Combinatorics Seminar

Upcoming talks


Past talks

  • Thursday (Dec 21, 2023), 3:00-4:00 pm: Xiaofan Yuan (Arizona State University)
    Rainbow Connectivity in Edge-colored Graphs
    Abstract(Click to expand) Fujita and Magnant proved that if the minimum color degree of an edge-colored graph $(G,c)$ of order $n (>2)$ is at least $n/2$, then each pair of vertices in $(G,c)$ is connected by a properly-colored path. We show that for large $n$, the same minimum color degree condition implies rainbow connectivity of the edge-colored graph; that is, each pair of vertices in $(G,c)$ is connected by a path whose edges are distinctly colored. This is joint work with Andrzej Czygrinow.

  • Tuesday (Jun 27, 2023), 10:30-11:30 pm: Peng Zhang (Rutgers University)
    Splitting a special class of circulant graphs
    Abstract(Click to expand) Given a graph $G = (V,E)$ where each edge has effective resistance (treating $G$ as an electrical network where each edge is a resistor) at most $\alpha > 0$, we want to partition the edges of $G$ into $E = E_1 \cup E_2$ so that both graphs $G_1 = (V,E_1)$ and $G_2 = (V,E_2)$ with doubled edge weights are $(1 +/- O(\sqrt{\alpha}))$ approximation to $G$. We say a graph $H$ is a $(1+/-\epsilon)$ approximation to another graph $G$ if the Laplacian matrix of $H$, denoted by $L_H$, and the Laplacian matrix of $G$, denoted by $L_G$, satisfies $(1-\epsilon) L_G \preceq L_H \preceq (1+\epsilon) L_G$ (the Loewner order). The proof of the Kadison-Singer problem by Marcus, Spielman, and Srivastava (Annals of Mathematics, 2015) implies the above edge partition exists, but no polynomial-time algorithms are known. We will describe a polynomial-time algorithm for a special class of circulant graphs.

  • Tuesday (May 30, 2023), 4-5 pm: Luoxin Zhang (National University of Singapore)
    Network Classes and Their Mathematical Properties
    Abstract(Click to expand) Phylogenetic networks over a set of taxa are rooted, directed acyclic graphs in which leaves represent the taxa, the non-leaf indegree-1 nodes represent speciation events and the nodes with multiple incoming edges represent reticulation events when they are used to model the evolutionary history of a set of genomes. In the study of algorithmic issues of phylogenetic networks, different classes of phylogenetic networks have been investigated. The popular network classes include galled network, tree-child network, tree-based network, etc. I will discuss our recent work on how to count tree-child networks and its subclasses.

  • Friday (Apr 7, 2023), 4:30-5:30 pm: Zhi-Wei Sun (Nanjing University)
    A new result similar to the Graham-Pollak theorem
    Abstract(Click to expand) In 1971, R.L. Graham and H.O. Pollak obtained a beautiful result on determinants of distance matrices for trees. In this talk, we introduce the speaker's recent work which is similar to the Graham-Pollak theorem. Namely, we will prove the following result:Let $n > 1$ be an integer, and let $T$ be any tree with $n+1$ vertices $v_1,\ldots,v_{n+1}$, where $v_1$ and $v_{n+1}$ are two leaves of $T$. Then $$\det[q^{d(v_{j+1},v_k)}-t]_{1\le j,k\le n}=t(q-1)^n(q+1)^{n-2},$$ where $d(v_{j+1},v_k)$ is the distance between $v_{j+1}$ and $v_k$ in the tree $T$.

    Talks from:   2022