Thu, 25 Jan 2024

12:00 - 13:00
L3

Collective motion and environmental path entropy

Matthew Turner
(University of Warwick)
Further Information

Matthew Turner is a Professor in the Physics department, attached to the Complexity center, at Warwick University. He works on Biological and Soft Matter Physics, amongst other things.

Abstract

 

We study “bottom-up” models for the collective motion of large groups of animals. Similar models can be encoded into (micro)robotic matter, capable of sensing light and processing information. Agents are endowed only with visual sensing and information processing. We study a model in which moving agents reorientate to maximise the path-entropy of their visual environment over paths into the future. There are general arguments that principles like this that are based on retaining freedom in the future may confer fitness in an uncertain world. Alternative “top-down” models are more common in the literature. These typically encode coalignment and/or cohesion directly and are often motivated by models drawn from physics, e.g. describing spin systems. However, such models can usually give little insight into how co-alignment and cohesion emerge because these properties are encoded in the model at the outset, in a top-down manner. We discuss how our model leads to dynamics with striking similarities with animal systems, including the emergence of coalignment, cohesion, a characteristic density scaling anddifferent behavioural phenotypes. The dynamics also supports a very unusual order-disorder transition in which the order (coalignment) initially increases upon the addition of sensory or behavioural noise, before decreasing as the noise becomes larger.

 

 

Thu, 25 Jan 2024

11:00 - 12:00
C3

Pre-seminar meeting on motivic integration

Margaret Bilu
(University of Oxford)
Abstract

This is a pre-seminar meeting for Margaret Bilu's talk "A motivic circle method", which takes place later in the day at 5PM in L3.

Wed, 24 Jan 2024
15:30

.

(Oxford University)
Tue, 23 Jan 2024

16:00 - 17:00
C2

Asymptotic freeness in tracial ultraproducts

Cyril Houdayer
(ENS Paris)
Abstract

I will present novel freeness results in ultraproducts of tracial von Neumann algebras. As a particular case, I will show that if a and b are the generators of the free group F_2, then the relative commutants of a and b in the ultraproduct of the free group factor are free with respect to the ultraproduct trace. The proof is based on a surprising application of Lp-boundedness results of Fourier multipliers in free group factors for p > 2. I will describe applications of these results to absorption and model theory of II_1 factors. This is joint work with Adrian Ioana.

Tue, 23 Jan 2024

16:00 - 17:00
L6

Combinatorial moment sequences

Natasha Blitvic
(Queen Mary University of London)
Abstract

We will look at a number of interesting examples — some proven, others merely conjectured — of Hamburger moment sequences in combinatorics. We will consider ways in which this positivity may be expected: for instance, in different types of combinatorial statistics on perfect matchings that encode moments of noncommutative analogues of the classical Central Limit Theorem. We will also consider situations in which this positivity may be surprising, and where proving it would open up new approaches to a class of very hard open problems in combinatorics.

Tue, 23 Jan 2024

15:30 - 16:30
Online

Paths in random temporal graphs

Nina Kamčev
(University of Zagreb)
Further Information

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

Abstract

Random temporal graphs are a version of the classical Erdős-Rényi random graph $G(n,p)$ where additionally, each edge has a distinct random time stamp, and connectivity is constrained to sequences of edges with increasing time stamps. We are interested in the asymptotics for the distances in such graphs, mostly in the regime of interest where the average degree $np$ is of order $\log n$ ('near' the phase transition).

More specifically, we will discuss the asymptotic lengths of increasing paths: the lengths of the shortest and longest paths between typical vertices, as well as the maxima between any two vertices; this also covers the (temporal) diameter. In the regime $np \gg \log n$, longest increasing paths were studied by Angel, Ferber, Sudakov and Tassion.

The talk contains joint work with Nicolas Broutin and Gábor Lugosi.

Tue, 23 Jan 2024
15:00
L6

Cocycle and orbit equivalence superrigidity for measure preserving actions

Daniel Drimbe
Abstract

The classification of measure preserving actions up to orbit equivalence has attracted a lot of interest in the last 25 years. The goal of this talk is to survey the major discoveries in the field, including Popa's cocycle and orbit equivalence superrigidity theorem and discuss some recent superrigidity results for dense subgroups of Lie groups acting by translation.

Tue, 23 Jan 2024

14:30 - 15:00
L6

Manifold-Free Riemannian Optimization

Boris Shustin
(Mathematical Institute (University of Oxford))
Abstract

Optimization problems constrained to a smooth manifold can be solved via the framework of Riemannian optimization. To that end, a geometrical description of the constraining manifold, e.g., tangent spaces, retractions, and cost function gradients, is required. In this talk, we present a novel approach that allows performing approximate Riemannian optimization based on a manifold learning technique, in cases where only a noiseless sample set of the cost function and the manifold’s intrinsic dimension are available.

Tue, 23 Jan 2024

14:00 - 15:00
Online

Sharpness of the phase transition for interlacements percolation

Augusto Teixeira
(Instituto Nacional de Matemática Pura e Aplicada (IMPA))
Further Information

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

Abstract

In this talk we will review the problem of sharpness in percolation, tracing its origins back to the seminal works of Menshikov, Grimmett-Marstrand and Aizenman-Barsky, which successfully settled the question in the context of Bernoulli independent percolation. Then we will present some recent advancements on the field, which have opened up the possibility of investigating dependent percolation models. Special emphasis will be given to the Interpolation technique, which has proved itself very effective. In particular, it has been used to establish the sharpness for Interlacements Percolation, a model introduced by Sznitman with notoriously intricate dependencies.

