We study certain subgroups
of
defined by nonsingular
-matrices
with integer coefficients. In the first nontrivial case when
,
we give necessary and sufficient conditions for two such groups to be
isomorphic. Namely, in the generic case when the characteristic polynomial of
is irreducible, we attach a generalized ideal class to
and
essentially, two groups are isomorphic if and only if the corresponding ideal classes are
equivalent. The obtained results can be applied to studying associated toroidal
solenoids.