For a planar graph H, let N_P(n,H) denote the maximum number of copies of H in an n-vertex planar graph. The case where H is the path on 3 vertices, H=P_3, was established by Alon and Caro. The case of H=P_4 was determined, also exactly, by Gy\H{o}ri, Paulos, Salia, Tompkins, and Zamora. In this talk, we will give the asymptotic values for H equal to P_5 and P_7 as well as the cycles C_6, C_8, C_{10} and C_{12} and discuss the general approach which can be used to compute the asymptotic value for many other graphs H.

Conference website: https://www.math.tamu.edu/conferences/combinatexas/

Structural decomposition theorems for graphs with forbidden minors and topological minors have been proved and led to many applications. Graph immersions is a notion related to graph minors and topological minors, and many analogous open problems about immersions have been proposed. In this talk we address the fundamental problem about the structure of a graph with forbidden immersions. We prove that every graph with no edge-cut of size 3 that forbids a fixed graph H as an immersion can be decomposed into graphs that are "nearly simpler" than H. The condition for having no 3-edge-cut is necessary to have a clean theorem.

Let a, b be freely independent random variables in a non-commutative probability space. Based on some considerations on bipartite graphs, we provide a formula to express the n-th free cumulant of the anticommutator ab+ba as a sum indexed by subset Y_{2n} of non-crossing partitions of {1,2,...,2n}. Specifically, Y_{2n} is the set of partitions that separate odd elements and where blocks with only even elements have even size. We will study the sets Y_{2n} and obtain new results regarding the distribution of ab+ba. For instance, the size |Y_{2n}| is closely related to the case when a,b have a Marchenko-Pastur (free Poisson) distribution of parameter 1. We will also provide a formula with the sum indexed by cacti graphs, and discuss a natural generalization to study quadratic forms in $k$ free random variables. The talk is based on the preprint arXiv:2101.09444.