Definability of band structures on posets

The idempotent semigroups (bands) that give rise to partial orders by defining a ≤ b a · b = a are the "right-regular" bands (RRB), which are axiomatized by x· y · x = y · x. In this work we consider the class of "associative posets", which comprises all partial orders underlying right-regular bands, and study to what extent the ordering determines the possible "compatible" band structures and their canonicity. We show that the class of associative posets in the signature \ ≤ \ is not first-order axiomatizable. We also show that the Axiom of Choice is equivalent over ZF to the fact that every tree with finite branches is associative. We also present an adjunction between the categories of RRBs and that of associative posets. We study the smaller class of "normal" posets (corresponding to right-normal bands) and give a structural characterization. As an application of the order-theoretic perspective on bands, we generalize results by the third author, obtaining "inner" direct product representations for RRBs having a central (commuting) element.