Solving homogeneous linear equations over polynomial semirings

Abstract

For a subset $B$ of $\mathbb{R}$, denote by $\operatorname{U}(B)$ be the semiring of (univariate) polynomials in $\mathbb{R}[X]$ that are strictly positive on $B$. Let $\mathbb{N}[X]$ be the semiring of (univariate) polynomials with non-negative integer coefficients. We study solutions of homogeneous linear equations over the polynomial semirings $\operatorname{U}(B)$ and $\mathbb{N}[X]$. In particular, we prove local-global principles for solving single homogeneous linear equations over these semirings. We then show PTIME decidability of determining the existence of non-zero solutions over $\mathbb{N}[X]$ of single homogeneous linear equations. Our study of these polynomial semirings is largely motivated by several semigroup algorithmic problems in the wreath product $\mathbb{Z} \wr \mathbb{Z}$. As an application of our results, we show that the Identity Problem (whether a given semigroup contains the neutral element?) and the Group Problem (whether a given semigroup is a group?) for finitely generated sub-semigroups of the wreath product $\mathbb{Z} \wr \mathbb{Z}$ is decidable when elements of the semigroup generator have the form $(y, \pm 1)$.