Towards the Distribution of the Size of a Largest Planar Matching and Largest Planar Subgraph in Random Bipartite Graphs

TL;DR

This paper investigates the distribution of maximum planar matchings and subgraphs in random bipartite graphs, generalizing the LIS problem and establishing combinatorial identities involving lattice walks and Young tableaux.

Contribution

It introduces new combinatorial identities linking bipartite graph properties with lattice walks and Young tableaux, extending Gessel's identity to bipartite multigraphs.

Findings

Derived identities relating bipartite graph parameters to lattice walks

Connected maximum planar matchings to Young tableaux properties

Initiated pattern avoidance study in bipartite multigraphs

Abstract

We address the following question: When a randomly chosen regular bipartite multi--graph is drawn in the plane in the ``standard way'', what is the distribution of its maximum size planar matching (set of non--crossing disjoint edges) and maximum size planar subgraph (set of non--crossing edges which may share endpoints)? The problem is a generalization of the Longest Increasing Sequence (LIS) problem (also called Ulam's problem). We present combinatorial identities which relate the number of r-regular bipartite multi--graphs with maximum planar matching (maximum planar subgraph) of at most d edges to a signed sum of restricted lattice walks in Z^d, and to the number of pairs of standard Young tableaux of the same shape and with a ``descend--type'' property. Our results are obtained via generalizations of two combinatorial proofs through which Gessel's identity can be obtained (an identity that is crucial in the derivation of a bivariate generating function associated to the distribution of LISs, and key to the analytic attack on Ulam's problem). We also initiate the study of pattern avoidance in bipartite multigraphs and derive a generalized Gessel identity for the number of bipartite 2-regular multigraphs avoiding a specific (monotone) pattern.