This talk is based on a joint work with Duminil-Copin, Goswami, Rodriguez and Severo

Tue, 23 Jan 2024

14:00 - 15:00
L5

On a quantitative version of Harish-Chandra's regularity theorem and singularities of representations

Yotam Hendel
(KU Leuven)
Abstract

Let G be a reductive group defined over a local field of characteristic 0 (real or p-adic). By Harish-Chandra’s regularity theorem, the character Θ_π of an irreducible representation π of G is given by a locally integrable function f_π on G. It turns out that f_π has even better integrability properties, namely, it is locally L^{1+r}-integrable for some r>0. This gives rise to a new singularity invariant of representations \e_π by considering the largest such r.

We explore \e_π, show it is bounded below only in terms of the group G, and calculate it in the case of a p-adic GL(n). To do so, we relate \e_π to the integrability of Fourier transforms of nilpotent orbital integrals appearing in the local character expansion of Θ_π. As a main technical tool, we use explicit resolutions of singularities of certain hyperplane arrangements. We obtain bounds on the multiplicities of K-types in irreducible representations of G for a p-adic G and a compact open subgroup K.

Based on a joint work with Itay Glazer and Julia Gordon.

Tue, 23 Jan 2024

14:00 - 14:30
L6

Scalable Gaussian Process Regression with Quadrature-based Features

Paz Fink Shustin
(Oxford)
Abstract

Gaussian processes provide a powerful probabilistic kernel learning framework, which allows high-quality nonparametric learning via methods such as Gaussian process regression. Nevertheless, its learning phase requires unrealistic massive computations for large datasets. In this talk, we present a quadrature-based approach for scaling up Gaussian process regression via a low-rank approximation of the kernel matrix. The low-rank structure is utilized to achieve effective hyperparameter learning, training, and prediction. Our Gauss-Legendre features method is inspired by the well-known random Fourier features approach, which also builds low-rank approximations via numerical integration. However, our method is capable of generating high-quality kernel approximation using a number of features that is poly-logarithmic in the number of training points, while similar guarantees will require an amount that is at the very least linear in the number of training points when using random Fourier features. The utility of our method for learning with low-dimensional datasets is demonstrated using numerical experiments.

Tue, 23 Jan 2024
13:00
L3

The Bethe-Gauge Correspondence for Superspin Chains

Faroogh Moosavian
(Oxford)
Abstract

The Bethe-Gauge Correspondence (BGC) of Nekrasov and Shatashvili, linking quantum integrable spin chains to two-dimensional supersymmetric gauge theories with N=2 supersymmetry, stands out as a significant instance of the deep connection between supersymmetric gauge theories and integrable models. In this talk, I will delve into this correspondence and its origins for superspin chains. To achieve this, I will first elucidate the Bethe Side and its corresponding Gauge Side of the BGC. Subsequently, it becomes evident that the BGC can be naturally realized within String Theory. I will initially outline the brane configuration for the realization of the Gauge Side. Through the use of string dualities, this brane configuration will be mapped to another, embodying the Bethe Side of the correspondence. The 4D Chern-Simons theory plays a crucial role in this latter duality frame, elucidating the integrability of the Bethe Side. Lastly, I will elaborate on computing the main object of interest for integrable superspin chains—the R-matrix—from the BGC. While this provides a rather comprehensive picture of the correspondence, some important questions remain for further clarification. I will summarize some of the most interesting ones at the end of the talk.


 

Tue, 23 Jan 2024

11:00 - 12:00

[Rescheduled] A new axiom for Q_p^ab and non-standard methods for perfectoid fields

Leo Gitin
(University of Oxford)
Abstract

The class of henselian valued fields with non-discrete value group is not well-understood. In 2018, Koenigsmann conjectured that a list of seven natural axioms describes a complete axiomatisation of Q_p^ab, the maximal extension of the p-adic numbers Q_p with abelian Galois group, which is an example of such a valued field. Informed by the recent work of Jahnke-Kartas on the model theory of perfectoid fields, we formulate an eighth axiom (the discriminant property) that is not a consequence of the other seven. Revisiting work by Koenigsmann (the Galois characterisation of Q_p) and Jahnke-Kartas, we give a uniform treatment of their underlying method. In particular, we highlight how this method can yield short, non-standard model-theoretic proofs of known results (e.g. finite extensions of perfectoid fields are perfectoid).

Tue, 23 Jan 2024
11:00
L5

Wilson-Ito diffusions

Massimiliano Gubinelli
(Mathematical Institute)
Abstract

In a recent preprint, together with Bailleul and Chevyrev we introduced a class of random fields which try to model the basic properties of quantum fields. I will try to explain the basic ideas and some of the many open problems.

To read the preprint, please click here.

