Minimal normal graph covers

Abstract

A graph is normal if it admits a clique cover $\mathcal C$ and a stable set cover $\mathcal S$ such that each clique in $\mathcal C$ and each stable set in $\mathcal S$ have a vertex in common. The pair $(\mathcal{C,S})$ is a normal cover of the graph. We present the following extremal property of normal covers. For positive integers $c,s$, if a graph with $n$ vertices admits a normal cover with cliques of sizes at most $c$ and stable sets of sizes at most $s$, then $c+s\geq\log_2(n)$. For infinitely many $n$, we also give a construction of a graph with $n$ vertices that admits a normal cover with cliques and stable sets of sizes less than $0.87\log_2(n)$. Furthermore, we show that for all $n$, there exists a normal graph with $n$ vertices, clique number $\Theta(\log_2(n))$ and independence number $\Theta(\log_2(n))$. When $c$ or $s$ are very small, we can describe all normal graphs with the largest possible number of vertices that allow a normal cover with cliques of sizes at most $c$ and stable sets of sizes at most $s$. However, such extremal graphs remain elusive even for moderately small values of $c$ and $s$.