# Seminars

There are three series of seminars: **Imperial pure analysis and PDE**, **London analysis and probability**, and **Paris-London analysis seminar**. The first two alternate weekly, the Paris-London series meets four times per year.

**Our PhD students also jointly organise the Junior Analysis seminar; which consists of informal talks by students and visitors. **

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## Upcoming seminars

**2019-2020 Spring term programme**

** **

**Thursday Jan 16, **London analysis and probability seminar, (Imperial college, Huxley 140)

3pm: Andrea Mondino (Oxford University)

Title: Optimal transport and quantitative geometric inequalities

Abstract: The goal of the talk is to discuss a quantitative version of the Levy-Gromov isoperimetric inequality (joint with Cavalletti and Maggi) as well as other geometric/functional inequalities (joint with Cavalletti and Semola). Given a closed Riemannian manifold with strictly positive Ricci tensor, one estimates the measure of the symmetric difference of a set with a metric ball with the deficit in the Levy-Gromov inequality. The results are obtained via a quantitative analysis based on the localisation method via L1-optimal transport.

4:30pm: Marina Iliopoulou (Kent)

Title: Sharp estimates for Hörmander-type operators

Abstract: The restriction conjecture, lying at the heart of harmonic analysis, suggests that the Fourier transform of a function defined on a curved surface "behaves better" than if the domain surface was flat. Hörmander further suggested that oscillatory integrals more general than the Fourier transform should satisfy similar properties. However, Bourgain showed that this is false, as the mass of these more general oscillatory integrals may cluster too close to low-degree algebraic varieties, and thus, roughly speaking, can have too many peaks in little space. In this talk we describe the extent to which Hörmander's conjecture was correct, by providing sharp estimates for Hörmander-type oscillatory integrals that resemble the oscillatory integrals related to the restriction conjecture. This is joint work with L. Guth and J. Hickman.

**Friday 17 January, **Joint seminar with applied PDE's, Huxley 130,

1:30pm: Professor Mayte Pérez-Llanos, Universidad de Sevilla

**Wednesday 22 January 2019, 1.30-5.30 pm** King's College London, King's Building, room S-3.20 (level minus three)

*Special session of the London Analysis Seminar dedicated to the memory of Yuri Safarov*

1:30-2:30 PM Dmitri Yafaev (Rennes) *Asymptotic behavior of orthogonal polynomials without the Carleman condition *abstract

3:00-4:00 PM Lev Kapitanski (Miami) *Lagrange, Euler, Hooke, and Cauchy *abstract

4:30-5:30 PM Eugene Shargorodsky (KCL) *Title: Quantitative results on continuity of the spectral factorisation mapping *abstract

**Thursday Jan 23,** Imperial analysis and PDE's seminar, (Huxley 140)

3pm: Ivan Gentil (Université Claude Bernard Lyon 1)

4:30pm: Fabio Cipriani, Politecnico di Milano

Title: How to hear the shape of a drum

Abstract: In a iconic 1912 paper Hermann Weyl, motivated by problems posed by the physicist H.A. Lorentz about J.H. Jeans's radiation theory, showed that the dimension and the volume of a Euclidean domain may be traced from

the asymptotic distribution of the eigenvalues of its Laplace operator.

In a as much famous 1966 paper titled "Can one hear the shape of a drum" Marc Kac popularized this and related problems connecting geometry and spectrum. He noticed that the hope to characterize isometrically, Euclidean domains or compact Riemannian manifolds by the spectrum of the Laplace operator, is vain: John Milnor in 1964 had showed the existence of non isometric 16 dimensional tori sharing a common (discrete) spectrum. The aim of the talk is to show how to recognize that a map between Euclidean domains is conformal if it is homeomorphism which transform multipliers of the Sobolev-Dirichlet spaces of a domain into multipliers of the other and leave invariant the fundamental tone or first nonzero eigenvalue of the Dirichlet integral with respect to the energy measures of any multiplier. Related results hold true for quasiconformal and bounded distortion maps. In the opposite direction, we prove that the trace of the Dirichlet integral, with respect to the energy measure of a multiplier, is a Dirichlet space that only depends upon the orbit of the conformal group of the Euclidean space on the multiplier algebra. The methods involve potential theory of Dirichlet

forms (changing of speed measure, multipliers) and the Li-Yau conformal volume of Riemannian manifolds.

This is a collaboration with Jean-Luc Sauvageot C.N.R.S. France et Universit'e Paris 7.

**Thursday Jan 30, **London analysis and probability seminar, (Imperial college, Huxley 140)

3pm: Georgi Raykov (Santiago)

*"Threshold Singularities of the Spectral Shift Function for Geometric Perturbations of a Magnetic Hamiltonian" *abstract

4:30pm : Sergey Bobkov (Minnesota)

**Thursday Feb 6,** Imperial analysis and PDE's seminar,

3pm: Giacomo Di Gesu (TU Wien)

Title: An Eyring-Kramers formula for the spectral gap of the stochastic Allen-Cahn equation

Abstract: We consider the Allen-Cahn equation on the one-dimensional torus, perturbed by a small spacetime white noise. The deterministic equation is a nonlinear PDE, which can be seen as a gradient flow with respect to a double-well energy. If a small noise is added, the typical picture of a metastable dynamics emerges: the system quickly reaches a local equilibrium in one of the two wells; this state endures for an exponentially long time, until a sufficiently large stochastic fluctuation enables the system to overcome the energetic barrier separating the two wells. This behavior produces a slowdown in the relaxation to the equilibrium measure, reflected e.g. by an exponentially small spectral gap. In this talk I will present a technique which provides a formula for the precise asymptotic behavior of the spectral gap, showing that the prefactor is given by a suitable Fredholm determinant. The formula shows that the gap behaves like twice the inverse of the metastable transition time from one well to the other.