Mon, 22 Jan 2024

16:30 - 17:30
L5

Cross-diffusion systems for segregating populations with incomplete diffusion

Ansgar Jungel
(TU Wien)
Abstract

Busenberg and Travis suggested in 1983 a population system that exhibits complete segregation of the species. This system can be rigorously derived from interacting particle systems in a mean-field-type limit. It consists of parabolic cross-diffusion equations with an indefinite diffusion matrix. It is known that this system can be formulated in terms of so-called entropy variables such that the transformed equations possess a positive semidefinite diffusion matrix. We consider in this talk the case of incomplete diffusion, which means that the diffusion matrix has zero eigenvalues, and the problem is not parabolic in the sense of Petrovskii. 

We show that the cross-diffusion equations can be written as a normal form of symmetric hyperbolic-parabolic type beyond the Kawashima-Shizuta theory. Using results for symmetric hyperbolic systems, we prove the existence of a unique local classical solution. As solutions may become discontinuous in finite time, only global solutions with very low regularity can be expected. We prove the existence of global dissipative measure-valued solutions satisfying a weak-strong uniqueness property. The proof is based on entropy methods and a finite-volume approximation with a mesh-dependent artificial diffusion. 

Mon, 22 Jan 2024
16:00
L2

Computing Tangent Spaces to Eigenvarieties

James Rawson
(University of Warwick)
Abstract

Many congruences between modular forms (or at least their q-expansions) can be explained by the theory of $p$-adic families of modular forms. In this talk, I will discuss properties of eigenvarieties, a geometric interpretation of the idea of $p$-adic families. In particular, focusing initially on the well-understood case of (elliptic) modular forms, before delving into the considerably murkier world of Bianchi modular forms. In this second case, this work gives numerical verification of a couple of conjectures, including BSD by work of Loeffler and Zerbes.

Mon, 22 Jan 2024
15:30

Surface automorphisms and elementary number theory

Greg McShane
(Universite Grenoble-Alpes)
Abstract
The modular surface $\mathbb{H}/\Gamma,\, \Gamma= \mathrm{SL}(2,\mathbb{Z})$ has many covers - for example the three punctured torus $\mathbb{H}/\Gamma(2)$ and the once punctured torus $\mathbb{H}/\Gamma'$. We will discuss how classical Diophantine approximation can be interpreted in terms of the behaviour of geodesics on the once punctured torus and a geometric reformulation of the Frobenius uniqueness conjecture.
We will then give an account of two theorems of Fermat in terms of   the automorphisms of $\mathbb{H}/\Gamma(2)$:
- if $p$ is a prime such that $4|(p-1)$ then  can be written as a   sum of squares $p = c^2 + d^2$
- if $p$ is a prime such that $3|(p-1)$ then  can be written as  $  p = c^2 +cd +  d^2$
Finally we will discuss possible extensions to surfaces of the for  m $\mathbb{H}/\Gamma_0(N)$.
 
Mon, 22 Jan 2024
15:30
Lecture room 5

Nonparametric generative modeling for time series via Schrödinger bridge

Professor Huyên Pham
(Université Paris Cité )
Abstract

We propose a novel generative model for time series based on Schrödinger bridge (SB) approach. This consists in the entropic interpolation via optimal transport between a reference probability measure on path space and a target measure consistent with the joint data distribution of the time series. The solution is characterized by a stochastic differential equation on finite horizon with a path-dependent drift function, hence respecting  the temporal dynamics of the time series distribution. We  estimate the drift function from data samples by nonparametric, e.g. kernel regression methods,  and the simulation of the SB diffusion  yields new synthetic data samples of the time series. The performance of our generative model is evaluated through a series of numerical experiments.  First, we test with autoregressive models, a GARCH Model, and the example of fractional Brownian motion,  and measure the accuracy of our algorithm with marginal, temporal dependencies metrics, and predictive scores. Next, we use our SB generated synthetic samples for the application to deep hedging on real-data sets. 

Mon, 22 Jan 2024
14:15
L4

A special class of $k$-harmonic maps inducing calibrated fibrations

Spiro Karigiannis
(University of Waterloo)
Abstract

Let $(M, g)$ be a Riemannian manifold equipped with a calibration $k$-form $\alpha$. In earlier work with Cheng and Madnick (AJM 2021), we studied the analytic properties of a special class of $k$-harmonic maps into $M$ satisfying a first order nonlinear PDE, whose images (away from a critical set) are $\alpha$-calibrated submanifolds of $M$. We call these maps Smith immersions, as they were originally introduced in an unpublished preprint of Aaron Smith. They have nice properties related to conformal geometry, and are higher-dimensional analogues of the $J$-holomorphic map equation. In new joint work (arXiv:2311.14074) with my PhD student Anton Iliashenko, we have obtained analogous results for maps out of $M$. Slightly more precisely, we define a special class of $k$-harmonic maps out of $M$, satisfying a first order nonlinear PDE, whose fibres (away from a critical set) are $\alpha$-calibrated submanifolds of $M$. We call these maps Smith submersions. I will give an introduction to both of these sets of equations, and discuss many future questions.

