Week of April 19, 2026

Mon Apr 20, 2026
12:00pm to 1:00pm - 340N - Mathematical Physics
Abel Klein - (UCI)
Many-body localization for the random XXZ spin chain in fixed energy intervals

A key signature of MBL (many-body localization) is the slow rate at which information spreads. In this talk I will describe my recent results with Elgart showing that the infinite random XXZ spin-1/2 chain exhibits slow propagation of information (logarithmic light cone) in any arbitrary but fixed energy interval. The relevant parameter regime, which covers both weak interaction and strong disorder, is determined solely by the energy interval.

I will not assume that the audience is familiar with random spin chains. I will introduce the infinite random XXZ spin-1/2 chain, state the main result, and describe some important ingredients for the proof.

2:00pm to 3:00pm - 340P Rowland Hall - Combinatorics and Probability
Haixiao Wang - (University of Wisconsin Madison)
Singular values and singular vectors of sparse random rectangular matrices at criticality

In modern machine learning applications, data matrices are always assumed to admit the signal-plus-noise structure. Typically, we assume that the spectra of signal and noise matrices are well-separated and that noise subspaces only produce a marginal influence. While these assumptions are readily verified for dense matrices via classical random matrix theory, real-world data is often sparse, posing significant theoretical challenges. We investigate the spectral properties of a matrix $X \in \mathbb{R}^{n \times m}$ with i.i.d. $\text{Bernoulli}(p)$ entries. Previous literature proved that when $np \gg \log(n)$, the singular values of $X$ almost surely remain within the compact support of the Marčenko-Pastur (MP) distribution. However, we identify a critical sparsity regime $p = b \log(n) / \sqrt{mn}$ where this classical result fails. We provide a quantitative characterization of the emergence of outlier singular values. For explicit thresholds $b_*$ and $b^*$ as functions of the aspect ratio $\gamma = n/m \ge 1$, we prove a three-phase transition: (1) for $b > b_*$, no outliers exist; (2) for $b^* < b < b_*$, outliers emerge only beyond the right edge of the MP law; and (3) for $b < b^*$, outliers appear on both sides of the bulk, all with high probability. The locations of those outliers are precisely determined by the largest and smallest degree vertices of the underlying random graph. Besides, behavior of singular vectors corresponding to bulk and edge singular values can be characterized precisely. Our approach follows the framework established by Alt, Ducatez, and Knowles (2021), which can be extended to sparse matrices with general bounded entries.

Tue Apr 21, 2026
1:00pm to 2:00pm - RH 340N - Dynamical Systems
Victor Kleptsyn - (CNRS, University of Rennes 1, France)
C^1-version of the Denjoy example that is affine on the wandering intervals

The talk will be devoted to the construction of a C^1-version of the Denjoy example that is still exactly affine on the intervals of the complement to the minimal set. This is our recent work with Andrés Navas.

3:00pm to 4:00pm - 340P - Differential Geometry
Robert Koirala - (UC San Diego)
Structure Theory of Parabolic Nodal and Singular Sets

We will discuss new estimates for the size and structure of the nodal set $\{u=0\}$ and the singular set $\{u=|\nabla u|=0\}$ of solutions to parabolic inequalities with parabolic Lipschitz coefficients. In particular, we show that almost all of these sets are covered by regular parabolic Lipschitz graphs, with quantitative control, and that both satisfy parabolic Minkowski bounds depending only on a doubling quantity at a point. Many of these results are new even in the case of the heat equation on $\mathbb{R}^n\times \mathbb{R}$. This is joint work with Max Hallgren and Zilu Ma.

3:00pm to 3:50pm - 440 R Rowland Hall - Logic Set Theory
Julian Eshkol - (UC Irvine)
Weakly Threading ideals on successor cardinals

Complete ineffability is a classical topic in Set Theory, much of it due to Baumgartner. A cardinal kappa being completely ineffable implies that kappa is inaccessible.  Work of Eshkol, Foreman and Magidor has shown that ineffability is equivalent to the existence of certain threading ideals. 

This talk describes a new kind of ideal: weakly threading ideals, and how to show that it is consistent that they exist on successor cardinals such as aleph_2 assuming the consistency of a measurable cardinal.

4:00pm to 5:00pm - RH 306 - Nonlinear PDEs
Filippo Gaia - (Stanford University)
CMC surfaces with bounded genus

We will discuss the following problem: given a closed $3$-manifold $M$ and $H \geq 0$, is there a surface with constant mean curvature $H$ in $M$ whose genus $g$ is controlled? We will show that for almost every $H$, one can construct a branched immersion with these properties, with $g$ bounded above by the Heegaard genus of $M$. The proof relies on a min-max construction based on a perturbation of the area functional involving the second fundamental form of the immersion, introduced by T. Rivière. This is joint work with Xuanyu Li (Cornell).