illingworth
(University College London)
Abstract

In 1975, Bollobás, Erdős, and Szemerédi asked the following Zarankiewicz-type problem. What is the smallest $\tau$ such that an $n \times n \times n$ tripartite graph with minimum degree $n + \tau$ must contain $K_{t, t, t}$? They further conjectured that $\tau = O(n^{1/2})$ when $t = 2$.

I will discuss our proof that $\tau = O(n^{1 - 1/t})$ (confirming their conjecture) and an infinite family of extremal examples. The bound $O(n^{1 - 1/t})$ is best possible whenever the Kővári-Sós-Turán bound $\operatorname{ex}(n, K_{t, t}) = O(n^{2 - 1/t})$ is (which is widely-conjectured to be the case).

This is joint work with Francesco Di Braccio (LSE).

Tue, 03 Dec 2024
14:00
L6

Hyperbolic intersection arrangements

Samuel Lewis
(University of Oxford)
Abstract

Consider a connected graph and choose a subset of its vertices. From this simple setup, Iyama and Wemyss define a collection of real hyperplanes known as an intersection arrangement, going on to classify all tilings of the affine plane that arise in this way. These "local" generalisations of Coxeter combinatorics also admit a nice wall-crossing structure via Dynkin involutions and longest Weyl elements. In this talk I give an analogous classification in the hyperbolic setting using the data of an "overextended" ADE diagram with three distinguished vertices. I then discuss ongoing work applying intersection arrangements to parametrise notions of stability conditions for preprojective algebras.

Tue, 03 Dec 2024
13:00
L2

Quantized axial charge of lattice fermions and the chiral anomaly

Arkya Chatterje
(MIT )
Abstract

Realizing chiral global symmetries on a finite lattice is a long-standing challenge in lattice gauge theory, with potential implications for non-perturbative regularization of the Standard Model. One of the simplest examples of such a symmetry is the axial U(1) symmetry of the 1+1d massless Dirac fermion field theory: it acts by equal and opposite phase rotations on the left- and right-moving Weyl components of the Dirac field. This field theory also has a vector U(1) symmetry which acts identically on left- and right-movers. The two U(1) symmetries exhibit a mixed anomaly, known as the chiral anomaly. In this talk, we will discuss how both symmetries are realized as ordinary U(1) symmetries of an "ultra-local" lattice Hamiltonian, on a finite-dimensional Hilbert space. Intriguingly, the anomaly of the Abelian U(1) symmetries in the infrared (IR) field theory is matched on the lattice by a non-Abelian Lie algebra. The lattice symmetry forces the low-energy phase to be gapless, closely paralleling the effects of the anomaly in the field theory.

Mon, 02 Dec 2024
16:30
L4

Introducing various notions of distances between space-times

Anna Sakovich
(University of Uppsala)
Abstract

I will introduce the class of causally-null-compactifiable spacetimes that can be canonically converted into compact timed-metric spaces using the cosmological time function of Andersson-Galloway-Howard and the null distance of Sormani-Vega. This class of space-times includes future developments of compact initial data sets and regions exhausting asymptotically flat space-times. I will discuss various intrinsic notions of distance between such space-times and show that some of them are definite in the sense that they are equal to zero if and only if there is a time-oriented Lorentzian isometry between the space-times. These definite distances allow us to define notions of convergence of space-times to limit space-times that are not necessarily smoothThis is joint work with Christina Sormani.

Mon, 02 Dec 2024
16:00
C3

TBC

Leo Gitin
(University of Oxford)
Abstract

TBC

Mon, 02 Dec 2024
15:30
L5

Building surfaces from equilateral triangles

Lasse Rempe
(Manchester University)
Abstract
In this talk, we consider the following question. Suppose that we glue a (finite or infinite) collection of closed equilateral triangles together in such a way that we obtain an orientable surface. The resulting surface is a Riemann surface; that is, it has a natural conformal structure (a way of measuring angles in tangent space). We ask which Riemann surfaces are *equilaterally triangulable*; i.e., can arise in this fashion.

The answer in the compact case is given by a famous classical theorem of Belyi, which states that a compact surface is equilaterally triangulable if and only if it is defined over a number field. These *Belyi surfaces* - and their associated “dessins d’enfants” - have found applications across many fields of mathematics, including mathematical physics.

