# Cuspidal representation

In number theory, cuspidal representations are certain representations of algebraic groups that occur discretely in $L^2$ spaces. The term cuspidal is derived, at a certain distance, from the cusp forms of classical modular form theory. In the contemporary formulation of automorphic representations, representations take the place of holomorphic functions; these representations may be of adelic algebraic groups.

When the group is the general linear group $\operatorname{GL}_2$, the cuspidal representations are directly related to cusp forms and Maass forms. For the case of cusp forms, each Hecke eigenform (newform) corresponds to a cuspidal representation.

## Formulation

Let G be a reductive algebraic group over a number field K and let A denote the adeles of K. Let Z denote the centre of G and let ω be a continuous unitary character from Z(K)\Z(A)× to C×. Fix a Haar measure on G(A) and let L20(G(K)\G(A), ω) denote the Hilbert space of measurable complex-valued functions, f, on G(A) satisfying

1. fg) = f(g) for all γ ∈ G(K)
2. f(gz) = f(g)ω(z) for all zZ(A)
3. $\int_{Z(\mathbf{A})G(K)\backslash G(\mathbf{A})}|f(g)|^2\,dg < \infty$
4. $\int_{U(K)\backslash U(\mathbf{A})}f(ug)\,du=0$ for all unipotent radicals, U, of all proper parabolic subgroups of G(A).

This is called the space of cusp forms with central character ω on G(A). A function occurring in such a space is called a cuspidal function. This space is a unitary representation of the group G(A) where the action of gG(A) on a cuspidal function f is given by

$(g\cdot f)(x)=f(xg).$

The space of cusp forms with central character ω decomposes into a direct sum of Hilbert spaces

$L^2_0(G(K)\backslash G(\mathbf{A}),\omega)=\hat{\bigoplus}_{(\pi,V_\pi)}m_\pi V_\pi$

where the sum is over irreducible subrepresentations of L20(G(K)\G(A), ω) and mπ are positive integers (i.e. each irreducible subrepresentation occurs with finite multiplicity). A cuspidal representation of G(A) is such a subrepresentation (π, V) for some ω.

The groups for which the multiplicities mπ all equal one are said to have the multiplicity-one property.

## References

• James W. Cogdell, Henry Hyeongsin Kim, Maruti Ram Murty. Lectures on Automorphic L-functions (2004), Chapter 5.