Petersson inner product

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In 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 \mathbb{M}_k be the space of entire modular forms of weight k and \mathbb{S}_k the space of cusp forms.

The mapping \langle \cdot , \cdot \rangle : \mathbb{M}_k \times \mathbb{S}_k \rightarrow \mathbb{C},

\langle f , g \rangle := \int_\mathrm{F} f(\tau) \overline{g(\tau)} (\operatorname{Im}\tau)^k d\nu (\tau)

is called Petersson inner product, where

\mathrm{F} = \left\{ \tau \in \mathrm{H} : \left| \operatorname{Re}\tau \right| \leq \frac{1}{2}, \left| \tau \right| \geq 1 \right\}

is a fundamental region of the modular group Γ and for τ = x + iy

dν(τ) = y − 2dxdy

is the hyperbolic volume form.

The integral is absolutely convergent and the Petersson inner product is a positive definite Hermite form.

For the Hecke operators Tn we have:

\langle T_n f , g \rangle = \langle f , T_n g \rangle

This can be used to show that the space of cusp forms has an orthonormal basis consisting of simultaneous eigenfunctions for the Hecke operators and the Fourier coefficients of these forms are all real.

  • T.M. Apostol, Modular Functions and Dirichlet Series in Number Theory, Springer Verlag Berlin Heidelberg New York 1990, ISBN 3-540-97127-0
  • M. Koecher, A. Krieg, Elliptische Funktionen und Modulformen, Springer Verlag Berlin Heidelberg New York 1998, ISBN 3-540-63744-3
  • S. Lang, Introduction to Modular Forms, Springer Verlag Berlin Heidelberg New York 2001, ISBN 3-540-07833-9
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