On Patterns and Languages in 1-11-Representations of Graphs
Biswajit Das, Ramesh Hariharasubramanian
TL;DR
The paper investigates encoding graphs through $1$-$11$-representations, focusing on repetition patterns such as cubes and squares. It shows that cubes cannot always be avoided in minimum-length $1$-$11$-representations, but in the permutational variant every cube can be removed to yield a cube-free encoding, with a length-multiplicity constraint guiding cube occurrences. It also reveals that squares may be unavoidable for some graphs in the minimum-length permutational setting. Furthermore, it proves that the set of all $1$-$11$-representations of a graph, and the subset of permutational representations, form regular languages via a DFA-based construction, linking graph representations to formal language theory. These results illuminate both combinatorial and language-theoretic dimensions of graph encodings and have implications for compact representations and pattern avoidance in combinatorics on words.
Abstract
A 1-11-representation of a graph $G(V,E)$ is a word over the alphabet $V$ such that two distinct vertices $x$ and $y$ are adjacent if and only if the restricted word $w{x,y}$ (obtained from $w$ by deleting all letters except $x$ and $y$) contains at most one occurrence of $xx$ or $yy$. Although every graph admits a 1-11-representation, the repetition patterns that may or must appear in such representations have not been fully studied. In this paper, we study cube-free and square-free 1-11-representations of graphs. We first show that cubes cannot always be avoided in 1-11-representations of minimum length by providing a graph for which every minimum-length 1-11-representation necessarily contains a cube. We then focus on permutational 1-11-representations, where the representing word is a concatenation of permutations of the vertex set. In this setting, we prove that any cube appearing in a permutational 1-11-representation can be removed without changing the represented graph. As a consequence, every permutational 1-11-representation attaining the permutational 1-11-representation number is cube-free. We further show that this behaviour does not extend to squares by providing a graph for which every permutational 1-11-representation with the minimum number of permutations necessarily contains a square. Finally, we prove that the language of all 1-11-representations of a given graph is regular. Moreover, we show that the language of all permutational 1-11-representations of a graph is also regular.