A characterization and an application of weight-regular partitions of graphs

TL;DR

This paper introduces the concept of weight-regular partitions in graphs, characterizes them through stochastic matrices and polynomials, and applies these findings to improve bounds on graph chromatic numbers.

Contribution

It provides new characterizations of weight-regular partitions using stochastic matrices and Hoffman-like polynomials, extending classical results and applications.

Findings

Characterization of weight-regular partitions via double stochastic matrices

New Hoffman-like polynomial characterization of weight-regularity

Application to graphs attaining equality in Hoffman's chromatic bound

Abstract

A natural generalization of a regular (or equitable) partition of a graph, which makes sense also for non-regular graphs, is the so-called weight-regular partition, which gives to each vertex $u\in V$ a weight that equals the corresponding entry $\nu_u$ of the Perron eigenvector $\mathbf{\nu}$. This paper contains three main results related to weight-regular partitions of a graph. The first is a characterization of weight-regular partitions in terms of double stochastic matrices. Inspired by a characterization of regular graphs by Hoffman, we also provide a new characterization of weight-regularity by using a Hoffman-like polynomial. As a corollary, we obtain Hoffman's result for regular graphs. In addition, we show an application of weight-regular partitions to study graphs that attain equality in the classical Hoffman's lower bound for the chromatic number of a graph, and we show that weight-regularity provides a condition under which Hoffman's bound can be improved.