Martins-Ferreira, Nelson2025-05-202025-05-202016-10-132193-84071512-2891http://hdl.handle.net/10400.8/12936We introduce the notion ofWhitehead sequence which is defined for a base category together with a system of abstract actions over it. In the classical case of groups and group actions theWhitehead sequences are precisely the crossed modules of groups. For a general setting we give sufficient conditions for the existence of a categorical equivalence between internal groupoids and Whitehead sequences.engWhitehead sequenceAction-systemOrganic morphismExact-right-patchProtomodular categoryCrossed moduleSimplicial objectInternal groupoidjournal article10.1007/s40062-016-0159-6