Polymorphism-homogeneity and universal algebraic geometry

TL;DR

This paper explores the relationship between polymorphism-homogeneity in finite algebras and their associated algebraic geometry, establishing equivalences with properties like solution set closure and injectivity.

Contribution

It introduces a canonical relational structure for finite algebras and characterizes polymorphism-homogeneity through multiple equivalent conditions, including algebraic and categorical properties.

Findings

Polymorphism-homogeneity is equivalent to solution sets being closed under the centralizer clone.

Algebras are polymorphism-homogeneous iff they are injective in their finite subpowers category.

Explicit classifications of finite semilattices, lattices, Abelian groups, and monounary algebras satisfying these properties.

Abstract

We assign a relational structure to any finite algebra in a canonical way, using solution sets of equations, and we prove that this relational structure is polymorphism-homogeneous if and only if the algebra itself is polymorphism-homogeneous. We show that polymorphism-homogeneity is also equivalent to the property that algebraic sets (i.e., solution sets of systems of equations) are exactly those sets of tuples that are closed under the centralizer clone of the algebra. Furthermore, we prove that the aforementioned properties hold if and only if the algebra is injective in the category of its finite subpowers. We also consider two additional conditions: a stronger variant for polymorphism-homogeneity and for injectivity, and we describe explicitly the finite semilattices, lattices, Abelian groups and monounary algebras satisfying any one of these three conditions.