Вариетальные гомологии в категории нильпотентных групп

The notion of group homology came from topological context, but has proved useful for the scope of an abstract algebra [4]. In particular, the second group (co)homology group has a vast number of natural interpretations [4]. One of the natural description of the second homology groups is a Brown-Loday formula [?]:
H2(G) = ker{G ∧ G → [G,G]}.
Analogous homology theories were introduced for other algebraic categories. Later, Quillen has shown [2] that the notion of (co)homology can be extended on a broad range of categories: the only two required things is an abelianization functor and a model structure on the category of simplicial objects such that the abelianization functor is a part of Quillen’s adjunction. This had further expanded the class of different homology theories.
In particular, subcategories of groups having the several properties (i.e. varieties of groups) can be described that way. Corresponding (co)homology theories are called varietal. The specificity of them is in the different model structure: whereas for a classical theory the cofibrant objects are free groups, for varietal theories the cofibrant objects are free objects of corresponding variety. Low dimensional varietal homology was studied by Leedham-Green [11], Stammbach [3] and Baues-Pirashvili [9]. At that time the Brown-Loday formula was unknown, and the naturally arising question is how does it looks for varietal case.
In this work, we give an answer for the case of variety of nilpotent groups of degree at most k in the theorem 6.6. As an application, we give a description of nilpotent groups of degree strictly equal to k in the theorem 7.1.
2. Preliminaries
2.1. Classical results on homology. The statement we mainly refer can be for-
mulated as a next theorem [2]:
Theorem 2.1 (Quillen). Let G be a group, ab ∶ Gr → Ab is an abelianization
functor. Then, there is a natural isomorphism:
Hn+1(G) = Lnab(G).
Here L is a left derived functor in the sense of Dold-Puppe, H (G) = TorZ[G](Z, Z) nnn
is a classical definition of group homology.
We also will mainly focus on second homology group. The famous Hopf’s theorem
gives its description as a group of relations lying in commutant modulo ”obvious” relations: