Square-free Word-representation of Word-representable Graphs

Abstract

A graph $G=(V,E)$ is word-representable, if there exists a word w over the alphabet V such that for letters $x,y\in V$ , $x$ and $y$ alternate in $w$ if and only if $xy\in E$. In this paper, we prove that any non-empty word-representable graph can be represented by a word containing no non-trivial squares. This result provides a positive answer to the open problem present in the book Words and graphs written by Sergey Kitaev, and Vadim Lozin. Also, we prove that for a word-representable graph $G$, if the representation number of $G$ is $k$, then every $k$-uniform word representing the graph $G$ is also square-free. Moreover, we prove that every minimal-length word representing a graph is square-free. Then, we count the number of possible square-free word-representations of a complete graph. At last, using the infinite square-free string generated from the Thue-Morse sequence, we prove that infinitely many square-free words represent a non-complete connected word-representable graph.

12 pages