**Thursday Feb 13, **London analysis and probability seminar,

3pm: Alexander Olevskii (Tel-Aviv University)

*Wiener algebra & Fourier multipliers*

Abstract: I'll survey some fundamental properties of the concepts mentioned in the title and discuss , to which extent the properties can be improved by a homeomorphic change of variable. Some open problems will be mentioned.

4:30pm: Roland Bauerschmidt (Cambridge University)

Title: Log-Sobolev inequality for the continuum Sine-Gordon model

Abstract: We derive a multiscale generalisation of the Bakry--Emery criterion for a measure to satisfy a Log-Sobolev inequality. Our criterion relies on the control of an associated PDE well known in renormalisation theory: the Polchinski equation. It implies the usual Bakry--Emery criterion, but we show that it remains effective for measures which are far from log-concave. Indeed, as an application, we prove that the massive continuum Sine-Gordon model with $\beta < 6\pi$ satisfies asymptotically optimal Log-Sobolev inequalities for Glauber and Kawasaki dynamics.

(This is joint work with Thierry Bodineau.)

**Thursday Feb 20, **Imperial analysis and PDE's seminar,

3pm: Spencer Becker-Kahn (Cambridge)

*n-*varifold is a critical point of the

*n*-dimensional area functional; it is a measure-theoretic generalization of the classical notion of a minimal surface and can have singularities. The main obstacle to gaining a good understanding of the singular set is the difficulty in analysing ‘higher multiplicity’ singular behaviour: Suppose that in an open ball in Euclidean space, the stationary integral

*n*-varifold

*V*is close to a plane of some integer multiplicity

*Q*passing through the centre of the ball. If

*Q=1,*then via pioneering work of De Giorgi and Allard, we know that near to the centre of the ball, the varifold is the graph of a smooth function over the plane with small gradient and satisfying estimates on all derivatives. But in a higher multiplicity setting,

*i.e.*when

*Q=2*or greater, exactly what can be said in this level of generality is a more subtle issue. Over the last few years Neshan Wickramaserkera and I have been considering these subtleties when

*Q=2*(to begin with). While the simple example of the catenoid shows that

*V*is not always equal to a 2-valued graph over the plane, we isolate a necessary and sufficient topological structural condition (on the open part of the varifold with density strictly less than 2) which guarantees that a multiplicity 2 analogue of Allard’s regularity theorem holds. The condition is satisfied when

*V*is a Lipschitz 2-valued graph (of arbitrary Lipschitz constant) or if the codimension is 1 and

*V*corresponds to a boundary-less current with stable regular part.

**Thursday Feb 27, **London analysis and probability seminar

3pm:** **Sergei Treil (Brown)

4.30pm:** **Dave Hewett (UCL)

*Sobolev spaces on non-Lipschitz subsets of Rn with application to boundary integral equations on fractal screens*

Abstract: We study properties of fractional Sobolev spaces (Bessel potential spaces) on non-Lipschitz subsets of \R^n. We investigate the extent to which some of the standard properties of these spaces (e.g. density, duality and interpolation) that hold in the well-studied case of a Lipschitz open set, generalise to non-Lipschitz cases. For instance, for a given smoothness parameter s\in\R, one might ask for which open sets \Omega\subset\R^n the set C^\infty_0(\Omega) is dense in the space of distributions in the Sobolev space H^s(\R^n) whose distributional support is contained in the closure of \Omega. This is always the case for Lipschitz (in fact, even for C^0) \Omega, but fails in general. One of our recent results is that the aforementioned density result holds for "thick" \Omega (in the sense of H. Triebel), which includes the Koch snowflake domain. Our studies are motivated by applications in PDE and integral equation theory, particularly to the analysis of boundary integral equation formulations of wave scattering by planar screens with fractal boundary. This is joint work with Simon Chandler-Wilde (Reading), Andrea Moiola (Pavia) and Antonio Caetano (Aveiro).

**Thursday Mar 5, **Imperial analysis and PDE's seminar

**Thursday Mar 12, **Imperial analysis and PDE's seminar (Imperial college, Huxley 140)

3pm: Jessica Guerand (Cambridge University)

Title: Quantitative regularity for parabolic De Giorgi classes

how it is useful to get Hölder regularity.

**Thursday Mar 19, ** London analysis and probability seminar (Imperial college, Huxley 140)

CANCELLED

## Past seminars

### 2019-2020

## 2019-2020 Autumn term programme

**Friday Oct 4,** Paris-London analysis seminar, at Imperial College

Whole day event, see http://www.london-analysis-seminar.org.uk/Paris-London/ for programme.

**Thursday Oct 10**, London analysis seminar, at UCL, room 706 (Department of Mathematics, 25 Gordon Street)

3pm: Alexander Pushnitski (King's), *Kato smoothness and functions of self-adjoint operators*

Abstract: The topic of the talk is a new approach to estimates of the operator norm and Schatten norms of f(A)-f(B), where A and B are self-adjoint operators in a Hilbert space. This approach was developed in my recent joint work with Rupert Frank. It utilises ideas of scattering theory and involves conditions on A and B in terms of Kato smoothness. As an important technical tool, a new notion of Schatten class valued Kato smoothness is proposed and a new framework for double operator integrals is developed.