Mon, 22 Jan 2024

14:00 - 15:00
Lecture Room 3

Kernel Limit of Recurrent Neural Networks Trained on Ergodic Data Sequences

Prof. Justin Sirignano
(Mathematical Institute University of Oxford)
Abstract

Mathematical methods are developed to characterize the asymptotics of recurrent neural networks (RNN) as the number of hidden units, data samples in the sequence, hidden state updates, and training steps simultaneously grow to infinity. In the case of an RNN with a simplified weight matrix, we prove the convergence of the RNN to the solution of an infinite-dimensional ODE coupled with the fixed point of a random algebraic equation. 
The analysis requires addressing several challenges which are unique to RNNs. In typical mean-field applications (e.g., feedforward neural networks), discrete updates are of magnitude O(1/N ) and the number of updates is O(N). Therefore, the system can be represented as an Euler approximation of an appropriate ODE/PDE, which it will converge to as N → ∞. However, the RNN hidden layer updates are O(1). Therefore, RNNs cannot be represented as a discretization of an ODE/PDE and standard mean-field techniques cannot be applied. Instead, we develop a fixed point analysis for the evolution of the RNN memory state, with convergence estimates in terms of the number of update steps and the number of hidden units. The RNN hidden layer is studied as a function in a Sobolev space, whose evolution is governed by the data sequence (a Markov chain), the parameter updates, and its dependence on the RNN hidden layer at the previous time step. Due to the strong correlation between updates, a Poisson equation must be used to bound the fluctuations of the RNN around its limit equation. These mathematical methods allow us to prove a neural tangent kernel (NTK) limit for RNNs trained on data sequences as the number of data samples and size of the neural network grow to infinity.

Sat, 20 Jan 2024
16:30
L4

TBC

Noureddine Igbida
(Université de Limoges)
Fri, 19 Jan 2024
16:00
L1

Mathematical Societies and Organisations

Chris Breward, Sam Cohen, Rebecca Crossley, Dawid Kielak and Ulrike Tillmann
(Mathematical Institute)
Abstract
Mathematical societies and organisations run exciting academic activities and provide important funding opportunities. This session will include presentations on the London Mathematical Society (by LMS Rep Dawid Kielak), the Institute of Mathematics and its Applications (by Chris Breward), the Society for Industrial and Applied Mathematics (by Sam Cohen and Becky Crossley) and the Isaac Newton Institute (by its Director, Ulrike Tillmann).
 
The event will be followed by free pizza.
Fri, 19 Jan 2024

15:00 - 16:00
L4

The Function-Rips Multifiltration as an Estimator

Steve Oudot
(INRIA - Ecole Normale Supérieure)
Abstract

Say we want to view the function-Rips multifiltration as an estimator. Then, what is the target? And what kind of consistency, bias, or convergence rate, should we expect? In this talk I will present on-going joint work with Ethan André (Ecole Normale Supérieure) that aims at laying the algebro-topological ground to start answering these questions.

Fri, 19 Jan 2024

14:00 - 15:00
L3

Modelling cells in one-dimension: diverse migration modes, emergent oscillations on junctions and multicellular "trains"

Professor Nir Gov
(Department of Chemical and Biological Physics Weizmann Institute of Science)
Abstract

Motile cells inside living tissues often encounter junctions, where their path branches into several alternative directions of migration. We present a theoretical model of cellular polarization for cells migrating along one-dimensional lines, exhibiting diverse migration modes. When arriving at a symmetric Y-junction and extending protrusions along the different paths that emanate from the junction. The model predicts the spontaneous emergence of deterministic oscillations between competing protrusions, whereby the cellular polarization and growth alternates between the competing protrusions. These predicted oscillations are found experimentally for two different cell types, noncancerous endothelial and cancerous glioma cells, migrating on patterned network of thin adhesive lanes with junctions. Finally we present an analysis of the migration modes of multicellular "trains" along one-dimensional tracks.

Fri, 19 Jan 2024

12:00 - 13:00
Common Room

Junior Algebra Social

Abstract

The Junior Algebra and Representation Theory Seminar will kick-off the start of Hilary term with a social event in the common room. Come to catch up with your fellow students and maybe play a board game or two. Afterwards we'll have lunch together.

Fri, 19 Jan 2024
12:00
L3

Topological Recursion: Introduction, Overview and Applications

Alex Hock
(Oxford)
Abstract
I will give a talk about the topological recursion (TR) of Eynard and Orantin, which generates from some initial data (the so-called the spectral curve) a family of symmetric multi-differentials on a Riemann surface. Symplectic transformations of the spectral curve play an important role and are conjectured to leave the free energies $F_g$ invariant. TR has nowadays a lot of applications ranging random matrix theory, integrable systems, intersection theory on the moduli space of complex curves $\mathcal{M}_{g,n}$, topological string theory over knot theory to free probability theory. I will highlight specific examples, such as the Airy curve (also sometimes called the Kontsevich-Witten curve) which enumerates $\psi$-class intersection numbers on $\mathcal{M}_{g,n}$, the Mirzakhani curve for computing Weil–Petersson volumes, the spectral curve of the hermitian 1-matrix model, and the topological vertex curve which derives the $B$-model correlators in topological string theory. Should time allow, I will also discuss the quantum spectral curve as a quantisation of the classical spectral curve annihilating a wave function constructed from the family of multi-differentials. 
 
 
Thu, 18 Jan 2024
16:00
L3

Multireference Alignment for Lead-Lag Detection in Multivariate Time Series and Equity Trading

Danni Shi
(Oxford Man Institute [OMI])
Further Information

Join us for refreshments from 330 outside L3.

