e/Congruence subgroup

Information
has gloss	eng: In mathematics, a congruence subgroup of a matrix group with integer entries is a subgroup defined by congruence conditions on the entries. A very simple example would be invertible 2x2 integer matrices of determinant 1, such that the off-diagonal entries are even.
lexicalization	eng: congruence subgroup
instance of	e/Modular form