On the BCI Problem

Ted Dobson, Gregory Robson

Published 2024-11-12Version 1

Let $G$ be a group. The BCI problem asks whether two Haar graphs of $G$ are isomorphic if and only if they are isomorphic by an element of an explicit list of isomorphisms. We first generalize this problem in a natural way and give a theoretical way to solve the isomorphism problem for the natural generalization. We then restrict our attention to abelian groups and, with an exception, reduce the problem to the isomorphism problem for a related quotient, component, or corresponding Cayley digraph. For Haar graphs of an abelian group of odd order with connection sets $S$ those of Cayley graphs (i.e. $S = -S$), the exception does not exist. For Haar graphs of cyclic groups of odd order with connection sets those of a Cayley graph, among others, we solve the isomorphism problem.