Cliques in the union of $C_4$-free graphs

TL;DR

This paper investigates the structure of the union of two $C_4$-free graphs, establishing conditions under which the union forms a large clique and deriving bounds on the clique number of the union graph.

Contribution

It provides new structural results and bounds on the clique number for the union of two $C_4$-free graphs, including conditions for the union to be obedient and bounds involving the intersection graph.

Findings

If $G(B,R)$ is complete, then $V$ can be covered by specific $B$- and $R$-cliques and a clique in both.

The clique number of $G(B,R)$ is bounded by the sum of the clique numbers of $B$ and $R$ plus half the minimum of these clique numbers.

If $G(B,R)$ contains no double $C_5$, then $V$ is obedient.

Abstract

Let $B$ and $R$ be two simple graphs with vertex set $V$, and let $G(B,R)$ be the simple graph with vertex set $V$, in which two vertices are adjacent if they are adjacent in at least one of $B$ and $R$. We prove that if $B$ and $R$ are two $C_4$-free graphs on the same vertex set $V$ and $G(B,R)$ is the complete graph, then there exists an $B$-clique $X$, an $R$-clique $Y$ and a clique $Z$ in $B$ and $R$, such that $V=X\cup Y\cup Z$. Further, if $x\in Z$ then $x$ is one of the vertices of some double $C_5$ in $G(B,R)$. In particular, if also $G(B,R)$ does not contains a double $C_5$, then $V$ is obedient. We obtain that if $B$ and $R$ are $C_4$-free graphs then $\omega(G(B,R))\leq \omega(B)+\omega(R)+\frac{1}{2}\min(\omega(B),\omega(R))$ and $\omega(G(B,R))\leq \omega(B)+\omega(R)+\omega(H(B,R))$ where $H(B,R)$ is the simple graph with vertex set $V$, in which two vertices are adjacent if they are adjacent in $B$ and $R$.