The colloquium offers survey talks in mathematics and related fields, the time for all the faculty and graduate students to get together to learn of somebody else's perspective on mathematics, and to broaden their own. A colloquium talk should be addressed to a general mathematical audience rather than technical details for specialists. The first 15 minutes should be completely understandable to graduate students.

The colloquium talks begin on Tuesdays at 2:00 p.m. Colloquium coffee succeeding talks around 3:00 pm.

Contact: Jiange LI (jiange.li AT hit.edu.cn)

**Upcoming talks**

**May16****,**** 2024. 16:00 (Beijing time)**

Speaker: Liping Zhu (Renmin University of China)

Title: 非线性相依与独立性检验

## Abstract

统计学“始于度量、兴于相关”。度量相关性和检验独立性是统计学领域基本问题，预测和推断是统计学和机器学习领域核心问题。本报告将会回顾统计学领域线性相关和非线性相关各类度量准则，以及独立性检验的最新进展。

**Past talks**

**May9****,**** 2024. 16:00 (Beijing time)**

Speaker: Xiaohua Yao (Central China Normal University)

Title: Some problems from scattering theory

## Abstract

In this talk, I will first review some classical results of scattering theory for Schrodinger operator ,including limiting absorption principle, generalized eigenfunction expansions and the existence of wave operators in the context of . These concepts/operators also have similar counterparts or estimates related with harmonic analysis, including uniform Sobolev estimates of resolvent, bounds of wave operators and so on. Finally, we also address some related -works associated with higher order Schrodinger operators . These are adjoint-works with Haruya Mizutani and Zijun Wan.

**May6****,**** 2024. 14:30 (Beijing time)**

Speaker: Naihuan Jing (North Carolina State University)

Title: Generalized Specht criteria and local unitary equivalence

## Abstract

Local unitary equivalence of quantum states is a basic theoretical problem in quantum information. Lots of efforts have been made on finding complete invariants for pure and fixed states. So far only bipartite mixed qubits are known by the Macklin's 18 invariants. We report on some recent progress using hypermatrix algebras and hyperdeterminants on 3-qubits. Joint work with Dobes.**May6****,**** 2024. 15:30 (Beijing time)**

Speaker: Olivier Mathieu (CNRS, Lyon)

Title: Systoles and Coxeter groups

## Abstract

## Given a closed hyperbolic surface, a systole is a closed geodesic of shortest length. A generic surface of genus has a unique systole which cuts it into one or two noncontractible pieces. However, there are closed hyperbolic surfaces of genus , whose set of systoles fills the surface, i.e. it cuts the surface into polygons. In general, it is easy to find surfaces with a large number of systoles which fill. A classical example is the Bolsa surface of genus 2 with 12 systoles. Here we are interested to find surfaces for which the set of systoles is filling while being small. In fact, our approach provides surfaces with significatively less systoles than the previously known examples. Then we will briefly outline a consequence in Teichmüller theory. Our construction is based on the theory of Coxeter groups, especially the arithmetic properties of the Tits representation. From a geometric viewpoint, it involves hyperbolic tessellations. Although this work mixes hyperbolic geometry and representation theory, our presentation will be quite elementary.

**April30****,**** 2024. 16:00 (Beijing time)**

Speaker: Shizan Fang (University of Burgundy)

Title: Navier-Stokes equations and geometric interpretation

## Abstract

We try to give geometric interpretations on Navier-Stokes equations

**April12****,**** 2024. 15:30 (Beijing time)**

Speaker: Vicentiu Radulescu(University of Craiova)

Title: Some striking phenomena in the analysis of (p,q)-equations

## Abstract

I shall discuss some new recent striking results that appear in the analysis of problems with unbalanced growth. In the isotropic case, I shall point out an interesting discontinuity property of the spectrum. In the second part my talk, I will discuss the anisotropic case and I shall discuss a new case corresponding to equations with mixed regime. Several new research directions will be highlighted in the final part of this talk.