4:30pm: Martin Taylor (Imperial), *The nonlinear stabillity of the Schwarzschild family of black holes*

Abstract: I will present a theorem on the full finite codimension asymptotic stability of the Schwarzschild family of black holes. The proof employs a double null gauge, is expressed entirely in physical space, and utilises the analysis of Dafermos--Holzegel--Rodnianski on the linear stability of the Schwarzschild family. This is joint work with M. Dafermos, G. Holzegel and I. Rodnianski.

**Thursday Oct 17**, Imperial analysis and PDE's seminar, at ICL (Huxleyroom 140)

3pm: Pierre Raphael (Nice)

**Thursday Oct 24**, London analysis seminar, at UCL, room 706 (Department of Mathematics, 25 Gordon Street)

3pm: Cyril Roberto (Paris Ouest Nanterre la Défense), *log Hessian estimates and the Talagrand Conjecture.*

Motivated by Talagrand's conjecture on the regularization effect of the Ornstein-Uhlenbeck semi-group, we investigate lower bounds on the log Hessian of a family of diffusion semi-group (essentially perturbation of the Ornstein-Uhlenbeck semi-group). We will also investigate similar questions for the (discrete) M/M/infinity queuing process on the integers. Joint work with N. Gozlan, X.-M Li, M. Madiman and P.-M. Samson.

4:30 Marco Marletta (Cardiff), *The essential numberical range of operators and pencils*

Abstract: We discuss several alternative characterisations of the essential numerical range We(T) for an unbounded operator T on a domain in a Hilbert space. Unlike the bounded case, studied by Stampfli and Williams in the 1960s, these definitions are no longer equivalent. We examine how they differ and which one(s) are of greatest use for applications. One of these definitions turns out to be the natural non-selfadjoint replacement for the extended essential spectrum used by Davies, and by Levitin and Shargorodsky, to analyse spectral pollution when approximating self-adjoint operators using projection methods. We also examine generalisations to linear operator pencils and generalised Morawetz tricks. These allow us to establish, e.g., substantial improvements of a result of Lewin and Sere on spectral pollution for Dirac systems.

This is joint work with Sabine Boegli (Durham) and Christiane Tretter (Bern).

**Thursday Oct 31**, Imperial analysis and PDE's seminar, at ICL (Huxley room 140)

3pm: Jose Antonio Carrillo de la Plata (Imperial College), *Nonlinear Aggregation-Diffusion Equations in the Diffusion-dominated and Fair competition regimes*

Abstract: We analyse under which conditions equilibration between two competing effects, repulsion modelled by nonlinear diffusion and attraction modelled by nonlocal interaction, occurs. I will discuss several regimes that appear in aggregation diffusion problemswith homogeneous kernels. I will first concentrate in the fair competition casedistinguishing among porous medium like cases and fast diffusion like ones. I will discuss the main qualitative properties in terms of stationary states and minimizers of the free energies. In particular, all the porous medium cases are critical while the fast diffusion are not. In the second part, I will discuss the diffusion dominated case in which this balance leads to continuous compactly supported radially decreasing equilibrium configurations for all masses. All stationary states with suitable regularity are shown to be radially symmetric by means of continuous Steiner symmetrisation techniques. Calculus of variations tools allow us to show the existence of global minimizers among these equilibria. Finally, in the particular case of Newtonian interaction in two dimensions they lead to uniqueness of equilibria for any given mass up to translation and to the convergence of solutions of the associated nonlinear aggregation-diffusion equations towards this unique equilibrium profile up to translations as time tends to infinity. This talk is based on works in collaboration with S. Hittmeir, B. Volzone and Y. Yao and with V. Calvez and F. Hoffmann.

4:30pm: Jean Lagace (UCL), *On recent approaches towards Pólya's conjecture*

Abstract : Eigenvalues of the Laplacian on a planar domain of unit area with either Dirichlet or Neumann boundary conditions form a sequence accumulating only at infinity. Pólya conjectured in 1962 that the k^th Dirichlet eigenvalue is bounded below by 4πk, while the k^th Neumann eigenvalue is bounded above by the same constant. The conjecture is

known to be true for domains that tile the plane, and is known to hold for the first few eigenvalues, or for k large enough, but in no other situations, even the disk.

The conjecture is notoriously hard to study in the intermediate regime, notably because we don't know whether or not optimising domains for the k^th eigenvalue exist amongst planar domains, so we cannot use properties of an extremiser. I will present two new approaches : the first one restrains the problem to a class of domains amongst which extremisers are known to exist. The conjecture then becomes equivalent to some specific properties of these extremmisers holding. The second approach relates the Neumann problem to the Steklov problem, and obtains universal upper bounds for the former in terms of the latter.

