T. Everaert and T. Van der Linden, Baer invariants in semi-abelian categories I: General theory, Theory Appl. Categ. 12 (2004), no. 1, 1-33.Additional comments
In Section 3, the context is not quite right for the stated results. It would probably be better to assume that the surrounding category A is pointed, exact and protomodular from the start. In particular, Proposition 3.9 actually needs that every regular epimorphism is normal, which is a consequence of protomodularity. This defect is repaired inT. Van der Linden, Homology and homotopy in semi-abelian categories, Ph.D. Thesis, Vrije Universiteit Brussel, 2006, arXiv:math/0607100There the context is also slightly enlarged (quasi-pointed instead of pointed).
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