The covariety of numerical semigroups with some prescribed pseudo-Frobenius numbers

Abstract

If $S$ is a numerical semigroup, then we denote by $m(S)$, $F(S)$ and $\mathrm{PF}(S)$ its multiplicity, its Frobenius number and its set of pseudo-Frobenius numbers, respectively. Let $\mathcal P$ be a finite nonempty set of positive integers. In this paper, we prove that $\mathcal C(\mathcal P)=\{S\mid F(S)=\max(\mathcal P),\ \mathcal P\subseteq\mathrm{PF}(S)\}$ is a covariety. The associated tree provides algorithms to compute $\mathcal C(\mathcal P)$, its elements with a prescribed type or genus, and the set of numerical semigroups $S$ satisfying $\mathrm{PF}(S)=\mathcal P$. We show that every element of $\mathcal C(\mathcal P)$ has a unique minimal system of generators relative to $\mathcal C(\mathcal P)$, and we call its cardinality the $\mathcal C(\mathcal P)$-rank. Finally, we characterize the elements of $\mathcal C(\mathcal P)$ with $\mathcal C(\mathcal P)$-rank one.