Based on joint work with Pedro Freitas (Lisbon) and Jordan Payette (Tel-Aviv), and with Alexandre Girouard (Laval) and Antoine Henrot (Nancy)

**Thursday Nov 7**, Imperial analysis and PDE's seminar, at ICL (Huxley room 140)

3pm: Max Renesse (University of Leipzig)

**Thursday Nov 14**, London analysis seminar, at UCL, room 706 (Department of Mathematics, 25 Gordon Street)

3pm: Igor Krasovsky (Imperial)

4:30pm: Alice Guionnet (Lyon)

**Thursday Nov 21**, Imperial analysis and PDE's seminar, at ICL (Huxley room 140)

3pm: John MacKay (Bristol University)

Title: Decompositions of hyperbolic groups and conformal dimension

Abstract:

The Hausdorff dimension of a metric space can vary under quasisymmetric or quasiconformal homeomorphisms; the infimal possible value it can take is the conformal dimension of the metric space, introduced by Pansu. For a Gromov hyperbolic group, the conformal dimension of its boundary at infinity is one of the fundamental invariants of the group. I will discuss this connection and its history, and describe new ways to compute this invariant when the group splits over two-ended subgroups.

Joint work with Matias Carrasco.

**Friday Nov 29**, Talk Cancelled

**Thursday Dec 5**, London analysis seminar, at UCL, room 706 (Department of Mathematics, 25 Gordon Street)

3pm: Alexander Grigoryan (Bielefeld)

4:30pm: Jeffrey Galkowski (UCL)

**Thursday Dec 12**, Imperial analysis and PDE's seminar, at ICL (Huxley room 140)

3pm: Marc Briant (Université Paris Descartes)

Title: Hypocoercive Techniques in Collisional Kinetic Theory

Abstract: The issue of long-time behaviour of solutions of a PDE, more precisely the convergence towards an equilibrium, can be viewed at a linear(ized) level by adopting a perturbative approach. In such a framework one expects the dynamics of the linear operator to take over higher order terms for small initial data. When the linear operator is symmetric negative then obvious Gronwall-type arguments apply to obtain explicit rate of decay for the solutions. Unfortunately for a lot of

collisional kinetic equations, the linear operator indeed offers a negative feedback but only outside of its kernel. In this talk we present different techniques used in order to recover a full coercivity thanks to the interplay between collision and transport operator : opertor commutators, weak ellipticity, L^2-L^\infty, extension methods... We shall present some models where such hypocoercivity proved itself useful to construct explicit Cauchy theories and rates of convergence.

### 2018-2019

## 2018-2019 - Summer term programme

♠ *Mira Shamis* (Queen Mary), Applications of the Ky Fan inequality to random (and almost periodic) operators

Thursday 16 May, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar

Abstract: We shall discuss the Ky Fan inequality for the eigenvalues of the sum of two Hermitian matrices. As an application, we shall derive a sharp version of a recent result of Hislop and Marx pertaining to the dependence of the integrated density of states of random Schroedinger operators on the distribution of the potential. Time permitting, we shall also discuss an application to quasiperiodic operators.

♠ *Jonathan Fraser* (St. Andrews), Approximate arithmetic structure in large sets of integers

Thursday 23 May, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar

Abstract: The celebrated Erdos conjecture on arithmetic progressions states that sets of positive integers whose reciprocals form a divergent series should contain arbitrarily long arithmetic progressions. I will discuss recent joint work with Han Yu, where we prove that such sets must at least approximate arithmetic progressions in a quantifiable sense.

♠ *Raffaella Carbone* (Uni. of Pavia), Stable subspaces and cyclic decomposition of a quantum channel.

Thursday 20 June, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar

Abstract: We consider a quantum channel acting on a von Neumann algebra of operators on a separable Hilbert space. Some probabilistic properties still need to be investigated for such channels, which can be seen as a non commutative version of Markov operators. When there exists an invariant normal faithful state, the cyclic properties of quantum channels can be studied passing through the decoherence free algebra and the fixed points domain. Both these spaces are proved to be images of normal conditional expectations so that their consequent atomic structure is analyzed in order to give a better description of the action of the channel and, for instance, of its Kraus form and invariant densities. This work is mainly in collaboration with Anna Jencova (Bratislava, Slovak Academy of Science)

## 2018-2019 - Spring term programme

♠** Anders Hansen (Cambridge), On non-computable problems in computer assisted proofs - Why foundations of computations may interest pure mathematicians**

Thursday 10 January, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar

Abstract: Computer assisted proofs have become increasingly popular over the last decades and turn out to be instrumental when proving many long standing conjectures. The recent computer assisted proof, led by T. Hales, of the more than four century old conjecture of Kepler (Hilbert’s 18th problem) on optimal packing of 3-spheres, is a striking example. Another fascinating case is the computer assisted proof of the Dirac-Schwinger conjecture, by C. Fefferman and L. Seco, on the asymptotic behaviour of the ground-state energy of certain Schrodinger operators. What may be surprising is that the computational problems used in these proofs are non-computable according to Turing. In this talk we will discuss this paradoxical phenomenon: Not only can non-computable problems be used in computer assisted proofs, they are crucial for proving important conjectures. A key tool for understanding this phenomenon is the Solvability Complexity Index (SCI) hierarchy, which allows for a classification theory for all types of computational problems. This classification theory may be of use to pure mathematicians for determining which computational problems that may be used in computer assisted proofs. In particular, there are non-computable problems that can be used and there are non-computable problems that are so difficult that they can never be used in computer assisted proofs. The question is: which ones are safe to utilise? Examples from mathematical physics and spectral theory will be highlighted.

♣** Leonid Parnovski (UCL), Floating mats and sloping beaches: spectral asymptotics**

Wednesday 23 January , 1.30 - 2.30pm, King's College London, King's Building, room K3.11, London Analysis and Probability seminar,

Abstract: I will discuss the asymptotic behaviour of the eigenvalues of the Steklov problem (aka Dirichlet-to-Neumann operator) on curvilinear polygons. The answer is completely unexpected and depends on the arithmetic properties of the angles of the polygon.

