Hecke operatorIn mathematics, in particular in the theory of modular forms, a Hecke operator, studied by , is a certain kind of "averaging" operator that plays a significant role in the structure of vector spaces of modular forms and more general automorphic representations. used Hecke operators on modular forms in a paper on the special cusp form of Ramanujan, ahead of the general theory given by .
Modular groupIn mathematics, the modular group is the projective special linear group of 2 × 2 matrices with integer coefficients and determinant 1. The matrices A and −A are identified. The modular group acts on the upper-half of the complex plane by fractional linear transformations, and the name "modular group" comes from the relation to moduli spaces and not from modular arithmetic. The modular group Γ is the group of linear fractional transformations of the upper half of the complex plane, which have the form where a, b, c, d are integers, and ad − bc = 1.
Petersson inner productIn mathematics the Petersson inner product is an inner product defined on the space of entire modular forms. It was introduced by the German mathematician Hans Petersson. Let be the space of entire modular forms of weight and the space of cusp forms. The mapping , is called Petersson inner product, where is a fundamental region of the modular group and for is the hyperbolic volume form. The integral is absolutely convergent and the Petersson inner product is a positive definite Hermitian form.
Cusp formIn number theory, a branch of mathematics, a cusp form is a particular kind of modular form with a zero constant coefficient in the Fourier series expansion. A cusp form is distinguished in the case of modular forms for the modular group by the vanishing of the constant coefficient a0 in the Fourier series expansion (see q-expansion) This Fourier expansion exists as a consequence of the presence in the modular group's action on the upper half-plane via the transformation For other groups, there may be some translation through several units, in which case the Fourier expansion is in terms of a different parameter.
Hilbert modular formIn mathematics, a Hilbert modular form is a generalization of modular forms to functions of two or more variables. It is a (complex) analytic function on the m-fold product of upper half-planes satisfying a certain kind of functional equation. Let F be a totally real number field of degree m over the rational field. Let be the real embeddings of F. Through them we have a map Let be the ring of integers of F. The group is called the full Hilbert modular group.