In joint work with Chris Bishop, we give a complete answer of the same question for the case of infinitely many triangles (i.e., for non-compact Riemann surfaces). The talk should be accessible to a general mathematical audience, including postgraduate students.


 

Mon, 02 Dec 2024
15:30
L3

Chasing regularization by noise of 3D Navier-Stokes equations

Dr Antonio Agresti
(Delft University of Technology )
Abstract

Global well-posedness of 3D Navier-Stokes equations (NSEs) is one of the biggest open problems in modern mathematics. A long-standing conjecture in stochastic fluid dynamics suggests that physically motivated noise can prevent (potential) blow-up of solutions of the 3D NSEs. This phenomenon is often referred to as `regularization by noise'. In this talk, I will review recent developments on the topic and discuss the solution to this problem in the case of the 3D NSEs with small hyperviscosity, for which the global well-posedness in the deterministic setting remains as open as for the 3D NSEs. An extension of our techniques to the case without hyperviscosity poses new challenges at the intersection of harmonic and stochastic analysis, which, if time permits, will be discussed at the end of the talk.

Mon, 02 Dec 2024
14:15
L4

Open Gromov-Witten invariants and Mirror symmetry

Kai Hugtenburg
(Lancaster)
Abstract

This talk reports on two projects. The first work (in progress), joint  with Amanda Hirschi, constructs (genus 0) open Gromov-Witten invariants for any Lagrangian submanifold using a global Kuranishi chart construction. As an application we show open Gromov-Witten invariants are invariant under Lagrangian cobordisms. I will then describe how open Gromov-Witten invariants fit into mirror symmetry, which brings me to the second project: obtaining open Gromov-Witten invariants from the Fukaya category.

Mon, 02 Dec 2024

14:00 - 15:00
Lecture Room 3

Enhancing Accuracy in Deep Learning using Marchenko-Pastur Distribution

Leonid Beryland
(Penn State University)
Abstract

We begin with a short overview of Random Matrix Theory (RMT), focusing on the Marchenko-Pastur (MP) spectral approach. 

Next, we present recent analytical and numerical results on accelerating the training of Deep Neural Networks (DNNs) via MP-based pruning ([1]). Furthermore, we show that combining this pruning with L2 regularization allows one to drastically decrease randomness in the weight layers and, hence, simplify the loss landscape. Moreover, we show that the DNN’s weights become deterministic at any local minima of the loss function. 
 

Finally, we discuss our most recent results (in progress) on the generalization of the MP law to the input-output Jacobian matrix of the DNN. Here, our focus is on the existence of fixed points. The numerical examples are done for several types of DNNs: fully connected, CNNs and ViTs. These works are done jointly with PSU PhD students M. Kiyashko, Y. Shmalo, L. Zelong and with E. Afanasiev and V. Slavin (Kharkiv, Ukraine). 

 

[1] Berlyand, Leonid, et al. "Enhancing accuracy in deep learning using random matrix theory." Journal of Machine Learning. (2024).

Mon, 02 Dec 2024
13:30
C4

Extended TQFT, gauge theory, and Measurement Based Quantum Computation

Gabriel Wong
Abstract

Measurement-Based Quantum Computation (MBQC) is a model of quantum computation driven by measurements instead of unitary gates.   In 2D it is capable of supporting universal quantum computations.   Interestingly, while all measurements are local, the computational output involves non local observables.   We will use the simpler case of 1D MBQC to illustrate how these features can be captured by ideas from gauge theory and extended TQFT. We will also explain  MBQC from the perspective of the extended Hilbert space construction in gauge theories, in which the entanglement edge modes play the role of the logical qubit.

Fri, 29 Nov 2024

14:00 - 15:00
L1

Combating Imposter Syndrome

Abstract

How can it be that so many clever, competent and capable people can feel that they are just one step away from being exposed as a complete fraud? Despite evidence that they are performing well they can still have that lurking fear that at any moment someone is going to tap them on the shoulder and say "We need to have a chat". If you've ever felt like this, or you feel like this right now, then this Friday@2 session might be of interest to you. We'll explore what "Imposter Feelings" are, why we get them and steps you can start to take to help yourself and others. This event is likely to be of interest to undergraduates and MSc students at all stages. 

Fri, 29 Nov 2024

12:00 - 13:00
C5

On Lusztig’s local Langlands correspondence and functoriality

Emile Okada
(National University of Singapore)
Abstract

In ’95 Lusztig gave a local Langlands correspondence for unramified representations of inner to split adjoint groups combining many deep results from type theory and geometric representation theory. In this talk, I will present a gentle reformulation of his construction revealing some interesting new structures, and with a view toward proving functoriality results in this framework. 