♣ *Valery Smyshlyaev* (UCL), Two-scale homogenisation for a general class of high-contrast PDE systems

Wednesday 23 January, 3.00 - 4.00pm, King's College London, King's Building, room K3.11, London Analysis and Probability seminar

Abstract: I present recent work with Ilia Kamotski [1] on two-scale homogenisation of general PDE systems with periodic coefficients with a critically scaled high contrast, which reflects certain interesting effects due to underlying ``micro-resonancesâ€™â€™. It appears that a strong two-scale resolvent convergence of associated high-contrast elliptic operators holds under a rather generic decomposition assumption. This implies in particular (two-scale) convergence of parabolic and hyperbolic semigroups with applications to a wide class of initial value problems. In the end I briefly discuss most recent stronger results with operator-type error estimates for high-contrast problems (with Shane Cooper and I. Kamotski), as well as situations where the micro-resonances display certain randomness. In simplest cases, the resulting two-scale limit behaviour appears to be rather explicit and the macroscopic equations display a form of wave trapping by the micro-resonances due to their randomness.

[1] I.V. Kamotski, V.P. Smyshlyaev, Two-scale homogenization for a general class of high contrast PDE systems with periodic coefficients, {\it Applicable Analysis} 98 (1-2), 64--90 (2019).

♣ *Dmitry Jakobson* (McGill), Quantum ergodicity for ray-splitting (branching) billiards

Wednesday 23 January, 4.30 - 5.30pm, King's College London, King's Building, room K3.11, London Analysis and Probability seminar.

Abstract: This is joint work with Yu. Safarov and A. Strohmaier.

After giving an overview of Quantum Ergodicity results on compact Riemannian manifolds with ergodic geodesic flow (due to Shnirelman, Zelditch, Colin de Verdiere and others), we discuss joint work with Yuri Safarov and Alex Strohmaier, which concerns the semiclassical limit of spectral theory on manifolds whose metrics have jump-like discontinuities. Such systems are quite different from manifolds with smooth Riemannian metrics because the semiclassical limit does not relate to a classical flow but rather to branching (ray-splitting) billiard dynamics. In order to describe this system we introduce a dynamical system on the space of functions on phase space. We prove a quantum ergodicity theorem for discontinuous systems. In order to do this we introduce a new notion of ergodicity for the ray-splitting dynamics. If time permits, we outline an example (provided by Y. Colin de Verdiere) of a system where the ergodicity assumption holds for the discontinuous system.

♠ **Alisa Knizel (Columbia), Log-gases on a quadratic lattice via discrete loop equations**

Thursday 31 January, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar.

Abstract: We study a general class of log-gas ensembles on a quadratic lattice. Using a variational principle we prove that the corresponding empirical measures satisfy a law of large numbers and that their global fluctuations are Gaussian with a universal covariance. We apply our general results to analyze the asymptotic behavior of a q-boxed plane partition model introduced by Borodin, Gorin and Rains. In particular, we show that the global fluctuations of the height function on a fixed slice are described by a one-dimensional section of a pullback of the two-dimensional Gaussian free field.

Our approach is based on a q-analogue of the Schwinger-Dyson (or loop) equations, which originate in the work of Nekrasov and his collaborators, and extends the methods developed by Borodin, Gorin and Guionnet to a quadratic lattice.

The talk is based on a joint work with Evgeni Dimitrov (Columbia University).

♣ *Paul Bourgade* (Courant), TBA

Thursday 7 February 2019 , 3-4pm, Imperial College London, Huxley Building, room 140, London Analysis and Probability seminar,

♣ *Mikhail Menshikov* (Durham)**, Non-homogeneous random walks in critical regimes**

Thursday 7 February 2019 , 4:30-5:30pm, Imperial College London, Huxley Building, room 140, London Analysis and Probability seminar.

Abstract: http://www.london-analysis-seminar.org.uk/abstracts/Menshikov_070219.pdf

**♠ Christopher Kauffman (Imperial), Global Stability for Charged Scalar Fields in Spacetimes close to Minkowski**

Thursday 14 February, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar.

**♣ Martin Hairer (Imperial), Reconciling Ito and Stratonovich**

Thursday 21 February, 3-4pm, Imperial College London, Huxley Building, room 140, London Analysis and Probability seminar,

**♣ Vlad Vysotsky (Sussex), Large deviations of convex hulls of planar random walks**

Thursday 21 February, 4:30-5.30pm, Imperial College London, Huxley Building, room 140, London Analysis and Probability seminar.

**♠ Disheng Xu (University of Chicago), Lyapunov exponents and spectral theory of Schrodinger operator.**

Thursday 28 February, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar.

**♣ Bassam Fayad (Paris), On the stability of quasi-periodic motion for analytic Hamiltonian systems**

Thursday 14 March, 3-4pm, Imperial College London, Huxley Building, room 140, London Analysis and Probability seminar,

Abstract: The stability of an elliptic fixed point of a Hamiltonian system is a central question in mathematical physics and is one of the founding problems of dynamical systems. We explore this question from three points of view : topological stability (Lyapunov stability), statistical stability (KAM theory), and effective stability (finite time stability). We introduce in particular new diffusion mechanisms that give the first examples of real analytic Hamiltonians (in three or more degrees of freedom) with unstable elliptic fixed points, or unstable invariant quasi-periodic tori. We produce examples with arbitrary (including Diophantine) frequency vectors and with divergent or convergent (in the case of tori) Birkhoff Normal Forms. We also give examples of analytic Hamiltonians that are integrable on half of the phase space and diffusive on the other half.

