Choiceless Polynomial Time on Structures with Small Abelian Colour Classes

Choiceless Polynomial Time (CPT) is one of the candidates in the quest for a logic for polynomial time. It is a strict extension of fixed-point logic with counting (FPC) but to date it is unknown whether it expresses all polynomial-time properties of finite structures. We study the CPT-definability of the isomorphism problem for relational structures of bounded colour class size q (for short, q-bounded structures). Our main result gives a positive answer, and even CPT-definable canonisation procedures, for classes of q-bounded structures with small Abelian groups on the colour classes. Such classes of q-bounded structures with Abelian colours naturally arise in many contexts. For instance, 2-bounded structures have Abelian colours which shows that CPT captures Ptime on 2-bounded structures. In particular, this shows that the isomorphism problem of multipedes is definable in CPT, an open question posed by Blass, Gurevich, and Shelah.