A Note on Threshold Dimension of Permutation Graphs

Abstract

A graph $G(V,E)$ is a threshold graph if there exist non-negative reals $w_v,v\in V$ and $t$ such that for every $U\subseteq V$, ${\sum }_{v\in U}{w}_v\le t$ if and only if $U$ is a stable set. The {\it threshold dimension} of a graph $G(V,E)$, denoted as $t(G)$, is the smallest integer $k$ such that $E$ can be covered by $k$ threshold spanning subgraphs of $G$. A permutation graph is a graph that can be represented as the intersection graph of a family of line segments that connect two parallel lines in the Euclidean plane. In this paper we will show that if $G$ is a permutation graph then $t(G)\le \alpha (G)$ (where $\alpha (G)$ is the cardinality of maximum independent set in $G$) and this bound is tight. As a corollary we will show that $t(G)\le \frac{n}{2}$ where $n$ is the number of vertices in the permutation graph $G$. This bound is also tight.

8 pages, 3 figures