# Topological properties of semigroup primes of a commutative ring

## TL;DR

This paper studies the topological structure of the space of semigroup primes of a commutative ring, revealing it as a spectral space and exploring its relationships with other spectral constructions and ring extensions.

## Contribution

It establishes that the space of semigroup primes is a spectral space, relates it to the Zariski topology on overrings, and characterizes when it is homeomorphic to other spectral spaces, especially for Bézout domains.

## Key findings

- $ extstyle ext{The space } ext{scal}(R) ext{ is spectral.}$
- $ extstyle ext{scal}(R) ext{ is a spectral retract of } oldsymbol{ ext{X}}(R).}$
- $ extstyle ext{For Bézout domains, } ext{scal}(R) ext{ is homeomorphic to } oldsymbol{ ext{X}}(R) ext{ and } ext{overr}(R).$

## Abstract

A semigroup prime of a commutative ring $R$ is a prime ideal of the semigroup $(R,\cdot)$. One of the purposes of this paper is to study, from a topological point of view, the space $\scal(R)$ of prime semigroups of $R$. We show that, under a natural topology introduced by B. Olberding in 2010, $\scal(R)$ is a spectral space (after Hochster), spectral extension of $\Spec(R)$, and that the assignment $R\mapsto\scal(R)$ induces a contravariant functor. We then relate -- in the case $R$ is an integral domain -- the topology on $\scal(R)$ with the Zariski topology on the set of overrings of $R$. Furthermore, we investigate the relationship between $\scal(R)$ and the space $\boldsymbol{\mathcal{X}}(R)$ consisting of all nonempty inverse-closed subspaces of $\spec(R)$, which has been introduced and studied in C.A. Finocchiaro, M. Fontana and D. Spirito, "The space of inverse-closed subsets of a spectral space is spectral" (submitted). In this context, we show that $\scal( R)$ is a spectral retract of $\boldsymbol{\mathcal{X}}(R)$ and we characterize when $\scal( R)$ is canonically homeomorphic to $\boldsymbol{\mathcal{X}}(R)$, both in general and when $\spec(R)$ is a Noetherian space. In particular, we obtain that, when $R$ is a B\'ezout domain, $\scal( R)$ is canonically homeomorphic both to $\boldsymbol{\mathcal{X}}(R)$ and to the space $\overr(R)$ of the overrings of $R$ (endowed with the Zariski topology). Finally, we compare the space $\boldsymbol{\mathcal{X}}(R)$ with the space $\scal(R(T))$ of semigroup primes of the Nagata ring $R(T)$, providing a canonical spectral embedding $\xcal(R)\hookrightarrow\scal(R(T))$ which makes $\xcal(R)$ a spectral retract of $\scal(R(T))$.