**♣ Eero Saksman (Helsinki), Decompositions of log-correlated fields with applications**

Thursday 14 March, 4:30-5.30pm, Imperial College London, Huxley Building, room 140, London Analysis and Probability seminar.

Abstract: We consider a simple idea to decompose of log-correlated Gaussian fields into two-parts, both of which behave well in suitable sense. Applications include Onsager type inequalities in all dimensions, analytic dependence and existence of critical chaos measures for a large class of log-correlated fields. Talk is based on Joint work with Janne Junnila (EPFL) and Christian Webb (Aalto University).

**♠ Diane Holcomb (KTH Stockholm); Random matrices through differential operators**

Thursday 21 March, 3:00-4:00, Imperial College London, Huxley 140, Pure analysis and PDE seminar.

Abstract: In 1984 Trotter described a tridiagonal random matrix model that has the same eigenvalues as the Gaussian Orthogonal Ensemble. This model and a later generalization share many structural properties with discrete Schrödinger operators. This led to the conjecture that the largest eigenvalues of the ensemble converged to the the eigenvalues a certain random differential operator. We will give an overview of where the conjecture comes from and a bit of the proof. We will then look at a process that appears when looking at eigenvalues of submatrices of the tridiagonal model. This process is notably different than the one that appears when considering submatrices of the full matrix model. This talk will survey work by Dumitriu-Edelman, Edelman-Sutton, and Ramirez-Rider-Virag. It finishes up with work that is joint A. Gonzalez.

♦ Program: http://www.london-analysis-seminar.org.uk/Paris-London/;

Friday 22 March, All day, Kings College London, The Paris-London Analysis Seminar.

## 2018-2019 - Autumn term programme

**♦ Sandrine Grellier (Orléans), Generic colourful tori and inverse spectral transform for Hankel operators**

5 October, 10:30-11:20, UCL (Room 706), Paris-London Analysis seminar

**♦**

*Tom Korner*(Cambridge),**Can we characterise sets of strong uniqueness**

5 October, 11:30-12:20, UCL (Room 706), London Analysis and Probability seminar

**♦ Emmanuel Fricain (Lille), **

**Multipliers between sub-Hardy Hilbert spaces**

5 October, 14:00-14:50, UCL (Room 706), Paris-London Analysis seminar

**♦ Tom Sanders (Oxford),****The Erdös Moser sum-free set problem**

5 October, 15:20-16:10, UCL (Room 706), London Analysis and Probability seminar,

For abstracts of the talks please visit here

**♠ Andrzej Zuk (CNRS- Paris), From PDEs to groups**

12 October, 3:00-4:00, Imperial College London, Huxley 140), Pure analysis and PDE seminar

Abstract: We present a construction which associates to a KdV equation the lamplighter group. At crucial steps of it appear automata and random walks on ultra discrete limits. It is also related to the L2 Betti numbers introduced by Atiyah which are homotopy invariants of closed manifolds.

**♣ Jürg Fröhlich (ETH), The Arrow of Time - Images of Irreversible Behavior**

18 October, 3:00-4:00, UCL (Room 706), London Analysis and Probability seminar,

Abstract:I sketch various examples of physical systems with time-reversal invariant dynamics exhibiting irreversible behavior. I start with deriving the Second Law of Thermodynamics in the formulation of Clausius from the existence of quantum-mechanical heat baths and then derive the Carnot bound for the degree of efficiency of heat engines. I continue with the analysis of a quantum-mechanical model with unitary time evolution describing a particle that exhibits diffusive motion when coupled to a suitably chosen (non-interacting) heat bath. A classical model with a Hamiltonian time evolution describing a particle coupled to a wave medium exhibiting friction is sketched next. I conclude with an attempt to draw the attention of the audience to the fact that the dynamics of isolated, open quantum systems featuring events is fundamentally irreversible.

**♣ Thomas Spencer (IAS), Edge reinforced random walk as a toy model of localization**

18 October, 4:30-5:30, UCL (Room 707), London Analysis and Probability seminar,

Abstract: I will present some results and conjectures about edge reinforced random walk (ERRW). This is a history dependent walk which favors edges it has visited in the past. In three dimensions the walk has a phase transition as the reinforcement is varied. The relation of ERRW to a toy model of quantum localization will also be discussed.

**♠ Jan Sbierski (Oxford), Uniqueness & non-uniqueness results for wave equations**

25 October, 3:00-4:00, Imperial College (Huxley 140), Pure analysis and PDE seminar

Abstract:A well-known theorem of Choquet-Bruhat and Geroch states that for given smooth initial data for the Einstein equations there exists a unique maximal globally hyperbolic development. In particular, time evolution of globally hyperbolic solutions is unique. This talk investigates whether the same result holds for quasilinear wave equations defined on a fixed background. After recalling the notion of global hyperbolicity, we first present an example of a quasilinear wave equation for which unique time evolution in fact fails and contrast this with the Einstein equations. We then proceed by presenting conditions on quasilinear wave equations which ensure uniqueness. This talk is based on joint work with Harvey Reall and Felicity Eperon.