**December28****,**** 2023. 16:00 (Beijing time)**

Speaker: Meiyue Jiang(Peking University)

Title: Uniqueness of Positive Solutions of Nonlinear Elliptic Equations

## Abstract

Let be a bounded and regular domain. In this talk we will discuss some uniqueness results of positive solutions of the following nonlinear elliptic equation

via a kind of convexity of the associated functional

where . Some related results for the -Laplacian equation and other equations will also be discussed.

**November**24**,**** 2023. 16:00 (Beijing time)**

Speaker: Zeqian Chen(Chinese Academy of Sciences)

Title: 量子人工智能的拓扑斯理论形式

## Abstract

本报告介绍近期我们用拓扑斯理论(Topos theory)研究人工智能的工作。依据图灵测试对智能给出的科学定义, 我们将人工智能系统定义为由拓扑斯理论所描述的物理系统(Isham等人在2008年建立的物理理论), 它们具有自身的高阶形式语言及逻辑推理系统。特别是,我们用Hilbert空间上算子理论构造相应的拓扑斯以给出量子人工智能系统的数学描述。我们还将介绍量子计算的拓扑斯理论模型。

**October27****,**** 2023. 15:00 (Beijing time)**

Speaker: Fabcrice Béthuel(Sorbonne Université)

Title: On Sobolev maps between manifolds and branched transportation

## Abstract

In the talk, I wish to stress the link between branched transportation theory, and some issues in the study of Sobolev maps between manifold. In particular, I will present a counterexample to the sequential weak density of smooth maps between two manifolds 𝑀 and 𝑁 in the Sobolev space 𝑊1,𝑝 (𝑀, 𝑁), in the case 𝑝 is an integer. It has been shown quite a while ago that, if 𝑝 < 𝑚 = 𝑑𝑖𝑚(𝑀) is not an integer and the [𝑝]-th homotopy group 𝜋[𝑝] (𝑁) of 𝑁 is not trivial, [𝑝] denoting the largest integer less than 𝑝, then smooth maps are not sequentially weakly dense in 𝑊1,𝑝 (𝑀, 𝑁). On the other hand, in the case 𝑝 < 𝑚 is an integer, examples of specific manifolds 𝑀 and 𝑁 have been provided where smooth maps are sequentially weakly dense in 𝑊1,𝑝 (𝑀, 𝑁)with 𝜋[𝑝] (𝑁) ≠ 0, although they are not dense for the strong convergence. This is the case for instance for 𝑀 = 𝐵 𝑚. Such a property does not hold for arbitrary manifolds 𝑁 and integers 𝑝.

The counterexample deals with the case 𝑝=3, 𝑚 ≥ 4 and 𝑁 = 𝑆 2 , for which 𝜋3 (𝑆 2 ) = 𝑍 is related to the Hopf fibration. We provide an explicit map which is not weakly approximable in 𝑊1,3 (𝑀, 𝑆 2 ), by smooth. One of the central ingredients in our argument is related to issues in branched transportation and irrigation theory in the critical exponent case.

**October27****,**** 2023. 16:15 (Beijing time)**

Speaker: Sami Mustapha (Sorbonne Université)

Title: Potential theory and combinatorics

## Abstract

“... But the most general and direct method for resolving questions of probability consists of making them depend on difference equations ...”

The aim of this talk is to illustrate this quote from Laplace (Philosophical essay on probabilities) by developing certain aspects of the theory of discrete potential theory attached to a random walk in 𝑍 𝑑 ; the difference equations playing in this framework the same role played by the PDEs in the theory of the classical potential theory. Although the presentation will be mainly limited to simple walks in quadrants, some extensions to walks in discrete Lipschitzian domains and to inhomogeneous walks will be discussed. The emphasis will be placed on the role that tools from discrete potential theory can play (harmonic functions and discrete caloric functions, maximum principle, Harnack inequalities, boundary Harnack inequalities at the boundary) in establishing optimal estimates for the number of paths confined to a region as well as the number of excursions.