Abstract

We introduce a methodology based on Multireference Alignment (MRA) for lead-lag detection in multivariate time series, and demonstrate its applicability in developing trading strategies. Specifically designed for low signal-to-noise ratio (SNR) scenarios, our approach estimates denoised latent signals from a set of time series. We also investigate the impact of clustering the time series on the recovery of latent signals. We demonstrate that our lead-lag detection module outperforms commonly employed cross-correlation-based methods. Furthermore, we devise a cross-sectional trading strategy that capitalizes on the lead-lag relationships uncovered by our approach and attains significant economic benefits. Promising backtesting results on daily equity returns illustrate the potential of our method in quantitative finance and suggest avenues for future research.

Thu, 18 Jan 2024
16:00
Lecture Room 4, Mathematical Institute

Traces of random matrices over F_q, and short character sums

Ofir Gorodetsky
(University of Oxford)
Abstract
Let g be a matrix chosen uniformly at random from the GL_n(F_q), where F_q is the field with q elements. We consider two questions:
1. For fixed k and growing n, how fast does Tr(g^k) converge to the uniform distribution on F_q?
2. How large can k be taken, as a function of n, while still ensuring that Tr(g^k) converges to the uniform distribution on F_q?
We will answer these two questions (as well as various variants) optimally. The questions turn out to be strongly related to the study of particular character sums in function fields.
Based on joint works with Brad Rodgers (arXiv:1909.03666) and Valeriya Kovaleva (arXiv:2307.01344).
Thu, 18 Jan 2024

16:00 - 17:00
C2

Morita equivalence for operator systems

Evgenios Kakariadis
(Newcastle University)
Abstract

In ring theory, Morita equivalence is an invariant for many properties, generalising the isomorphism of commutative rings. A strong Morita equivalence for selfadjoint operator algebras was introduced by Rieffel in the 60s, and works as a correspondence between their representations. In the past 30 years, there has been an interest to develop a similar theory for nonselfadjoint operator algebras and operator spaces with much success. Taking motivation from recent work of Connes and van Suijlekom, we will present a Morita theory for operator systems. We will give equivalent characterizations of Morita equivalence via Morita contexts, bihomomoprhisms and stable isomorphisms, while we will highlight properties that are preserved in this context. Time permitted we will provide applications to rigid systems, function systems and non-commutative graphs. This is joint work with George Eleftherakis and Ivan Todorov.

Thu, 18 Jan 2024

14:00 - 15:00
Rutherford Appleton Laboratory, nr Didcot

A preconditioner with low-rank corrections based on the Bregman divergence

Andreas Bock
(Danish Technical University)
Abstract

We present a general framework for preconditioning Hermitian positive definite linear systems based on the Bregman log determinant divergence. This divergence provides a measure of discrepancy between a preconditioner and a target matrix, giving rise to

the study of preconditioners given as the sum of a Hermitian positive definite matrix plus a low-rank correction. We describe under which conditions the preconditioner minimises the $\ell^2$ condition number of the preconditioned matrix, and obtain the low-rank 

correction via a truncated singular value decomposition (TSVD). Numerical results from variational data assimilation (4D-VAR) support our theoretical results.

 

We also apply the framework to approximate factorisation preconditioners with a low-rank correction (e.g. incomplete Cholesky plus low-rank). In such cases, the approximate factorisation error is typically indefinite, and the low-rank correction described by the Bregman divergence is generally different from one obtained as a TSVD. We compare these two truncations in terms of convergence of the preconditioned conjugate gradient method (PCG), and show numerous examples where PCG converges to a small tolerance using the proposed preconditioner, whereas PCG using a TSVD-based preconditioner fails. We also consider matrices arising from interior point methods for linear programming that do not admit such an incomplete factorisation by default, and present a robust incomplete Cholesky preconditioner based on the proposed methodology.

The talk is based on papers with Martin S. Andersen (DTU).

 

Thu, 18 Jan 2024

12:00 - 13:00
L3

Coupling rheology and segregation in granular flows

Nico Gray
(University of Manchester)
Further Information

Professor Nico Gray is based in the Department of Mathematics at the University of Manchester. 

This is from his personal website:

My research interests lie in understanding and modelling the flow of granular materials, in small scale experiments, industrial processes and geophysical flows.

[Mixing in a rotating drum][Flow past a rearward facing pyramid]

Current research is aimed at understanding fundamental processes such as the flow past obstacles, shock waves, dead-zones, fluid-solid phase transitions, particle size segregation and pattern formation. A novel and important feature of all my work is the close interplay of theory, numerical computation and experiment to investigate these nonlinear systems. I currently have three active experiments which are housed in two laboratories at the Manchester Centre for Nonlinear Dynamics. You can click on the videos and pictures as well as the adjacent toolbar to find out more about specific problems that I am interested in.

