Actions Arising from Intersection and Union

Abstract

An action is a pair of sets, C and S, and a function $f:C\times S \rightarrow C$. Rothschild and Yalcin gave a simple axiomatic characterization of those actions arising from set intersection, i.e. for which the elements of C and S can be identified with sets in such a way that elements of S act on elements of C by intersection. We introduce and axiomatically characterize two natural classes of actions which arise from set intersection and union. In the first class, the $\uparrow \!\!\downarrow$-actions, each element of S is identified with a pair of sets $(s^\downarrow ,s^\uparrow )$, which act on a set c by intersection with $s^\downarrow$ and union with $s^\uparrow$. In the second class, the $\uparrow \!\!\downarrow$-biactions, each element of S is labeled as an intersection or a union, and acts accordingly on C. We give intuitive examples of these actions, one involving conversations and another a university’s changing student body. The examples give some motivation for considering these actions, and also help give intuitive readings of the axioms. The class of $\uparrow \!\!\downarrow$-actions is closely related to a class of single-sorted algebras, which was previously treated by Margolis et al., albeit in another guise (hyperplane arrangements), and we note this connection. Along the way, we make some useful, though very general, observations about axiomatization and representation problems for classes of algebras.