**September26****,**** 2023. 14:00 (Beijing time)**

Speaker: Adam Skalski (IMPAN)

Title: `What is a good definition? In search of quantum groups'

## Abstract

Definitions form a fundamental part of the mathematical study. I will discuss the requirements that mathematicians put on `good' definitions and explain how such definitions develop, based on the notion of a locally compact quantum group, originating in 1970s and 1980s, and reaching a (possibly?) final form in the work of Kustermans and Vaes in 2000. Later developments will also be mentioned, but in general the talk will be accessible to general mathematical audience.

About the speaker: Adam Skalski is a full professor and the scientific director of the Institute of Mathematics of of the Polish Academy of Sciences, and the chairman of ERCOM (European Research Centers on Mathematics). His areas of interest include topological quantum groups, operator algebras and quantum stochastic processes. He has been awarded the Kuratowski Prize of the Polish Mathematical Society in 2008 and the Sierpinski Prize of the Polish Academy of Sciences in 2014. He is one of the executive editors of Studia Mathematica and Banach Center Publications.

**June12****,**** 2023. 14:00 (Beijing time)**

Speaker: Xiangdong Li（Academy of Methematics and System Science，Chinese Academy of Sciences）

Title: 最优传输问题与经济资源的最优分配

## Abstract

1781年，法国数学家G. Monge从军事工程问题中提出了最优传输映射问题。1942年，苏联数学家L. Kantorovich提出了最优传输计划问题，并将其应用于国民经济资源最优分配问题的研究。1975年，Kantorovich与美国数理经济学家T. Koopmans共同分享了Nobel经济学奖。2010年和2018年，法国数学家C. Villani与意大利数学家A. Figalli先后获得菲尔兹奖，其研究工作均与最优传输问题密切有关。在本报告中，我们将回顾最优传输问题的历史发展和重要的标志性成果，并介绍Kantorovich等人关于经济资源最优分配问题的研究成果。

**May30****,**** 2023. 16:00 (Beijing time)**

Speaker: Jean-Paul Allouche（CNRS, IMJ-PRG）

Title: Playing with Kempner-like harmonic series

## Abstract

Though the series of the inverses of integers diverges, a nice,now classical, result of Kempner (in 1914) states that the sum of the inverses of the integers whose expansion in base 10 contains no occurrence of a given digit converges. Several generalizations or extensions of this result have been given. In particular a paper of Farhi (in 2008) proves the somehow unexpected result that

where s10(n) is the sum of the digits of n in base 10.

We show how to extend this result, first by replacing s10(n) with s2(n), the sum of the binary digits of the integer n; then by replacing s2(n) by any binary pattern counting sequence (i.e., any sequence aw(n) that counts the number of occurrences of a fixed block w of 0’s and 1’s in the binary expansion of the integer n).

**April 11****,**** 2023. 16:00 (Beijing time)**

Speaker: Liming Wu（Harbin Institute of Technology，l'Université Clermont-Auvergne）

Title: 马氏过程的集中不等式

## Abstract

这个报告的主题是马氏过程观察量的经验均值趋于真实均值的集中不等式，或者说随机算法的错误概率的绝对估计。我将介绍与集中不等式等价的“运费vs Fisher信息量”不等式，以及怎样使用Poincare或logSobolev不等式，与最优传输，熵这些工具去证明“运费vs Fisher信息量”不等式。作为应用例子，我将介绍高维分布Gibbs算法的错误概率绝对估计。

**March ****7****, 2023. 16:00 (Beijing time)**

Speaker: Changgui Zhang (University of Lille)

Title: Some problems about a family of linear functional differential equations

## Abstract

Given q between 0 and 1, one considers the following problems about the q-difference-differential equation y′(x) = ay(qx) + by(x) + f (x),where a and b are two complex numbers and where f is a rational function:

(1) The pantograph equations following Kato and McLeod;

(2) The indexes of the associated operator d/dx − aσ_q − b;

(3) The connection formulas between zero and infinity;

(4) The asymptotic behavior of the solutions at infinity.

This talk is partially based on a joint work with H. Dai and G. Chen (HITSZ).

**November 26****, 2022. 10:00 (Beijing time)**

Speaker: Yitang Zhang (University of California)

Title: 关于朗道-西格尔零点猜想