# Cusp form

In 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 *a*_{0} 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. In all cases, though, the limit as *q* → 0 is the limit in the upper half-plane as the imaginary part of *z* → ∞. Taking the quotient by the modular group, this limit corresponds to a cusp of a modular curve (in the sense of a point added for compactification). So, the definition amounts to saying that a cusp form is a modular form that vanishes at a cusp. In the case of other groups, there may be several cusps, and the definition becomes a modular form vanishing at *all* cusps. This may involve several expansions.

The dimensions of spaces of cusp forms are, in principle, computable via the Riemann–Roch theorem. For example, the Ramanujan tau function *τ*(*n*) arises as the sequence of Fourier coefficients of the cusp form of weight 12 for the modular group, with *a*_{1} = 1. The space of such forms has dimension 1, which means this definition is possible; and that accounts for the action of Hecke operators on the space being by scalar multiplication (Mordell's proof of Ramanujan's identities). Explicitly it is the **modular discriminant**

which represents (up to a normalizing constant) the discriminant of the cubic on the right side of the Weierstrass equation of an elliptic curve; and the 24-th power of the Dedekind eta function. The Fourier coefficients here are written

and called 'Ramanujan's tau function', with the normalization *τ*(1) = 1.

In the larger picture of automorphic forms, the cusp forms are complementary to Eisenstein series, in a *discrete spectrum*/*continuous spectrum*, or *discrete series representation*/*induced representation* distinction typical in different parts of spectral theory. That is, Eisenstein series can be 'designed' to take on given values at cusps. There is a large general theory, depending though on the quite intricate theory of parabolic subgroups, and corresponding cuspidal representations.