**♣ Thierry Lévy (Paris 6), Quantum spanning forests**

1 November, 3:00-4:00, UCL (Room 706), London Analysis and Probability seminar,

Abstract: I will report on a work in progress with Adrien Kassel (ENS Lyon) about an extension of Kirchhoffâ€™s matrix-tree theorem and determinantal point processes, to the framework of vector bundles over graphs. While trying to understand in combinatorial terms the determinant of the covariant Laplacian on the space of sections of a vector bundle over a graph endowed with a connection, we were led to the definition of a family of probability measures on the Grassmannian of a Euclidean or Hermitian space, associated with an orthogonal splitting of this space and a self-adjoint contraction on it. This family of measures contains and extends the family of determinantal point processes.

**♣ Thomas Bothner (KCL), When J. Ginibre met E. Schrödinger**

1 November, 4:30-5:30, UCL (Room 706), London Analysis and Probability seminar,

Abstract: The real Ginibre ensemble consists of square real matrices whose entries are i.i.d. standard normal random variables. In sharp contrast to the complex and quaternion Ginibre ensemble, real eigenvalues in the real Ginibre ensemble attain positive likelihood. In turn, the spectral radius of a real Ginibe matrix follows a different limiting law for purely real eigenvalues than for non-real ones. Building on previous work by Rider, Sinclair and Poplavskyi, Tribe, Zaboronski, we will show that the limiting distribution of the largest real eigenvalue admits a closed form expression in terms of a distinguished solution to an inverse scattering problem for the Zakharov-Shabat system. This system is directly related to several of the most interesting nonlinear evolution equations in 1+1 dimensions which are solvable by the inverse scattering method, for instance the nonlinear Schro ̈dinger equation. The results of this talk are based on the recent preprint arXiv:1808.02419, joint with Jinho Baik.

**♠ Vedran Sohinger (Warwick) Gibbs measures of nonlinear Schrödinger equations as limits of many-body quantum states in dimension d <= 3**

8 November, 3:00-4:00, Imperial College (Huxley 140), Pure Analysis and PDE seminar

Abstract: Gibbs measures of nonlinear Schrödinger equations are a fundamental object used to study low-regularity solutions with random initial data. In the dispersive PDE community, this point of view was pioneered by Bourgain in the 1990s. We prove that Gibbs measures of nonlinear Schrödinger equations arise as high-temperature limits of appropriately modified thermal states in many-body quantum mechanics. We consider bounded defocusing interaction potentials and work either on the d-dimensional torus or on R^d with a confining potential. The analogous problem for d=1 and in higher dimensions with smooth non translation-invariant interactions was previously studied by Lewin, Nam, and Rougerie by means of variational techniques.

In our work, we apply a perturbative expansion of the interaction, motivated by ideas from field theory. The terms of the expansion are analysed using a diagrammatic representation and their sum is controlled using Borel resummation techniques. When d=2,3, we apply a Wick ordering renormalisation procedure. Moreover, in the one-dimensional setting our methods allow us to obtain a microscopic derivation of time-dependent correlation functions for the cubic nonlinear Schrödinger equation. This is joint work with Jürg Fröhlich, Antti Knowles, and Benjamin Schlein.

**♠ Edward Crane (Bristol), Circle Packing and Uniformizations**

15 November, 3:00-4:00, Imperial College (Huxley 140), Pure analysis and PDE seminar

Abstract: Koebe discovered his circle packing theorem in the 1930s as a limiting case of his uniformization theorem for multiply-connected plane domains. After Thurston had interpreted circle packing as a discretization of conformal structure, Rodin and Sullivan showed how one could deduce the Riemann mapping theorem as a limiting case of the circle packing theorem. I will explain conformal welding and its circle packing analogue. I will show how this technique can be used to approximate an unusual uniformization of multiply-connected domains, in which each complementary component is a disc in the hyperbolic metric associated to the complement of all the other complementary components.

**♣ Jonathan Bennett (Birmingham) The nonlinear Brascamp-Lieb inequality and applications**

22 November, 3:00-4:00, UCL (Room tba), London Analysis and Probability seminar,

Abstract: The Brascamp--Lieb inequality is a broad generalisation of many well-known multilinear inequalities in analysis, including the multilinear H\"older, Loomis--Whitney and sharp Young convolution inequalities. There is by now a rich theory surrounding this inequality, along with diverse applications in convex geometry, partial differential equations, number theory and beyond. Of particular importance is Lieb's Theorem (1990), which states that the best constant in this inequality is exhausted by centred gaussian functions. In this talk we present a recent "nonlinear" variant of the Brascamp--Lieb inequality, and describe some of its applications in harmonic analysis and PDE. A key ingredient in our proof is a certain effective version of Lieb's theorem, providing information about the shapes of gaussian near-extremisers for the classical Brascamp--Lieb inequality. This is joint work with Stefan Buschenhenke, Neal Bez, Michael Cowling and Taryn Flock.

**♣ Herbert Koch (Bonn), A continuous family of conserved energies for the Gross-Pitaevskii equation,**

22 November, 4:30-5:30, UCL (Room tba), London Analysis and Probability seminar,

Abstract: The Gross-Pitaevskii equation is the defocusing cubic nonlinear Schrödinger equation with the boundary conditions |u(t,x)| -> 1 at infinity. A difficulty in the study of the Gross-Pitaevskii equation is that the state space is nonlinear. In joint work with Xian Liao we study the equation in one space dimension, equip it with a new metric, and construct a continuous family of conserved energies.

