Extremal factorization lengths of elements in commutative, cancellative semigroups

Abstract

For a numerical semigroup $S:=⟨n_1,\dots ,n_k⟩$ with minimal generators $n_1<\cdots <n_k$, Barron, O'Neill, and Pelayo showed that $L(s+n_1)=L(s)+1$ and $\ell (s+n_k)=\ell (s)+1$ for all sufficiently large $s\in S$, where $L(s)$ and $\ell (s)$ are the longest and shortest factorization lengths of $s\in S$, respectively. For some numerical semigroups, $L(s+n_1)=L(s)+1$ for all $s\in S$ or $\ell (s+n_k)=\ell (s)+1$ for all $s\in S$. In a general commutative, cancellative semigroup $S$, it is also possible to have $L(s+m)=L(s)+1$ for some atom $m$ and all $s\in S$ or to have $\ell (s+m)=\ell (s)+1$ for some atom $m$ and all $s\in S$. We determine necessary and sufficient conditions for these two phenomena. We then generalize the notions of Kunz posets and Kunz polytopes. Each integer point on a Kunz polytope corresponds to a commutative, cancellative semigroup. We determine which integer points on a given Kunz polytope correspond to semigroup in which $L(s+m)=L(s)+1$ for all $s$ and similarly which integer points yield semigroups for which $\ell (s+m)=\ell (s)+1$ for all $s$.