Geometrical equivalence of nilpotent torsion free groups
  The variety of nilpotent groups is Noetherian. That is why two nilpotent class s groups are geometrically equivalent if and only if they have same quasi-identities ([Pl3]). Therefore, we can describe classes of geometrical equivalence of nilpotent groups instead of describing quasivarieties generated by single nilpotent group. The problem of geometrical equivalence can be researched by the technique of approximation of groups ([Pl2]). In this paper only nilpotent torsion free groups are considered. In the first part two sufficient conditions are presented (Theorem 1 and Theorem 2) when the nilpotent torsion free group is geometrically equivalent to its Mal'tsev completion and so it belongs to the same quasi-variety. In the second part of this paper some results are achieved by the easy methods of approximation of groups in the describing of classes of geometrical equivalence of nilpotent class 2 torsion free groups with the small rank of center. 
A.Tsurkov