Past: Winter 2018, Spring 2018, Fall 2018, Winter 2019, Spring 2019
Future: Winter 2020
Friday Oct 4 

4:305:20 Caltech, Linde 310 Svetlana Jitormirskaya (UC Irvine) Fractal properties of the Hofstadter's butterfly and singular continuous spectrum of the critical almost Mathieu operator Harper's operator  the 2D discrete magnetic Laplacian  is the model behind the Hofstadter's butterfly. It reduces to the critical almost Mathieu family, indexed by phase. We discuss the proof of singular continuous spectrum for this family, for all phases, finishing a program with a long history. We also present a result (with I. Krasovsky) that proves one half of the Thouless' one half conjecture from the early 80s: that Hausdorff dimension of the spectrum of Harper's operator is bounded by 1/2 for all irrational fluxes. 

5:306:20 Caltech, Linde 310 Khang Huynh (UCLA) Hodgetheoretic analysis on manifolds with boundary, heatable currents, and Onsager's conjecture in fluid dynamics We use Hodge theory and functional analysis to develop a clean approach to heat flows and Onsager's conjecture on Riemannian manifolds with boundary, where the weak solution lies in the tracecritical Besov space $B_{13}^{3,1}$. We also introduce heatable currents as the natural analogue to tempered distributions and justify their importance in Hodge theory. 

Friday Oct 18 

4:004:50 UCLA, MS 6627 Tarek Elgindi (UCSD) Singularity formation in incompressible fluids We will discuss recent results on singularity formation for solutions to the incompressible Euler equation in various settings. We will discuss in some detail a construction of selfsimilar blowup for C^{1,a} solutions to the 3D Euler equation as well as a proof of stability that yields finiteenergy local selfsimilar blowup. Some of the results are joint with I. Jeong, T. Ghoul, and N. Masmoudi. 

5:005:50 UCLA, MS 6627 Eyvindur Palsson (Virginia Tech) Falconer type theorems and integral operators Finding and understanding patterns in data sets is of significant importance in many applications. One example of a simple pattern is the distance between data points, which can be thought of as a 2point configuration. Two classic questions, the Erdos distinct distance problem, which asks about the least number of distinct distances determined by N points in the plane, and its continuous analog, the Falconer distance problem, explore that simple pattern. Questions similar to the Erdos distinct distance problem and the Falconer distance problem can also be posed for more complicated patterns such as various 3point configurations. In this talk I will explore such generalizations. The key focus will be integral operators, that arise naturally from such point configuration questions. In addition, a novel grouptheoretic viewpoint, which has allowed for much progress recently, will be highlighted. 

Friday Nov 1 

4:305:20 Caltech, Linde 310 Vladimir Markovic (Caltech) Random quasiconformal mappings and Delauney triangulations I shall discuss my very recent work with Oleg Ivrii regarding two problems dealing with the homogenization of random media. We show that a random quasiconformal mapping is close to an affine mapping, while a circle packing of a random Delauney triangulation is close to a conformal map. Moreover, on a Riemann surface equipped with a conformal metric, a random Delauney triangulation is close to being circle packed. 

5:306:20 Caltech, Linde 310 Danielle Hilhorst (CNRS and U. ParisSud) A reactiondiffusionODE model in anthropology A reactiondiffusionODE model for the Neolithic spread of farmers in Europe has been recently proposed by Elias, Kabir and Mimura. This model involves huntergatherers and farmers, which are divided into two subpopulations, namely sedentary and migrating farming populations. The conversion between the farming subpopulations depends on the total density of the farmers and it is superimposed on the classical LotkaVolterra competition model; it is therefore described by a threecomponent reactiondiffusionODE system. In this talk, we study its singular limit as the conversion rate tends to infinity and prove that solutions of the threecomponent system converge to solutions of a twocomponent system with linear diffusion in one of the equations and nonlinear degenerate diffusion in the other. This is joint work with Jan Elias, Masayasu Mimura and Yoshihisa Morita. We then study the limiting two component system. From an ecological viewpoint, the nonlinear diffusion takes into account the population density pressure of the farmers on their dispersal. The interaction between farmers and huntergatherers is of the LotkaVolterra preypredator type. We present an alternative method for proving the existence and uniqueness of the globalintime solution and study its asymptotic behavior as time tends to infinity. This is joint work with Jan Elias and Masayasu Mimura. 

Friday Nov 15 

4:004:50 UCLA, MS 6627 Florian Richter (Northwestern) A sumset conjecture of Erdős A longstanding conjecture of Erdős stated that every subset of the integers with positive density contains a sumset $B+C=\{b+c: b\in B, c\in C\}$ for two infinite sets $B$ and $C$. I will talk about joint work with Joel Moreira and Donald Robertson in which we resolve this conjecture. Our proof utilizes ideas and methods coming from Ergodic Theory, including an intersectivity lemma of Bergelson and a novel decomposition theorem for arithmetic functions into structured and random components. 

5:005:50 UCLA, MS 6627 Susan Friedlander (USC) The Joy of Small Parameters Many equations that model fluid behavior are derived from systems that encompass multiple physical forces. When the equations are written in non dimensional form appropriate to the physics of the situation, the resulting PDEs often involve multiple nondimensional parameters. Frequently some of these parameters are very small and they enter into the analysis in different ways. We will discuss one such system which has been proposed as a model for magnetostrophic turbulence and describe results that can be obtained in several different small parameter limits. In this talk we will concentrate on a forced driftdiffusion equation for the temperature where the fluid viscosity enters via the drift velocity. We examine the convergence of solutions in the limit as the viscosity goes to zero. We introduce a natural notion of ”vanishing viscosity ” weak solutions and prove the existence of a compact global attractor for the critical driftdiffusion equation. This is joint work with Anthony Suen. 

Friday Dec 6 

4:305:20 Caltech, Linde 310 Vjekoslav Kovac (U. Zagreb/Georgia Tech) On maximal and variational Fourier restriction The problem of restricting the Fourier transform of a nonintegrable function to a hypersurface gained attention in the 1970s and is an active research field today. On the other hand, maximal Fourier restriction estimates were inaugurated recently by Müller, Ricci, and Wright in order to give a pointwise meaning to the restriction of the Fourier transform of a function. We will discuss variational Fourier restriction estimates, which quantify the pointwise convergence in question. Moreover, we will sketch a simple abstract argument that deduces certain maximal and variational Fourier restriction estimates from ordinary a priori estimates. 

5:306:20 Caltech, Linde 310 Vladimir Sverak (U. Minnesota) PDE aspects of incompressible fluid flows and simpler model equations We will discuss some of the open questions arising in connection with equations of the incompressible fluid mechanics in parallel with related problems for simpler models for which our knowledge is better. 

Last update: Nov 26, 2019 