Torsion factors of commutative monoid semirings

Abstract

Let $P$ be a finitely generated commutative semiring. It was shown recently that if $P$ is a parasemifield (i.e. the multiplicative reduct of $P$ is a group) then $P$ cannot contain the positive rationals $\mathbb{Q}^+$ as its subsemiring. Equivalently, a commutative parasemifield $P$ finitely generated as a semiring is additively divisible if and only if $P$ is additively idempotent. We generalize this result using weaker forms of these additive properties to a broader class of commutative semirings in the following way. Let $S$ be a semiring that is a factor of a monoid semiring $\mathbb{N}[\mathcal{C}]$ where $\mathcal{C}$ is a submonoid of a free commutative monoid of finite rank. Then the semiring $S$ is additively almost-divisible if and only if $S$ is torsion. In particular, we show that if $S$ is a ring then $S$ cannot contain any non-finitely generated subring of $\mathbb{Q}$.