Equivariant Gauss sum of finite quadratic forms

The classical quadratic Gauss sum can be thought of as an exponential sum attached to a quadratic form on a cyclic group. We introduce an equivariant version of Gauss sum for arbitrary finite quadratic forms, which is an exponential sum twisted by the action of the orthogonal group. We prove that simple arithmetic formulae hold for some basic classes of quadratic forms. In application, such invariant appears in the dimension formula for certain vector-valued modular forms.