$H$-product and $H$-threshold graphs

TL;DR

This paper introduces the concept of $H$-product and $H$-threshold graphs, providing a canonical decomposition framework for graphs based on a binary operation influenced by a digraph $H$, and characterizes graphs with small threshold-widths.

Contribution

It defines $H$-product and $H$-threshold graphs, proving unique factorization and extending properties of threshold and difference graphs.

Findings

Every $H$-product operation has a unique prime factorization.

$H$-threshold graphs generalize threshold and difference graphs.

Threshold-width is well-defined for all graphs and characterized for small values.

Abstract

This paper is the continuation of the research of the author and his colleagues of the {\it canonical} decomposition of graphs. The idea of the canonical decomposition is to define the binary operation on the set of graphs and to represent the graph under study as a product of prime elements with respect to this operation. We consider the graph together with the arbitrary partition of its vertex set into $n$ subsets ($n$-partitioned graph). On the set of $n$-partitioned graphs distinguished up to isomorphism we consider the binary algebraic operation $\circ_H$ ($H$-product of graphs), determined by the digraph $H$. It is proved, that every operation $\circ_H$ defines the unique factorization as a product of prime factors. We define $H$-threshold graphs as graphs, which could be represented as the product $\circ_{H}$ of one-vertex factors, and the threshold-width of the graph $G$ as the minimum size of $H$ such, that $G$ is $H$-threshold. $H$-threshold graphs generalize the classes of threshold graphs and difference graphs and extend their properties. We show, that the threshold-width is defined for all graphs, and give the characterization of graphs with fixed threshold-width. We study in detail the graphs with threshold-widths 1 and 2.