Word-representability of split graphs generated by morphisms

TL;DR

This paper characterizes when split graphs are word-representable, especially those generated by morphisms, providing a matrix-based classification and advancing understanding of their combinatorial properties.

Contribution

It offers a new characterization of word-representable split graphs using permutation matrices and classifies those generated by 2x2 morphisms.

Findings

Characterization of word-representable split graphs via permutation matrices

Complete classification of split graphs generated by 2x2 morphisms

Theoretical framework connecting morphisms and word-representability

Abstract

A graph $G=(V,E)$ is word-representable if and only if there exists a word $w$ over the alphabet $V$ such that letters $x$ and $y$, $x\neq y$, alternate in $w$ if and only if $xy\in E$. A split graph is a graph in which the vertices can be partitioned into a clique and an independent set. There is a long line of research on word-representable graphs in the literature, and recently, word-representability of split graphs has attracted interest. In this paper, we first give a characterization of word-representable split graphs in terms of permutations of columns of the adjacency matrices. Then, we focus on the study of word-representability of split graphs obtained by iterations of a morphism, the notion coming from combinatorics on words. We prove a number of general theorems and provide a complete classification in the case of morphisms defined by $2\times 2$ matrices.