This seminar is organised jointly with the Junior Algebra and Representation Theory Seminar - all are very welcome!

Fri, 29 Nov 2024

12:00 - 13:00
C5

On Lusztig’s local Langlands correspondence and functoriality

Emile Okada
(National University of Singapore)
Abstract

In ’95 Lusztig gave a local Langlands correspondence for unramified representations of inner to split adjoint groups combining many deep results from type theory and geometric representation theory. In this talk I will present a gentle reformulation of his construction revealing some interesting new structures, and with a view toward proving functoriality results in this framework. 

Fri, 29 Nov 2024
12:00
L2

Towards a mathematical definition of superstring scattering amplitudes

Alexander Polishchuk
(University of Oregon)
Abstract

This is a report on the ongoing joint project with Giovanni Felder and David Kazhdan. I'll describe a conjectural way to set up the integration of the superstring measure on the moduli space of supercurves, including a brief review of the necessary supergeometry. The main theorem is that this setup works for genus 2 with no punctures.

Fri, 29 Nov 2024

11:00 - 12:00
L5

Algebraic approaches in the study of chemical reaction networks

Dr Murad Banaji
(Mathematical Institute University of Oxford)
Abstract

Underlying many biological models are chemical reaction networks (CRNs), and identifying allowed and forbidden dynamics in reaction networks may 
give insight into biological mechanisms. Algebraic approaches have been important in several recent developments. For example, computational 
algebra has helped us characterise all small mass action CRNs admitting certain bifurcations; allowed us to find interesting and surprising 
examples and counterexamples; and suggested a number of conjectures.   Progress generally involves an interaction between analysis and 
computation: on the one hand, theorems which recast apparently difficult questions about dynamics as (relatively tractable) algebraic problems; 
and on the other, computations which provide insight into deeper theoretical questions. I'll present some results, examples, and open 
questions, focussing on two domains of CRN theory: the study of local bifurcations, and the study of multistationarity.

Thu, 28 Nov 2024
17:00
L4

The Index of Constant Mean Curvature Surfaces in Three-Manifolds

Luca Seemungal
(University of Leeds)
Abstract
Constant mean curvature (CMC) surfaces are special geometric variational objects, closely related to minimal surfaces. The key properties of a CMC surface are its area, mean curvature, genus, and index. The index of a CMC surface measures its stability: the index counts how many ways one can perturb the surface to decrease the area while keeping the enclosed volume constant. In this talk we discuss relationships between these key properties. In particular we present recent joint work with Ben Sharp, where we bound the index of CMC surfaces linearly from above by genus and the correct scale-invariant quantity involving mean curvature and area.

 
Thu, 28 Nov 2024
16:00
L4

Regurgitative Training in Finance: Generative Models for Portfolios

Adil Rengim Cetingoz
(Centre d'Economie de la Sorbonne)
Further Information

Please join us for refreshments outside the lecture room from 15:30.

Abstract
Simulation methods have always been instrumental in finance, but data-driven methods with minimal model specification, commonly referred to as generative models, have attracted increasing attention, especially after the success of deep learning in a broad range of fields. However, the adoption of these models in practice has not kept pace with the growing interest, probably due to the unique complexities and challenges of financial markets. This paper aims to contribute to a deeper understanding of the development, use and evaluation of generative models, particularly in portfolio and risk management. To this end, we begin by presenting theoretical results on the importance of initial sample size, and point out the potential pitfalls of generating far more data than originally available. We then highlight the inseparable nature of model development and the desired use case by touching on a very interesting paradox: that generic generative models inherently care less about what is important for constructing portfolios (at least the interesting ones, i.e. long-short). Based on these findings, we propose a pipeline for the generation of multivariate returns that meets conventional evaluation standards on a large universe of US equities while providing interesting insights into the stylized facts observed in asset returns and how a few statistical factors are responsible for their existence. Recognizing the need for more delicate evaluation methods, we suggest, through an example of mean-reversion strategies, a method designed to identify bad models for a given application based on regurgitative training, retraining the model using the data it has itself generated.
 

