Tensor products and relation quantales

TL;DR

This paper explores generalized tensor products of lattices and posets, revealing their structure as quantales under certain conditions, and establishing connections to relation algebras and categorical frameworks.

Contribution

It introduces new tensor product constructions for closure spaces and posets, characterizes when these form quantales, and links pseudocomplemented lattices to categorical and algebraic structures.

Findings

Tensor products of complete lattices relate to Galois maps and join-meet duality.

A tensor product becomes a quantale iff the lattice is pseudocomplemented.

The category of atomic boolean lattices is equivalent to sets and relations.

Abstract

A classical tensor product $A \,\otimes\, B$ of complete lattices $A$ and $B$, consisting of all down-sets in $A \times B$ that are join-closed in either coordinate, is isomorphic to the complete lattice $Gal(A,B)$ of Galois maps from $A$ to $B$, turning arbitrary joins into meets. We introduce more general kinds of tensor products for closure spaces and for posets. They have the expected universal property for bimorphisms (separately continuous maps or maps preserving restricted joins in the two components) into complete lattices. The appropriate ingredient for quantale constructions is here distributivity at the bottom, a generalization of pseudo\-complementedness. We show that the truncated tensor product of a complete lattice $B$ with itself becomes a quantale with the closure of the relation product as multiplication iff $B$ is pseudocomplemented, and the tensor product has a unit element iff $B$ is atomistic. The pseudocomplemented complete lattices form a semicategory in which the hom-set between two objects is their tensor product. The largest subcategory of that semicategory has as objects the atomic boolean complete lattices, which is equivalent to the category of sets and relations. More general results are obtained for closure spaces and posets.