# Residually-finite group

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A group that can be approximated by finite groups. Let be a group and a relation (in other words, a predicate) between elements and sets of elements, defined on and all homomorphic images of it (for example, the binary relation of equality of elements, the binary relation "the element x belongs to the subgroup y" , the binary relation of conjugacy of elements, etc.). Let be a class of groups. One says that can be approximated by groups in relative to (or: is residual in relative to ) if for any elements and sets of elements of that are not in relation there is a homomorphism of onto a group in under which the images of these elements and sets are also not in relation . Approximability relative to the relation of equality of elements is simply called approximability. A group can be approximated by groups in a class if and only if it is contained in a Cartesian product of groups in . Residual finiteness relative to is denoted by ; in particular, if runs through the predicates of equality, conjugacy, belonging to a subgroup, belonging to a finitely-generated subgroup, etc., then one obtains the properties (and classes) , , , , etc. The presence of these properties in a group implies the solvability of the corresponding algorithmic problem.

#### References

 [1] M.I. Kargapolov, J.I. [Yu.I. Merzlyakov] Merzljakov, "Fundamentals of the theory of groups" , Springer (1979) (Translated from Russian)