Counter-example to Conjectures on Complemented Zero-Divisor Graphs of Semigroups

Abstract

In this paper, we are motivated by two conjectures proposed by C. Bender et al.\ in 2024, which have remained open questions. The first conjecture states that if the complemented zero-divisor graph \( G(S) \) of a commutative semigroup \( S \) with a zero element has the clique number three or greater, then the reduced graph \( G_r(S) \) is isomorphic to the graph \( G(\mathcal{P}(n)) \). The second conjecture asserts that if \( G(S) \) is a complemented zero-divisor graph with the clique number three or greater, then \( G(S) \) is uniquely complemented. In this work, we construct a commutative semigroup \( S \) with a zero element that serves as a counter-example to both conjectures.