 
Thu, 28 Nov 2024
16:00
Lecture Room 3

Large sieve inequalities for exceptional Maass forms and applications

Alexandru Pascadi
(University of Oxford)
Abstract

A number of results on classical problems in analytic number theory rely on bounds for multilinear forms of Kloosterman sums, which in turn use deep inputs from the spectral theory of automorphic forms. We’ll discuss our recent work available at arxiv.org/abs/2404.04239, which uses this interplay between counting problems, exponential sums, and automorphic forms to improve results on the greatest prime factor of $n^2+1$, and on the exponents of distribution of primes and smooth numbers in arithmetic progressions.
The key ingredient in this work are certain “large sieve inequalities” for exceptional Maass forms, which improve classical results of Deshouillers-Iwaniec in special settings. These act as on-average substitutes for Selberg’s eigenvalue conjecture, narrowing (and sometimes completely closing) the gap between previous conditional and unconditional results.

Thu, 28 Nov 2024
16:00
C3

On the (Local) Lifting Property

Tatiana Shulman
(University of Gothenburg)
Abstract

The (Local) Lifting Property ((L)LP) is introduced by Kirchberg and deals with lifting completely positive maps. We will discuss various examples, characterizations, and closure properties of the (L)LP and, if time permits, connections with some other lifting properties of C*-algebras.  Joint work with Dominic Enders.

Thu, 28 Nov 2024

14:00 - 15:00
Lecture Room 3

Unleashing the Power of Deeper Layers in LLMs

Shiwei Liu
(Oxford University)
Abstract

Large Language Models (LLMs) have demonstrated impressive achievements. However, recent research has shown that their deeper layers often contribute minimally, with effectiveness diminishing as layer depth increases. This pattern presents significant opportunities for model compression. 

In the first part of this seminar, we will explore how this phenomenon can be harnessed to improve the efficiency of LLM compression and parameter-efficient fine-tuning. Despite these opportunities, the underutilization of deeper layers leads to inefficiencies, wasting resources that could be better used to enhance model performance. 

The second part of the talk will address the root cause of this ineffectiveness in deeper layers and propose a solution. We identify the issue as stemming from the prevalent use of Pre-Layer Normalization (Pre-LN) and introduce Mix-Layer Normalization (Mix-LN) with combined Pre-LN and Post-LN as a new approach to mitigate this training deficiency.

Thu, 28 Nov 2024
12:00
C6

Magnetic Brunn-Minkowski and Borell-Brascamp-Lieb inequalities on Riemannian manifolds

Rotem Assouline
(The Weizmann Institute of Science)
Abstract

The Brunn-Minkowski inequality gives a lower bound on the volume of the set of midpoints of line segments joining two sets. On a Riemannian manifold, line segments are replaced by geodesic segments, and the Brunn-Minkowski inequality characterizes manifolds with nonnegative Ricci curvature. I will present a generalization of the Riemannian Brunn-Minkowski inequality where geodesics are replaced by magnetic geodesics, which are minimizers of a functional given by length minus the integral of a fixed one-form on the manifold. The Brunn-Minkowski inequality is then equivalent to nonnegativity of a suitably defined magnetic Ricci curvature. More generally, I will present a magnetic version of the Borell-Brascamp-Lieb inequality of Cordero-Erausquin, McCann and Schmuckenschläger. The proof uses the needle decomposition technique.

Thu, 28 Nov 2024

12:00 - 12:30
Lecture Room 6

​​​​​Preconditioners for Multicomponent Flows

Kars Knook
(University of Oxford)
Abstract

Multicomponent flows, i.e. mixtures, can be modeled effectively using the Onsager-Stefan-Maxwell (OSM) equations. The OSM equations can account for a wide variety of phenomena such as diffusive, convective, non-ideal mixing, thermal, pressure and electrochemical effects for steady and transient multicomponent flows. I will first introduce the general OSM framework and a finite element discretisation for multicomponent diffusion of ideal gasses. Then I will show two ways of preconditioning the multicomponent diffusion problem for various boundary conditions. Time permitting, I will also discuss how this can be extended to the non-ideal, thermal, and nonisobaric settings.

Thu, 28 Nov 2024

11:00 - 12:00
TCC VC

Probability logic

Ehud Hrushovski
(University of Oxford)
Wed, 27 Nov 2024

17:00 - 18:30
L5

Truth Be Told: How To Interpret Past Mathematicians

A.C. Paseau and Fabian Pregel
(Department of Philosophy, University of Oxford)
Abstract

How should we interpret past mathematicians who may use the same vocabulary as us but with different meanings, or whose philosophical outlooks differ from ours? Errors aside, it is often assumed that past mathematicians largely made true claims—but what exactly justifies that assumption?


