The Structure of a Graph Inverse Semigroup

TL;DR

This paper explores the algebraic structure of graph inverse semigroups derived from directed graphs, classifying their congruences, representations, and homomorphisms to deepen understanding of their semigroup-theoretic properties.

Contribution

It provides a detailed classification of congruences on G(E), characterizes when G(E) has only Rees congruences, and establishes conditions for extending graph homomorphisms to semigroup homomorphisms.

Findings

Classified non-Rees congruences on G(E)

Proved quotient of G(E) by Rees congruence is another G(E)

Determined minimal degree of faithful partial transformation representations

Abstract

Given any directed graph E one can construct a graph inverse semigroup G(E), where, roughly speaking, elements correspond to paths in the graph. In this paper we study the semigroup-theoretic structure of G(E). Specifically, we describe the non-Rees congruences on G(E), show that the quotient of G(E) by any Rees congruence is another graph inverse semigroup, and classify the G(E) that have only Rees congruences. We also find the minimum possible degree of a faithful representation by partial transformations of any countable G(E), and we show that a homomorphism of directed graphs can be extended to a homomorphism (that preserves zero) of the corresponding graph inverse semigroups if and only if it is injective.