Abstract

During the last fifteen years, there has been a paradigm shift in the continuum modelling of granular materials; most notably with the development of rheological models, such as the μ(I)-rheology (where μ is the friction and I is the inertial number), but also with significant advances in theories for particle segregation. This talk details theoretical and numerical frameworks (based on OpenFOAM®) which unify these disconnected endeavours. Coupling the segregation with the flow, and vice versa, is not only vital for a complete theory of granular materials, but is also beneficial for developing numerical methods to handle evolving free surfaces. This general approach is based on the partially regularized incompressible μ(I)-rheology, which is coupled to a theory for gravity/shear-driven segregation (Gray & Ancey, J. Fluid Mech., vol. 678, 2011, pp. 353–588). These advection–diffusion–segregation equations describe the evolving concentrations of the constituents, which then couple back to the variable viscosity in the incompressible Navier–Stokes equations. A novel feature of this approach is that any number of differently sized phases may be included, which may have disparate frictional properties. The model is used to simulate the complex particle-size segregation patterns that form in a partially filled triangular rotating drum. There are many other applications of the theory to industrial granular flows, which are the second most common material used after fluids. The same processes also occur in geophysical flows, such as snow avalanches, debris flows and dense pyroclastic flows. Depth-averaged models, that go beyond the μ(I)-rheology, will also be derived to capture spontaneous self-channelization and levee formation, as well as complex segregation-induced flow fingering effects, which enhance the run-out distance of these hazardous flows.

 

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Wed, 17 Jan 2024

16:00 - 17:00
L6

Spectra of surfaces and MCG actions on random covers

Adam Klukowski
(University of Oxford)
Abstract

The Ivanov conjecture is equivalent to the statement that every covering map of surfaces has the so-called Putman-Wieland property. I will discuss my recent work with Vlad Marković, where we prove it for asymptotically all coverings as the degree grows. I will give some overview of our main tool: spectral geometry, which is related to objects like the heat kernel of a hyperbolic surface, or Cheeger connectivity constant.

Wed, 17 Jan 2024
12:00
L6

A new understanding of the grazing limit

Prof Tong Yang
(Department of Applied Mathematics, The Hong Kong Polytechnic University)
Abstract

The grazing limit of the Boltzmann equation to Landau equation is well-known and has been justified by using cutoff near the grazing angle with some suitable scaling. In this talk, we will present a new approach by applying a natural scaling on the Boltzmann equation. The proof is based on an improved well-posedness theory for the Boltzmann equation without angular cutoff in the regime with an optimal range of parameters so that the grazing limit can be justified directly that includes the Coulomb potential. With this new understanding, the scaled Boltzmann operator in fact can be decomposed into two parts. The first one converges to the Landau operator when the parameter of deviation angle tends to its singular value and the second one vanishes in the limit. Hence, the scaling and limiting process exactly capture the grazing collisions. The talk is based on a recent joint work with Yu-Long Zhou.

Tue, 16 Jan 2024

16:00 - 17:00
L6

Branching selection particle systems and the selection principle.

Julien Berestycki
(Department of Statistics, University of Oxford)
Abstract
The $N$-branching Brownian motion with selection ($N$-BBM) is a particle system consisting of $N$ independent particles that diffuse as Brownian motions in $\mathbb{R}$, branch at rate one, and whose size is kept constant by removing the leftmost particle at each branching event. It is a very simple model for the evolution of a population under selection that has generated some fascinating research since its introduction by Brunet and Derrida in the early 2000s.
 
If one recentre the positions by the position of the left most particle, this system has a stationary distribution. I will show that, as $N\to\infty$ the stationary empirical measure of the $N$-particle system converges to the minimal travelling wave of an associated free boundary PDE. This resolves an open question going back at least to works of e.g. Maillard in 2012.
It follows a recent related result by Oliver Tough (with whom this is joint work) establishing a similar selection principle for the so-called Fleming-Viot particle system.
 
With very best wishes,
Julien
Tue, 16 Jan 2024
15:00
L6

Profinite invariants of fibered groups

Monika Kudlinska
Abstract

A central question in infinite group theory is to determine how much global information about a group is encoded in its set of finite quotients. In this talk, we will discuss this problem in the case of algebraically fibered groups, which naturally generalise fundamental groups of compact manifolds that fiber over the circle. The study of such groups exploits the relationships between the geometry of the classifying space, the dynamics of the monodromy map, and the algebra of the group, and as such draws from all of these areas.

Tue, 16 Jan 2024

14:00 - 15:00
L4

Heights of random trees

Louigi Addario-Berry
(McGill University)
Abstract

A rooted tree $T$ has degree sequence $(d_1,\ldots,d_n)$ if $T$ has vertex set $[n]$ and vertex $i$ has $d_i$ children for each $i$ in $[n]$. 

I will describe a line-breaking construction of random rooted trees with given degree sequences, as well as a way of coupling random trees with different degree sequences that also couples their heights to one another. 

The construction and the coupling have several consequences, and I'll try to explain some of these in the talk.

First, let $T$ be a branching process tree with criticalmean oneoffspring distribution, and let $T_n$ have the law of $T$ conditioned to have size $n$. Then the following both hold.
1) $\operatorname{height}(T_n)/\log(n)$ tends to infinity in probability. 
2) If the offspring distribution has infinite variance then $\operatorname{height}(T_n)/n^{1/2}$ tends to $0$ in probability. This result settles a conjecture of Svante Janson.

The next two statements relate to random rooted trees with given degree sequences. 
1) For any $\varepsilon > 0$ there is $C > 0$ such that the following holds. If $T$ is a random tree with degree sequence $(d_1,\ldots,d_n)$ and at least $\varepsilon n$ leaves, then $\mathbb{E}(\operatorname{height}(T)) < C \sqrt{n}$. 
2) Consider any random tree $T$ with a fixed degree sequence such that $T$ has no vertices with exactly one child. Then $\operatorname{height}(T)$ is stochastically less than $\operatorname{height}(B)$, where $B$ is a random binary tree of the same size as $T$ (or size one greater, if $T$ has even size). 