In this talk, we will explore these questions through general philosophical considerations and three case studies: 19th-century analysis, 18th-century geometry, and 19th-century matricial algebra.  In each case, we encounter a significant challenge to supposing that the mathematicians in question made true claims. We will show how these challenges can be addressed and overcome.

Wed, 27 Nov 2024
16:00
L6

Floer Homology and Square Peg Problem

Soheil Azarpendar
(University of Oxford)
Abstract

In 1911, Otto Toeplitz posed the intriguing "Square Peg Problem," asking whether every Jordan curve admits an inscribed square. Despite over a century of study, the problem remains unsolved in its full generality. However, significant progress has been made over the years. In this talk, we explore recent advancements by Andrew Lobb and Joshua Greene, who approach the problem through the lens of Lagrangian Floer homology. Specifically, we outline a proof of their result: every smooth Jordan curve inscribes every rectangle up to similarity.

Tue, 26 Nov 2024
16:00
L6

Level repulsion and the Floquet quantum Ising model beyond integrability

Felix von Oppen
(Freie Universität Berlin)
Abstract

Motivated by a recent experiment on a superconducting quantum
information processor, I will discuss the Floquet quantum Ising model in
the presence of integrability- and symmetry-breaking random fields. The
talk will focus on the relation between boundary spin correlations,
spectral pairings, and effects of the random fields. If time permits, I
will also touch upon self-similarity in the dynamic phase diagram of
Fibonacci-driven quantum Ising models.
 

Tue, 26 Nov 2024
16:00
C3

Quantum expanders from quantum groups.

Mike Brannan
(University of Waterloo)
Abstract

I will give a light introduction to the concept of a quantum expander, which is an analogue of an expander graph that arises in quantum information theory.  Most examples of quantum expanders that appear in the quantum information literature are obtained by random matrix techniques.  I will explain another, more algebraic approach to constructing quantum expanders, which is based on using actions and representations of discrete quantum groups with Kazhdan's property (T).  This is joint work with Eric Culf (U Waterloo) and Matthijs Vernooij (TU Delft).   

Tue, 26 Nov 2024

15:30 - 16:30
Online

Optimizing the Campos-Griffiths-Morris-Sahasrabudhe upper bound on Ramsey numbers

Sergey Norin
(McGill University)
Further Information

Part of the Oxford Discrete Maths and Probability Seminar, held via Zoom. Please see the seminar website for details.

Abstract

In a recent breakthrough Campos, Griffiths, Morris and Sahasrabudhe obtained the first exponential improvement of the upper bound on the classical Ramsey numbers since 1935. I will outline a reinterpretation of their proof, replacing the underlying book algorithm with a simple inductive statement. In particular, I will present a complete proof of an improved upper bound on the off-diagonal Ramsey numbers and describe the main steps involved in improving their upper bound for the diagonal Ramsey numbers to $R(k,k)\le(3.8)^k$ for sufficiently large $k$.

Based on joint work with Parth Gupta, Ndiame Ndiaye, and Louis Wei.

Tue, 26 Nov 2024
15:00
L6

Embedding products of trees into higher rank

Oussama Bensaid
Abstract

I will present a joint work with Thang Nguyen where we show that there exists a quasi-isometric embedding of the product of n copies of the hyperbolic plane into any symmetric space of non-compact type of rank n, and there exists a bi-Lipschitz embedding of the product of n copies of the 3-regular tree into any thick Euclidean building of rank n. This extends a previous result of Fisher--Whyte. The proof is purely geometrical, and the result also applies to the non Bruhat--Tits buildings. I will start by describing the objects and the embeddings, and then give a detailed sketch of the proof in rank 2.

