Events

Title: Which graphs are determined by their total numbers of walks?

Abstract: Walks and closed-walks are ubiquitous in graph theory, which capture lots of important structural information of graphs. In this talk, we shall consider the question in the title. The same question for closed walks is precisely the problem of spectral characterizations of graphs. We show that for most graphs, the number of walks determines the generalized spectrum of graphs and vice versa. Then the above question is equivalent to the problem of generalized spectral characterizations of graphs, a topic which has been studied extensively in recent years. Some simple criterions are provided for graphs G to be determined by their total number of walks, based on some recent results on the generalized spectral characterizations of graphs. Numerical results are also provided to illustrate the effectiveness of the proposed method. This is a joint work with Weifang Lv, Finjin Liu and Wei Wang.

Title: Multicommodity Flows and Cuts

Abstract: Flow and cut problems occupy a central place in discrete optimzation and algorithms. The well known max-flow min-cut theorem states that there exists a s-t flow of value d in a network if and only if the minimum capacity of edges whose removal separates s and t is at least d. In this talk, we will discuss a generalization of this problem to the multicommodity setting and study natural necessary and sufficient conditions for the existence of a feasible flow. We will discuss connections to approximation algorithms, metric embeddings and partitions, and survey some of the classical results, open problems and recent progress.

Title: Central Limit Theorems via Analytic Combinatorics in Several Variables

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm.

Abstract: Analytic combinatorics in several variables is a field of study focused on the derivation of limit behaviours of multivariate sequences. In this talk, I will describe a local central limit theorem and its applications to a variety of examples. Included in these examples will be a family of permutations with restricted cycles, integer compositions with tracked summands and n-colour compositions with tracked summands. The proof of this new, automated, local central limit theorem will also be briefly discussed, using techniques from the theory of analytic combinatorics in several variables. I will also provide background for proving central limit theorems probabilistically.

Title: Recent Progresses on Correlation Clustering

Abstract: Correlation Clustering is one of the most well-studied graph clustering problems. The input is a complete graph where each edge is labeled either "+" or "-", and the goal is to partition the vertex set into (an arbitrary number of) clusters to minimize (the number of + edges between different clusters) + (the number of - edges within the same cluster). Until recently, the best polynomial-time approximation ratio was 2.06, nearly matching the integrality gap of 2 for the standard LP relaxation. Since 2022, we have bypassed this barrier and progressively improved the approximation ratio, with the current ratio being 1.44. Based on a new relaxation inspired by the Sherali-Adams hierarchy, the algorithm introduces and combines several tools considered in different contexts, including "local to global correlation rounding" and "combinatorial preclusering". In this talk, I will describe the current algorithm as well as how it has been inspired by various viewpoints. Joint work with Nairen Cao, Vincent Cohen-Addad, Shi Li, Alantha Newman, and Lukas Vogl.

Title: A general theorem in spectral extremal graph theory

Abstract: The extremal graphs $\mathrm{EX}(n,\mathcal F)$ and spectral extremal graphs $\mathrm{SPEX}(n,\mathcal F)$ are the sets of graphs on $n$ vertices with maximum number of edges and maximum spectral radius, respectively, with no subgraph in $\mathcal F$. We prove a general theorem which allows us to characterize the spectral extremal graphs for a wide range of forbidden families $\mathcal F$ and implies several new and existing results. In particular, whenever $\mathrm{EX}(n,\mathcal F)$ contains the complete bipartite graph $K_{k,n-k}$ (or certain similar graphs) then $\mathrm{SPEX}(n,\mathcal F)$ contains the same graph when $n$ is sufficiently large. We prove a similar theorem which relates $\mathrm{SPEX}(n,\mathcal F)$ and $\mathrm{SPEX}_\alpha(n,\mathcal F)$, the set of $\mathcal F$-free graphs which maximize the spectral radius of the matrix $A_\alpha=\alpha D+(1-\alpha)A$, where $A$ is the adjacency matrix and $D$ is the diagonal degree matrix.

Title: Rate-distortion theory for quantum data compression

Abstract: Quantum data compression is a fundamental quantum information processing task, where the sender compresses many copies of a given quantum state into the smallest possible storage space before transmitting it to the receiver. Among various quantum data compression setups and analyses, the trade-off between the efficiency (rate) and the error of the compression has been investigated using the framework of quantum rate-distortion theory, in which a small error is tolerated to improve the rate. In this talk, adopting the quantum rate-distortion theory, we reveal the optimal rate-error trade-off of quantum data compression with and without the assistance of quantum entanglement. Moreover, we generalize these results to figure out the full rate region where both the communication and entanglement rates vary. Thus, we successfully obtain a complete form of the rate-distortion theory of quantum data compression with entanglement.

Title: Self-dual axioms for infinite matroids on lattices

Abstract: In an informal sequel to my recent Tutte colloquium on the topic, I will talk about axiomatizing infinite matroids with a modular lattice in place of the usual ground set. I will present three sets of axioms that characterize these objects; two of the axiom sets are self-dual.

Title: Chord diagrams, triangulations, and $\phi^p$ amplitudes.

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm.

Abstract: I will discuss some combinatorics of non-crossing chord diagrams that arose in the context of proving a conjecture from my coauthor's thesis which said that the global Schwinger formula of Cachazo and Early could be decomposed into a sum over cones indexed by non-crossing chord diagrams and that the amplitudes can be read off the chord diagrams by a triangulation construction.

For this talk I will focus on the combinatorics, but will hand wave over the physics at some point between the seminar and the pre-seminar.

Joint work with Bruno Giménez Umbert.

Title: Stochastic Minimum Norm Combinatorial Optimization

Abstract: We develop a framework for designing approximation algorithms for a wide class of (1-stage) stochastic-optimization problems with norm-based objective functions. We introduce the model of stochastic minimum-norm combinatorial optimization, wherein the costs involved are random variables with given distributions, and we are given a monotone, symmetric norm f. Each feasible solution induces a random multidimensional cost vector whose entries are independent random variables, and the goal is to find a solution that minimizes the expected f-norm of the induced cost vector. This is a very rich class of objectives, containing all l_p norms, as also Top-l norms (sum of l largest coordinates in absolute value), which enjoys various closure properties.

Title: Webs and quantum representations

Abstract: A web is a directed, labeled plane graph satisfying certain conditions coming from representation theory. Each web corresponds to a specific invariant vector in a tensor product of fundamental representations of a quantum group. In this talk, we introduce a process called stranding, which encodes as the monomial terms in a web’s associated vector as a collection of paths in the web graph.  We also describe how the strandings connect seemingly-unrelated ideas in combinatorics (e.g. noncrossing matchings) and geometry (e.g. certain algebraic varieties called Springer fibers).