Organizers

The SUSTech Discrete Mathematics Seminar is organized by Ferdinand Ihringer, Caiheng Li, Qing Xiang, and Ziqing Xiang.

You can contact us under discretemath@sustech.edu.cn.

Talks

Speaker: Chi Hoi Yip (Hong Kong University of Science and Technology)
Room: College of Science M1001
Time: 2026/11/19, Thursday, 10:00 - 11:00
Tencent Meeting: 549 433 668

TBA

Speaker: Weigen Yan (Jimei University)
Room: College of Science M1001
Time: 2026/11/26, Thursday, 10:00 - 11:00
Tencent Meeting: 926 251 280

TBA

Recent Talks

On r-cross t-intersecting families for vector spaces

Speaker: Benjian Lv (Beijing Normal University)
Room: College of Science M1001
Time: 2026/11/05, Thursday, 10:00 - 11:00
Tencent Meeting: 112 151 550

Let V be an n-dimensional vector space over the finite field 𝔽q, and denote by [V over k] the family of all k-dimensional subspaces of V. We say that the families 𝓕₁ ⊆ [V over k₁], 𝓕₂ ⊆ [V over k₂], …, 𝓕ᵣ ⊆ [V over kᵣ] are r-cross t-intersecting ifdim(F₁ ∩ F₂ ∩ ⋯ ∩ Fᵣ) ≥ t for all Fᵢ ∈ 𝓕ᵢ, i = 1, 2, …, r. In this talk, I will introduce some results about the extremal structures of such families.

Eigenvalue Multiplicities and Eigenvalue Distributions of Graphs

Speaker: Zhibin Du (South of China Normal University)
Room: College of Science M1001
Time: 2026/10/29, Thursday, 10:00 - 11:00
Tencent Meeting: 589 762 461

The study of eigenvalues is a central topic in spectral graph theory. Eigenvalue multiplicities and eigenvalue distributions reveal important structural properties of graphs and have deep connections with graph invariants such as connectivity, diameter, independence number, and symmetry. In this talk, we present several classical and recent results concerning the multiplicities of graph eigenvalues and the distribution of eigenvalues in graph spectra. We discuss methods for determining eigenvalue multiplicities, characterizing graphs with prescribed spectral properties, and establishing relationships between spectral distributions and graph structures. Applications to graph characterization will also be highlighted.

Nonregular graphs with a given maximum degree attaining the maximum spectral radius

Speaker: Zejun Huang (Shenzhen University)
Room: College of Science M1001
Time: 2026/10/22, Thursday, 10:00 - 11:00
Tencent Meeting: 480 463 529

Let G be a connected nonregular graphs of order n with maximum degree k that attains the maximum spectral radius. In 2008, Liu and Li proposed a conjecture stating that G has a degree sequence (k,…,k,t) with t=k-1 or k-2. In this talk, I will present some progress on this conjecture. This talk is based on joint work with Jiahui Liu and Chenxi Yang.

Hardness and Algorithms for Packing Steiner Trees in Digraphs

Speaker: Yuefang Sun (Ningbo University)
Room: College of Science M1001
Time: 2026/10/15, Thursday, 10:00 - 11:00
Tencent Meeting: 267 575 018

Packing combinatorial objects—such as graphs, digraphs, and hypergraphs—by smaller ones is one of the central problems in graph theory and combinatorial optimization. Among these, the Steiner tree packing problem stands out not only for its theoretical significance but also for its practical relevance, particularly in VLSI circuit design. Over the past decades, it has attracted much attention from researchers across graph theory, combinatorial optimization, and theoretical computer science, and has matured into a well-established area. In this talk, we survey known hardness and algorithmic results for the directed Steiner tree packing problem, along with several related topics. This presentation is based on joint work with Anders Yeo, Shanshan Yu, and Xiaoyan Zhang.

Coxeter Condorcet domains

Speaker: Yibo Gao (Peking University)
Room: College of Science M1001
Time: 2026/10/08, Thursday, 10:00 - 11:00
Tencent Meeting: 876 147 079

In election, the Condorcet paradox says that pairwise majority comparisons can yield cyclic outcomes. The prototypical example is a voting profile consisting of three candidates a,b,c and three votes a>b>c, b>c>a and c>a>b. A Condorcet domain is a set of votes where such local configurations do not appear. In this project, we extend the theory of Condorcet domains to the broader setting of finite Coxeter groups. The core contribution of our approach is the introduction of Condorcet root posets, defined on the chosen root systems. Notably, we establish a natural bijection between closed Condorcet domains and Condorcet root posets, Furthermore, these posets give a unified language that efficiently captures a wide range of desirable properties of Condorcet domains, such as being maximal, connected, peak-pit, and of tiling type. Most of our results are novel in type A. This is joint work with Yulin Peng.