Tue, 26 Nov 2024
14:00
C3

Rohit Sahasrabuddhe: Concise network models from path data

Rohit Sahasrabuddhe
(Mathematical Institute (University of Oxford))
Abstract

Networks provide a powerful language to model and analyse interconnected systems. Their building blocks are  edges, which can  then be combined to form walks and paths, and thus define indirect relations between distant nodes and model flows across the system. In a traditional setting, network models are first-order, in the sense that flow across nodes is made of independent sequences of transitions. However, real-world systems often exhibit higher-order dependencies, requiring more sophisticated models. Here, we propose a variable-order network model that captures memory effects by interpolating between first- and second-order representations. Our method identifies latent modes that explain second-order behaviors, avoiding overfitting through a Bayesian prior. We introduce an interpretable measure to balance model size and description quality, allowing for efficient, scalable processing of large sequence data. We demonstrate that our model captures key memory effects with minimal state nodes, providing new insights beyond traditional first-order models and avoiding the computational costs of existing higher-order models.

Tue, 26 Nov 2024

14:00 - 15:00
Online

Boundedness of discounted tree sums

Élie Aïdékon
(Fudan University)
Further Information

Part of the Oxford Discrete Maths and Probability Seminar, held via Zoom. Please see the seminar website for details.

Abstract

Let $(V(u))$ be a branching random walk and $(\eta(u))$ be i.i.d marks on the vertices. To each path $\xi$ of the tree, we associate the discounted sum $D(\xi)$ which is the sum of the $\exp(V(u))\eta_u$ along the path. We study conditions under which $\sup_\xi D(\xi)$ is finite, answering an open question of Aldous and Bandyopadhyay. We will see that this problem is related to the study of the local time process of the branching random walk along a path. It is a joint work with Yueyun Hu and Zhan Shi.

Tue, 26 Nov 2024
14:00
L6

Probabilistic laws on groups

Guy Blachar
(Weizmann Institute)
Abstract

Suppose a finite group satisfies the following property: If you take two random elements, then with probability bigger than 5/8 they commute. Then this group is commutative. 

Starting from this well-known result, it is natural to ask: Do similar results hold for other laws (p-groups, nilpotent groups...)? Are there analogous results for infinite groups? Are there phenomena specific to the infinite setup? 

We will survey known and new results in this area. New results are joint with Gideon Amir, Maria Gerasimova and Gady Kozma.

Tue, 26 Nov 2024
13:00
L2

Late time saturation of the Einstein-Rosen bridge dual to the Double Scaled SYK model

Vijay Balasubramanian
(UPenn and Oxford)
Abstract

In this talk I will explain how the size of the Einstein-Rosen (ER) bridge dual to the Double Scaled SYK (DSSYK) model saturates at late times because of finiteness of the underlying quantum Hilbert space.  I will extend recent work implying that the ER bridge size equals the spread complexity of the dual DSSYK theory with an appropriate initial state.  This work shows that the auxiliary "chord basis'' used to solve the DSSYK theory is the physical Krylov basis of the spreading quantum state.  The ER bridge saturation follows from the vanishing of the Lanczos spectrum, derived by methods from Random Matrix Theory (RMT).

Mon, 25 Nov 2024
16:30
L4

Infinite Dyson Brownian Motion as a Gradient Flow

Kohei Suzuki (Durham)
Abstract

The Dyson Brownian motion (DMB) is a system of interacting Brownian motions with logarithmic interaction potential, which was introduced by Freeman Dyson '62 in relation to the random matrix theory. In this talk, we discuss the case where the number of particles is infinite and show that the DBM induces a diffusion structure on the configuration space having the Bakry-Émery lower Ricci curvature bound. As an application, we show that the DBM can be realised as the unique Benamou-Brenier-type gradient flow of the Boltzmann-Shannon entropy associated with the sine_beta point process. 

Mon, 25 Nov 2024
16:00
C3

Gap distributions and the Metric Poissonian Property 

Sophie Maclean
(King's College London)
Abstract
When studying dilated arithmetic sequences, it is natural to wonder about their distribution. Whilst it is relatively achievable to ascertain whether the resulting sequence is equidistributed, is it much more difficult to say much about gap size between consecutive elements of the new set? In this talk I will explore the gap distributions in dilated arithmetic sequences modulo 1, including what it means for a sequence to have the metric poissonian property. I will also give an overview of the current progress and what I am aiming to discover in my own work.
 
