On star-$k$-PCGs: Exploring class boundaries for small $k$ values

TL;DR

This paper investigates star-$k$-PCGs, a class of graphs defined by interval-based weight functions, determining the minimum $k$ (star number) for various small graph classes and establishing exact values for graphs up to 7 vertices.

Contribution

It precisely computes the star number for small graphs, characterizes classes like caterpillars and cycles, and explores bounds for grid graphs, advancing understanding of star-$k$-PCG class boundaries.

Findings

Small graphs with star number 2 have 4 or 5 vertices.

Small graphs with star number 3 have 7 vertices.

Caterpillars have star number 1.

Abstract

A graph $G=(V,E)$ is a star-$k$-PCG if there exists a weight function $w: V \rightarrow R^+$ and $k$ mutually exclusive intervals $I_1, I_2, \ldots I_k$, such that there is an edge $uv \in E$ if and only if $w(u)+w(v) \in \bigcup_i I_i$. These graphs are related to two important classes of graphs: PCGs and multithreshold graphs. It is known that for any graph $G$ there exists a $k$ such that $G$ is a star-$k$-PCG. Thus, for a given graph $G$ it is interesting to know which is the minimum $k$ such that $G$ is a star-$k$-PCG. We define this minimum $k$ as the star number of the graph, denoted by $\gamma(G)$. Here we investigate the star number of simple graph classes, such as graphs of small size, caterpillars, cycles and grids. Specifically, we determine the exact value of $\gamma(G)$ for all the graphs with at most 7 vertices. By doing so we show that the smallest graphs with star number 2 are only 4 and have exactly 5 vertices; the smallest graphs with star number 3 are only 3 and have exactly 7 vertices. Next, we provide a construction showing that the star number of caterpillars is one. Moreover, we show that the star number of cycles and two dimensional grid graphs is 2 and that the star number of $4$-dimensional grids is at least 3. Finally, we conclude with numerous open problems.