Determine isomorphism in the non-normal-subgroup case

Determine whether the derived graphs of voltage graphs whose voltage groups contain non-normal subgroups are isomorphic in cases where the common-cover method does not decide the question.

Background

The paper develops a method for deciding when derived graphs of finite connected voltage graphs are isomorphic. The method constructs a common cover and lifts the voltage assignments; its strongest results apply to Dedekind groups, including abelian groups.

The authors explicitly exhibit a pair of voltage graphs with voltage groups containing non-normal subgroups for which their method does not determine whether the resulting derived graphs are isomorphic. This identifies unresolved instances outside the normal-subgroup setting.