In mathematics, the genus is a classification of quadratic forms and lattices over the ring of integers. An integral quadratic form is a quadratic form on Z<sup>n</sup>, or equivalently a free Z-module of finite rank. Two such forms are in the same genus if they are equivalent over the local rings Z<sub>p</sub> for each prime p and also equivalent over R.
Equivalent forms are in the same genus, but the converse does not hold. For example, x<sup>2</sup> + 82y<sup>2</sup> and 2x<sup>2</sup> + 41y<sup>2</sup> are in the same genus but not equivalent over Z. Forms in the same genus have equal discriminant and hence there are only finitely many equivalence classes in a genus.
The SmithâÂÂMinkowskiâÂÂSiegel mass formula gives the weight or mass of the quadratic forms in a genus, the count of equivalence classes weighted by the reciprocals of the orders of their automorphism groups.
For binary quadratic forms there is a group structure on the set C of equivalence classes of forms with given discriminant. The genera are defined by the generic characters. The principal genus, the genus containing the principal form, is precisely the subgroup C<sup>2</sup> and the genera are the cosets of C<sup>2</sup>: so in this case all genera contain the same number of classes of forms.