Monads on Categories of Relational Structures

TL;DR

This paper develops a universal algebra framework for categories of relational structures, establishing a correspondence between enriched monads and algebraic theories with operations of bounded arity, applicable to structures like partial orders and metric spaces.

Contribution

It introduces a bijective correspondence between accessible enriched monads and algebraic theories in relational categories, extending algebraic syntax and semantics to structures like metric spaces.

Findings

Established a bijective correspondence between monads and algebraic theories.

Provided a sound and complete derivation system for relational algebraic theories.

Presented an $oldsymbol{oldsymbol{oldsymbol{ ext{ extomega}_1}}}$-ary algebraic theory of metric completion.

Abstract

We introduce a framework for universal algebra in categories of relational structures given by finitary relational signatures and finitary or infinitary Horn theories, with the arity $\lambda$ of a Horn theory understood as a strict upper bound on the number of premisses in its axioms; key examples include partial orders ($\lambda=\omega$) or metric spaces ($\lambda=\omega_1$). We establish a bijective correspondence between $\lambda$-accessible enriched monads on the given category of relational structures and a notion of $\lambda$-ary algebraic theories (i.e. with operations of arity $<\lambda$), with the syntax of algebraic theories induced by the relational signature (e.g. inequations or equations-up-to-$\epsilon$). We provide a generic sound and complete derivation system for such relational algebraic theories, thus in particular recovering (extensions of) recent systems of this type for monads on partial orders and metric spaces by instantiation. In particular, we present an $\omega_1$-ary algebraic theory of metric completion. The theory-to-monad direction of our correspondence remains true for the case of $\kappa$-ary algebraic theories and $\kappa$-accessible monads for $\kappa<\lambda$, e.g. for finitary theories over metric spaces.