Structural Properties of Word Representable Graphs

Abstract

Given a word $w=w_1w_2\cdots w_n$ of length n over an ordered alphabet $\Sigma _k$, we construct a graph $G(w)=(V(w), E(w))$ such that V(w) has n vertices labeled $1, 2,\ldots , n$ and for $i, j \in V(w)$, $(i, j) \in E(w)$ if and only if $w_iw_j$ is a scattered subword of w of the form $a_{t}a_{t+1}$, $a_t \in \Sigma _k$, for some $1 \le t \le k-1$ with the ordering $a_t<a_{t+1}$. A graph is said to be Parikh word representable if there exists a word w over $\Sigma _k$ such that $G=G(w)$. In this paper we characterize all Parikh word representable graphs over the binary alphabet in terms of chordal bipartite graphs. It is well known that the graph isomorphism (GI) problem for chordal bipartite graph is GI complete. The GI problem for a subclass of (6, 2) chordal bipartite graphs has been addressed. The notion of graph powers is a well studied topic in graph theory and its applications. We also investigate a bipartite analogue of graph powers of Parikh word representable graphs. In fact we show that for G(w), $G(w)^{[3]}$ is a complete bipartite graph, for any word w over binary alphabet.