This is based on joint work with Serte Donderwinkel and Igor Kortchemski.

Tue, 16 Jan 2024
13:00
L2

Defect two-point functions in 6d (2,0) theories

Xinan Zhou
(Beijing)
Abstract

In this talk, I will discuss correlation functions in 6d (2, 0) theories of two 1/2-BPS operators inserted away from a 1/2-BPS surface defect. In the large central charge limit the leading connected contribution corresponds to sums of tree-level Witten diagram in AdS7×S4 in the presence of an AdS3 defect. I will show that these correlators can be uniquely determined by imposing only superconformal symmetry and consistency conditions, eschewing the details of the complicated effective Lagrangian. I will present the explicit result of all such two-point functions, which exhibits remarkable hidden simplicity.

Tue, 16 Jan 2024
11:00
Lecture room 5

Random surfaces and higher algebra (Part II)

Darrick Lee
Abstract

A representation on the space of paths is a map which is compatible with the concatenation operation of paths, such as the path signature and Cartan development (or equivalently, parallel transport), and has been used to define characteristic functions for the law of stochastic processes. In this talk, we consider representations of surfaces which are compatible with the two distinct algebraic operations on surfaces: horizontal and vertical concatenation. To build these representations, we use the notion of higher parallel transport, which was first introduced to develop higher gauge theories. We will not assume any background in geometry or category theory. This is a continuation of the previous talk based on a recent preprint (https://arxiv.org/abs/2311.08366) with Harald Oberhauser.

Mon, 15 Jan 2024

16:30 - 17:30
L5

Functions of bounded variation and nonlocal functionals

Panu Lathi
(Academy of Mathematics and Systems Science of the Chinese Academy of Sciences)
Abstract

In the past two decades, starting with the pioneering work of Bourgain, Brezis, and Mironescu, there has been widespread interest in characterizing Sobolev and BV (bounded variation) functions by means of non-local functionals. In my recent work I have studied two such functionals: a BMO-type (bounded mean oscillation) functional, and a functional related to the fractional Sobolev seminorms. I will discuss some of my results concerning the limits of these functionals, the concept of Gamma-convergence, and also open problems. 

Mon, 15 Jan 2024
16:00
L2

A friendly introduction to Shimura curves

Håvard Damm-Johnsen
(University of Oxford)
Abstract

Modular curves play a key role in the Langlands programme, being the simplest example of so-called Shimura varieties.  Their less famous cousins, Shimura curves, are also very interesting, and very concrete. 
In this talk I will give a gentle introduction to the arithmetic of Shimura curves, with lots of explicit examples. Time permitting, I will say something about recent work about intersection numbers of geodesics on Shimura curves.

Mon, 15 Jan 2024
15:30

Invariant splittings of HFK of satellite knots

Sungkyung Kang
(Oxford University)
Abstract

Involutive knot Floer homology, a refinement of knot Floer theory, is a powerful knot invariant which was used to solve several long-standing problems, including the one-is-not-enough result for 4-manifolds with boundary. In this talk, we show that if the involutive knot Floer homology of a knot K admits an invariant splitting, then the induced splitting if the knot Floer homology of P(K), for any pattern P, can be made invariant under its \iota_K involution. As an application, we construct an infinite family of examples of pairs of exotic contractible 4-manifolds which survive one stabilization, and observe that some of them are potential candidates for surviving two stabilizations.
 

Mon, 15 Jan 2024
15:30
Lecture room 5

The Critical 2d Stochastic Heat Flow and other critical SPDEs

Professor Nikolaos Zygouras
(Dept. Mathematics, University of Warwick)
Abstract
Thanks to the theories of Paracontrolled Distributions and Regularity structures we now have a complete theory of  singular SPDEs, which are “sub-critical” in the sense of renormalisation. Recently, there have been efforts to approach the situation of “critical” SPDEs and statistical mechanics models. A first such treatment has been through the study of the two-dimensional stochastic heat equation, which has revealed a certain phase transition and has led to the construction of the novel object called the Critical 2d Stochastic Heat Flow. In this talk we will present some aspects of this model and its construction. We will also present developments relating to other critical SPDEs.
Parts of this talk are based on joint works with Caravenna and Sun and others with Rosati and Gabriel.  
Mon, 15 Jan 2024
14:15
L4

Stability conditions for line bundles on nodal curves

Nicola Pagani
(University of Liverpool)
Abstract

Mathematicians have been interested in the problem of compactifying the Jacobian variety of curves since the mid XIX century. In this talk we will discuss how all 'reasonable' compactified Jacobians of nodal curves can be classified combinatorically. This suffices to obtain a combinatorial classification of all 'reasonable' compactified universal (over the moduli spaces of stable curves) Jacobians. This is a joint work with Orsola Tommasi.

Mon, 15 Jan 2024

14:00 - 15:00
Lecture Room 3

On sketches and corruptions: devising adaptive randomized iterative methods for large linear systems

Elizaveta Rebrova
(Princeton University, NJ)
Abstract

When the data is large, or comes in a streaming way, randomized iterative methods provide an efficient way to solve a variety of problems, including solving linear systems, finding least square solutions, solving feasibility problems, and others. Randomized Kaczmarz algorithm for solving over-determined linear systems is one of the popular choices due to its efficiency and its simple, geometrically intuitive iterative steps. 
In challenging cases, for example, when the condition number of the system is bad, or some of the equations contain large corruptions, the geometry can be also helpful to augment the solver in the right way. I will discuss our recent work with Michal Derezinski and Jackie Lok on Kaczmarz-based algorithms that use external knowledge about the linear system to (a) accelerate the convergence of iterative solvers, and (b) enable convergence in the highly corrupted regime.