**♠ Benjamin Fahs (Imperial College), Toeplitz determinants with Fisher-Hartwig singularities**

29 November, 3:00-4:00, Imperial College (Huxley 140), Pure analysis and PDE seminar

Abstract: We consider the large n asymptotics of n dimensional Toeplitz determinants with Fisher-Hartwig singularities, uniformly as the location of the singularities are allowed to merge with each other. We discuss applications to moments of averages of the characteristic polynomials of the Circular Unitary Ensemble.

**♣ Horst Knörrer (ETH) Construction of oscillatory singular homogenuous space times**

6 December, 3:00-4:00, UCL (Room tba), London Analysis and Probability seminar,

Abstract: The vacuum Einstein equations for Bianchi space times (that is space times that can be foliated into three dimensional space like slices that are all homogenuous spaces) reduce to a system of ordinary differential equations. The conjectures of Belinskii, Khalatnikov and Lifshitz predict that for almost all initial data the solutions of these differential equation behave like trajectories of a billiard in a Farey triangle in the hyperbolic plane, that is, a triangle whose three vertices are ideal points. In joint work with M.Reiterer and E.Trubowitz we show that, for a set of initial data that has positive measure, this is indeed the case. We use ideas inspired by scattering theory for approximations of the system. The fact that billiard in a Farey triangle is chaotic leads us to small divisor problems similiar to those of KAM theory in Hamiltonian dynamics.

**♣ Tuomas Sahsten (Manchester), Delocalisation of waves under scaling limits**

6 December, 4:30-5:30, UCL (Room tba), London Analysis and Probability seminar,

Abstract: We establish quantitative quantum ergodicity type delocalisation theorem for waves on hyperbolic surfaces of large genus. In the compact setting our assumptions hold for random surfaces in the sense of Weil-Petersson volume in the TeichmÃ¼ller space due to the work of Mirzakhani and in non-compact setting for arithmetic surfaces coming from congruence covers of the modular surface. The methods are based on Benjamini-Schramm scaling limits of metric measure spaces and Stein type harmonic analysis ergodic theorems, and are inspired by similar results on graphs. We plan to give a gentle introduction to the field before going to our results. Joint work with Etienne Le Masson (Cergy-Pontoise University, France).

**♠ Matthew Jacques (Open University), Semigroups of hyperbolic isometries and their parameter spaces**

13 December , 3:00-4:00, Imperial College London, (Huxley 140), Pure analysis and PDE seminar

Abstract: Let M denote the collection of orientation-preserving isometries of the hyperbolic plane. Given an n-tuple x=(x_1, ..., x_n) in M^n, let S(x) denote the semigroup generated by the ordinates of x under composition. In a 2010 paper of Avila, Bochi and Yoccoz the authors define the hyperbolic locus, H, as the set of points in M^n whose ordinates generate composition sequences that grow exponentially. They also define the elliptic locus, E, as the set of those x in M^n for which S(x) contains an elliptic isometry. They show that both H and E are open, and that the closure of E is equal to the complement of H. Motivated by this work and by the theory of discrete (Fuchsian) subgroups contained in M, we introduce the term semidiscrete to describe a semigroup that does not contain the identity within its closure. The semidiscreteness property on semigroups appears to be a good analogue of the discreteness property on groups, and we give theorems that have familiar counterparts in the theory of Fuchsian groups. For instance, we find that every semigroup is one of four standard types: elementary, exceptional, semidiscrete, or dense in M. We use these ideas to characterise the set H in terms of the semidiscreteness property. Finally, we give an example of a point on the boundary of E but not on the boundary of H, and an example of a point on the boundary of H that does not lie on the boundary of any of its connected components, answering two questions posed by Avila, Bochi and Yoccoz.

**♠ Tom Claeys (Uni Louvain-la-Neuve), TBA**

13 December , 4:30-5:30, Imperial College London, (Huxley 140), Pure analysis and PDE seminar

**♠ Ivan Gentil (Lyon), Analytic point of view of the Schrödinger problem : a review on the subject.**

20 December , 3:00-4:00, Imperial College London, (Huxley 140), Pure analysis and PDE seminar

Abstract:We are going to describe the Schrödinger problem as a minimisation of a cost along paths. This point of view allows us to simplify the problem and to see how the Schrödinger problem approaches the optimal transportation problem and also dual formulation. This is a joint work with C. Léonard and L. Ripani.

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### Diversity Policy of the Pure Analysis and Probability Seminars

#### Department of Mathematics, Imperial College London

Imperial College London, its Department of Mathematics, and its Pure Mathematics Section are committed to fostering a welcoming, open, and diverse research community. We recognize that there are groups who remain under-represented in the discipline, particularly among senior academics and other professionals. We actively promote equality, diversity and inclusion among the speakers in this seminar series. To this end:

- We seek a diverse set of speakers that reflects the diversity of the worlds’community of mathematicians in terms of gender and ethnicity/race, etc.
- We invite speakers from all around the world
- We include speakers who are at various career stages.
- We make the list of speakers public. When soliciting names of potential speakers the organizers emphasize our aim to present a crossection of speakers that is diverse in terms of gender, ethnicity/race, geography, and career stage.
- We also take advantage of resources such as LMS Women in Mathematics, European Women in Mathematics Members List...and others.