On proportionally modular numerical semigroups that are generated by arithmetic progressions

Abstract

A numerical semigroup is a submonoid of ${\mathbb Z}_{\ge 0}$ whose complement in ${\mathbb Z}_{\ge 0}$ is finite. For any set of positive integers $a,b,c$, the numerical semigroup $S(a,b,c)$ formed by the set of solutions of the inequality $ax \bmod{b} \le cx$ is said to be proportionally modular. For any interval $[\alpha,\beta]$, $S\big([\alpha,\beta]\big)$ is the submonoid of ${\mathbb Z}_{\ge 0}$ obtained by intersecting the submonoid of ${\mathbb Q}_{\ge 0}$ generated by $[\alpha,\beta]$ with ${\mathbb Z}_{\ge 0}$. For the numerical semigroup $S$ generated by a given arithmetic progression, we characterize $a,b,c$ and $\alpha,\beta$ such that both $S(a,b,c)$ and $S\big([\alpha,\beta]\big)$ equal $S$.