 

Mon, 15 Jan 2024

13:00 - 14:00
N3.12

Mathematrix: Interview Discussion

Abstract

Join us for a discussion about preparing for PhD and PostDoc Interviews. We will be talking to Melanie Rupflin and Mura Yakerson.

Thu, 11 Jan 2024
11:00
C2

L-open and l-closed C*-algebras

Aaron Tikuisis
(University of Ottawa)
Abstract

This talk concerns some ideas around the question of when a *-homomorphism into a quotient C*-algebra lifts. Lifting of *-homomorphisms arises prominently in the notions of projectivity and semiprojectivity, which in turn are closely related to stability of relations. Blackadar recently defined the notions of l-open and l-closed C*-algebras, making use of the topological space of *-homomorphisms from a C*-algebra A to another C*-algebra B, with the point-norm topology. I will discuss these properties and present new characterizations of them, which lead to solutions of some problems posed by Blackadar. This is joint work with Dolapo Oyetunbi.

Wed, 10 Jan 2024
09:30
St Hilda’s College, University of Oxford

Workshop on Climate Change and Epidemics

Oxford-based organiser: Robin Thompson
Further Information

To sign up, please register your interest using this sign-up form by Thursday 30th November 2023 at the latest. Places will be confirmed by 5th December 2023. This workshop will take place at St Hilda's College, and is funded by the JUNIPER Consortium and Isaac Newton Institute.

Climate change is the key threat to this and future generations. With the Earth warming faster than ever before, we face inter-linked migration, infrastructure and public health challenges. In 2023, parts of Europe saw their hottest summer on record while other places have experienced unprecedented levels of rainfall and devastating floods. 

Many infectious diseases are climate-sensitive. For example, the locations and sizes of mosquito populations are linked to climate, which in turn affects the transmission of mosquito-borne diseases such as dengue and malaria. Therefore, changes in climate are altering the spatial and seasonal patterns of infections over time, putting millions of people at risk. In order to be more resilient to the health challenges posed by climate change, it is critical to understand its impacts on infectious diseases, both in the UK and globally.

The aim of this workshop is to bring together mathematical modellers, epidemiologists, climate scientists and public health specialists to identify key open challenges in our understanding of how climate change affects infectious diseases. The one-day workshop will consist of a series of talks and sessions covering the following themes:

  • Changes to infectious disease threats under a changing climate and regions most affected
  • Measures and initiatives to mitigate and build resilience in the UK and globally
  • Knowledge gaps that need to be filled to limit the impact of climate-sensitive infectious diseases
  • Challenges presented by climate-sensitive infectious diseases that provide opportunities to improve public health

The main aim of this event is to catalyse discussion between individuals in the research areas of climate science, infectious disease modelling and public health, fostering collaborations that address key challenges relating to climate-sensitive infectious diseases. Please note: this workshop is in-person only.

Organisers:

Robin Thompson (University of Oxford), Helena Stage (University of Bristol), Alexander Kaye (University of Warwick)

Fri, 08 Dec 2023
12:00
L3

A Positive Way to Scatter Strings and Particles

Hadleigh Frost
(Oxford)
Abstract

We present a new formulation of string and particle amplitudes that emerges from simple one-dimensional models. The key is a new way to parametrize the positive part of Teichmüller space. It also builds on the results of Mirzakhani for computing Weil-Petterson volumes. The formulation works at all orders in the perturbation series, including non-planar contributions. The relationship between strings and particles is made manifest as a "tropical limit". The results are well adapted to studying the scattering of large numbers of particles or amplitudes at high loop order. The talk will in part cover results from arXiv:2309.15913, 2311.09284.

Thu, 07 Dec 2023
18:00
The Auditorium, Citigroup Centre, London, E14 5LB

Frontiers in Quantitative Finance: Large Language Models for Quantitative Finance

Dr Ioana Boier
Abstract

This event is free but requires prior registration. To register, please click here.

 

Abstract
In the contemporary AI landscape, Large Language Models (LLMs) stand out as game-changers. They redefine not only how we interact with computers via natural language but also how we identify and extract insights from vast, complex datasets. This presentation delves into the nuances of training and customizing LLMs, with a focus on their applications to quantitative finance.


About the speaker
Ioana Boier is a senior principal solutions architect at Nvidia. Her background is in Quantitative Finance and Computer Science. Prior to joining Nvidia, she was the Head of Quantitative Portfolio Solutions at Alphadyne Asset Management, and led research teams at Citadel LLC, BNP Paribas, and IBM T.J. Watson Research. She has a Ph.D. in Computer Science from Purdue University and is the author of over 30 peer-reviewed publications, 15 patents, and the winner of several awards for applied research delivered into products.
View her LinkedIn page

 

Frontiers in Quantitative Finance is brought to you by the Oxford Mathematical and Computational Finance Group and sponsored by CitiGroup and Mosaic SmartData.