Krull dimension of monomial ideals in polynomial rings with real exponents

Abstract

We develop a new technique for studying monomial ideals in the standard polynomial rings $A[X_1,\ldots,X_d]$ where $A$ is a commutative ring with identity. The main idea is to consider induced ideals in the semigroup ring $R=A[\mathbb{M}^1_{\geq 0}\times\cdots\times\mathbb{M}^d_{\geq 0}]$ where $\mathbb{M}^1,\ldots,\mathbb{M}^d$ are non-zero additive subgroups of $\mathbb{R}$. We prove that the set of non-zero finitely generated monomial ideals in $R$ has the structure of a metric space, and we prove that a version of Krull dimension for this setting is lower semicontinuous with respect to this metric space structure. We also show how to use discrete techniques to study certain monomial ideals in this context.