References

We will recall basic properties and some examples of generalized central exten-sions defined via Huq commutators [2] (see also [1]) in semi-abelian categories. Those properties will be used to prove that the composites of central extensions determine a relative semi-abelian category structure [3] on the ground category. We will then briefly describe how the relative versions of basic homological lem-mas [4,5] apply to this case