Convex spaces, affine spaces, and commutants for algebraic theories

Abstract

Certain axiomatic notions of $\textit{affine space}$ over a ring and $\textit{convex space}$ over a preordered ring are examples of the notion of $\mathcal{T}$-algebra for an algebraic theory $\mathcal{T}$ in the sense of Lawvere. Herein we study the notion of $\textit{commutant}$ for Lawvere theories that was defined by Wraith and generalizes the notion of $\textit{centralizer clone}$. We focus on the Lawvere theory of $\textit{left $R$-affine spaces}$ for a ring or rig $R$, proving that this theory can be described as a commutant of the theory of pointed right $R$-modules. Further, we show that for a wide class of rigs $R$ that includes all rings, these theories are commutants of one another in the full finitary theory of $R$ in the category of sets. We define $\textit{left $R$-convex spaces}$ for a preordered ring $R$ as left affine spaces over the positive part $R_+$ of $R$. We show that for any $\textit{firmly archimedean}$ preordered algebra $R$ over the dyadic rationals, the theories of left $R$-convex spaces and pointed right $R_+$-modules are commutants of one another within the full finitary theory of $R_+$ in the category of sets. Applied to the ring of real numbers $\mathbb{R}$, this result shows that the connection between convex spaces and pointed $\mathbb{R}_+$-modules that is implicit in the integral representation of probability measures is a perfect `duality' of algebraic theories.