A note on the multiplicative semigroups \(\mathbb{Z}_n\) whose bi-ideals are quasi-ideals
A bi-ideal of a semigroup \(S\) is a subsemigroup \(B\) such that \(BSB\subseteq B\). In the present note it is shown that in the multiplicative semigroup of integers modulo \(n\), every bi-ideal is an ideal if and only if \(n=4\) or \(n\) is square-free.
multiplicative semigroups of integers modulo \(n\)
scientific article; zbMATH DE number 1747998