 
Mon, 25 Nov 2024
15:30
L5

Frobenius categories and Homotopy Quantum Field Theories

Paul Großkopf
(Oxford University)
Abstract

Topological Quantum Field Theories (TQFTs) have been studied as mathematical toy models for quantum field theories in physics and are described by a functor out of some bordism category. In dimension 2, TQFTs are fully classified by Frobenius algebras. Homotopy Quantum Field Theories (HQFTs), introduced by Turaev, consider additional homotopy data to some target space X on the bordism categories. For homotopy 1-types Turaev also gives a classification via crossed G-Frobenius algebras, where G denotes the fundamental group of X.
In this talk we will introduce a multi-object generalization of Frobenius algebras called Frobenius categories and give a version of this classification theorem involving the fundamental groupoid. Further, we will give a classification theorem for HQFTs with target homotopy 2-types by considering crossed modules (joint work with Alexis Virelizier).
 

Mon, 25 Nov 2024
15:30
L3

Stochastic quantization of fractional $\Phi^4_3$ model of Euclidean quantum field theory

Dr Paweł Duch
(Ecole Polytechnique Federale de Lausanne)
Abstract

The construction of the measure of the $\Phi^4_3$ model in the 1970s has been one of the major achievements of constructive quantum field theory. In the 1980s Parisi and Wu suggested an alternative way of constructing quantum field theory measures by viewing them as invariant measures of certain stochastic PDEs. However, the highly singular nature of these equations prevented their application in rigorous constructions until the breakthroughs in the area of singular stochastic PDEs in the past decade. After explaining the basic idea behind stochastic quantization proposed by Parisi and Wu I will show how to apply this technique to construct the measure of a certain quantum field theory model generalizing the $\Phi^4_3$ model called the fractional $\Phi^4$ model. The measure of this model is obtained as a perturbation of the Gaussian measure with covariance given by the inverse of a fractional Laplacian. Since the Gaussian measure is supported in the space of Schwartz distributions and the quartic interaction potential of the model involves pointwise products, to construct the measure it is necessary to solve the so-called renormalization problem. Based on joint work with M. Gubinelli and P. Rinaldi.

Mon, 25 Nov 2024
14:15
L4

CANCELLED

Simon Felten
(Oxford)
Abstract

A well-known problem in algebraic geometry is to construct smooth projective Calabi--Yau varieties $Y$. In the smoothing approach, we construct first a degenerate (reducible) Calabi--Yau scheme $V$ by gluing pieces. Then we aim to find a family $f\colon X \to C$ with special fiber $X_0 = f^{-1}(0) \cong V$ and smooth general fiber $X_t = f^{-1}(t)$. In this talk, we see how infinitesimal logarithmic deformation theory solves the second step of this approach: the construction of a family out of a degenerate fiber $V$. This is achieved via the logarithmic Bogomolov--Tian--Todorov theorem as well as its variant for pairs of a log Calabi--Yau space $f_0\colon X_0 \to S_0$ and a line bundle $\mathcal{L}_0$ on $X_0$.
 

Mon, 25 Nov 2024

14:00 - 15:00
Lecture Room 3

Ease-controlled Incremental Gradient- type Algorithm for nonconvex finite sum optimization

Laura Palagi
(Sapienza University of Rome)
Abstract

We consider minimizing the sum of a large number of smooth and possibly non-convex functions, which is the typical problem encountered in the training of deep neural networks on large-size datasets. 

Improving the Controlled Minibatch Algorithm (CMA) scheme proposed by Liuzzi et al. (2022), we propose CMALight, an ease-controlled incremental gradient (IG)-like method. The control of the IG iteration is performed by means of a costless watchdog rule and a derivative-free line search that activates only sporadically to guarantee convergence. The schemes also allow controlling the updating of the learning rate used in the main IG iteration, avoiding the use of preset rules, thus overcoming another tricky aspect in implementing online methods.

Convergence to a stationary point holds under the lonely assumption of Lipschitz continuity of the gradients of the component functions without knowing the Lipschitz constant or imposing any growth assumptions on the norm of the gradients.

We present two sets of computational tests. First, we compare CMALight against state-of-the-art mini-batch algorithms for training standard deep networks on large-size datasets, and deep convolutional neural networks and residual networks on standard image classification tasks on CIFAR10 and CIFAR100. 

Results shows that CMALight easily scales up to problem with order of millions  variables and has an advantage over its state-of-the-art competitors.

Finally, we present computational results on generative tasks, testing CMALight scaling capabilities on image generation with diffusion models (U-Net architecture). CMA Light achieves better test performances and is more efficient than standard SGD with weight decay, thus reducing the computational burden (and the carbon footprint of the training process).

Laura Palagi, @email

Department of Computer, Control and Management Engineering,

Sapienza University of Rome, Italy

 

Joint work with 

Corrado Coppola, @email

Giampaolo Liuzzi, @email

Lorenzo Ciarpaglini, @email

 

 

Mon, 25 Nov 2024
13:30
C4

The Evaporation of Charged Black Holes

Clément Virally
Abstract

Since Hawking first discovered that black holes radiate, the evaporation of black holes has been a subject of great interest. In this talk, based on [2411.03447], we review some recent results about the evaporation of charged (Reissner-Nordström) black holes. We consider in particular the difference between neutral and charged particle emission, and explain how this drives the black hole near extremality, as well as how evaporation is then changed in that limit.

Fri, 22 Nov 2024
16:00
L1

Fridays@4 – Trading Options: Predicting the Future in More Ways Than One

Chris Horrobin
(Optiver)
Abstract

In the fast-paced world of trading, where exabytes of data and advanced mathematical models offer powerful insights, how do you harness these to anticipate market shifts and evolving prices? Numbers alone only tell part of the story. Beneath the surface lies the unpredictable force of human behaviour – the delicate balance of buyers and sellers shaping the market’s course. 

In this talk, we’ll uncover how these forces intertwine, revealing insights that not only harness data but challenge conventional thinking about the future of trading.

Speaker: Chris Horrobin (Head of European and US people development for Optiver)

 

Trading options: predicting the future in more ways than one. Fridays @4. AI generated image

 

Speaker bio

Chris Horrobin is Head of European and US people development for Optiver. Chris started his career trading US and German bond options, adding currency and European index options into the mix before moving to focus primarily on index options. Chris spent his first three years in Amsterdam before transferring to Sydney. 

During these years, Chris traded some of the biggest events of his career including Brexit and Trump (first time around) and before moving back to Europe led the positional team in his last year. Chris then moved out of trading and into our training team running our trading education space for four years, owning both the design and execution of our renowned internship and grad programs. 

The Education Team at Optiver is central to the Optiver culture and focus on growth both of employees and the company. Chris has now extended his remit to cover the professional development of hires throughout the business.

Fri, 22 Nov 2024

14:00 - 15:00
L1

Mathematics: past, present, future - "The theory of knots"

Prof. Marc Lackenby
Further Information
Knots are just simple closed curves in 3-dimensional space. They are some of the most basic objects in topology, but are surprisingly tricky to understand. Knot theory started in 19th century with knot tabulation, and has expanded into a huge subject with connections to many other areas of modern mathematics. I will give some of the history of knot theory and will describe some of its basic results. I will also present some fascinating unsolved problems in the field.
Fri, 22 Nov 2024

12:00 - 13:00
Quillen Room

Non-nilpotent graphs of groups

Radosław Żak
(University of Oxford)
Abstract

A non-nilpotent graph Γ(G) of a finite group G has elements of G as vertices, with x and y joined by an edge iff a subgroup generated by these two elements is non-nilpotent. During the talk we will prove several (often unrelated) properties of this construction; for instance, any simple graph can be found as an induced subgraph of Γ(G) for some (solvable) group G. The talk is based on my article "A few remarks on the theory of non-nilpotent graphs" (May 2023).

Fri, 22 Nov 2024
12:00
L2

C for Carroll

Saikat Mondal
(Kanpur)
Abstract

Physics beyond relativistic invariance and without Lorentz (or Poincaré) symmetry and the geometry underlying these non-Lorentzian structures have become very fashionable of late. This is primarily due to the discovery of uses of non-Lorentzian structures in various branches of physics, including condensed matter physics, classical and quantum gravity, fluid dynamics, cosmology, etc. In this talk, I will be talking about one such theory - Carrollian theory, where the Carroll group replaces the Poincare group as the symmetry group of interest. Interestingly, any null hypersurface is a Carroll manifold and the Killing vectors on the null manifold generate Carroll algebra. Historically, Carroll group was first obtained from the Poincaré group via a contraction by taking the speed of light going to zero limit as a “degenerate cousin of the Poincaré group”.  I will shed some light on Carrollian fermions, i.e. fermions defined on generic null surfaces. Due to the degenerate nature of the Carroll manifold, there exist two distinct Carroll Clifford algebras and, correspondingly, two different Carroll fermionic theories. I will discuss them in detail. Then, I will show some examples; when the dispersion relation becomes trivial, i.e. energy bands flatten out, there can be a possibility of the emergence of Carroll symmetry.