Maass wave form

Short description: Complex-valued smooth functions of the upper half plane (harmonic analysis topic)

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In mathematics, Maass forms or Maass wave forms are studied in the theory of automorphic forms. Maass forms are complex-valued smooth functions of the upper half plane, which transform in a similar way under the operation of a discrete subgroup Γ of SL2(ℝ) as modular forms. They are eigenforms of the hyperbolic Laplace operator Δ defined on the upper half plane and satisfy certain growth conditions at the cusps of a fundamental domain of Γ. In contrast to modular forms, Maass forms need not be holomorphic. They were studied first by Hans Maass in 1949.

## Contents

* 1 General remarks
* 2 Classic Maass forms
* 2.1 Definition of the hyperbolic Laplace operator
* 2.2 Definition of a Maass form
* 2.3 Relation between Maass forms and Dirichlet series
* 2.3.1 Theorem: The L-function of a Maass form
* 3 Example: The non-holomorphic Eisenstein-series E
* 3.1 E is a Maass form
* 3.1.1 The Fourier-expansion of E
* 4 Maass forms of weight k
* 4.1 Congruence subgroups
* 4.2 Maass forms of weight k
* 4.3 The spectral problem
* 4.4 Embedding into the space L2(Γ \ G)
* 5 Maass cusp form
* 5.1 Definition
* 5.2 Major results
* 5.3 Higher dimensions
* 6 Automorphic representations of the adele group
* 6.1 The group GL2(A)
* 6.2 Adelisation of cuspforms
* 6.3 Cusp forms of the adele group
* 6.4 Cuspidal representations of the adele group
* 6.5 Automorphic L-functions
* 7 See also
* 8 References

## General remarks

The group

G:=SL2(ℝ)={(abcd)∈M2(ℝ):ad−bc=1}

operates on the upper half plane

ℋ={z∈ℂ:Im⁡(z)>0}

by fractional linear transformations:

(abcd)⋅z:=az+bcz+d.

It can be extended to an operation on ℋ∪{∞}∪ℝ by defining:

(abcd)⋅z:={az+bcz+dif cz+d≠0,∞if cz+d=0,

(abcd)⋅∞:=limIm⁡(z)->∞(abcd)⋅z={acif c≠0∞if c=0

The Radon measure

dμ(z):=dxdyy2

defined on ℋ is invariant under the operation of SL2(ℝ).

Let Γ be a discrete subgroup of G. A fundamental domain for Γ is an open set F⊂ℋ, so that there exists a system of representatives R of Γ∖ℋ with

F⊂R⊂F‾ and μ(F‾∖F)=0.

A fundamental domain for the modular group Γ(1):=SL2(ℤ) is given by

F:={z∈ℋ∣|Re⁡(z)|<12,|z|>1}

(see Modular form).

A function f:ℋ->ℂ is called Γ-invariant, if f(γz)=f(z) holds for all γ∈Γ and all z∈ℋ.

For every measurable, Γ-invariant function f:ℋ->ℂ the equation

∫Ff(z)dμ(z)=∫Γ∖ℋf(z)dμ(z),

holds. Here the measure dμ on the right side of the equation is the induced measure on the quotient Γ∖ℋ.

## Classic Maass forms

### Definition of the hyperbolic Laplace operator

The hyperbolic Laplace operator on ℋ is defined as

Δ:C∞(ℋ)->C∞(ℋ),
Δ=−y2(∂2∂x2+∂2∂y2)

### Definition of a Maass form

A Maass form for the group Γ(1):=SL2(ℤ) is a complex-valued smooth function f on ℋ satisfying

1. f(γz)=f(z) for all γ∈Γ(1),z∈ℋ
2. there exists λ∈ℂ with Δ(f)=λf
3. there exists N∈ℕ with f(x+iy)=𝒪(yN) for y≥1

∫01f(z+t)dt=0 for all z∈ℋ

we call f Maass cusp form.

### Relation between Maass forms and Dirichlet series

Let f be a Maass form. Since

γ:=(1101)∈Γ(1)

we have:

∀z∈ℋ:f(z)=f(γz)=f(z+1).

Therefore f has a Fourier expansion of the form

f(x+iy)=∑n=−∞∞an(y)e2πinx,

with coefficient functions an,n∈ℤ.

It is easy to show that f is Maass cusp form if and only if a0(y)=0∀y>0.

We can calculate the coefficient functions in a precise way. For this we need the Bessel function Kv.

Definition: The Bessel function Kv is defined as

Ks(y):=12∫0∞e−y(t+t−1)2tsdtt,s∈ℂ,y>0.

The integral converges locally uniformly absolutely for y>0 in s∈ℂ and the inequality

Ks(y)≤e−y2KRe⁡(s)(2)

holds for all y>4.

Therefore, |Ks| decreases exponentially for y->∞. Furthermore, we have K−s(y)=Ks(y) for all s∈ℂ,y>0.

Theorem (Fourier coefficients of Maass forms) -- Let λ∈ℂ be the eigenvalue of the Maass form f corresponding to Δ. There exist ν∈ℂ, unique up to sign, such that λ=14−ν2. Then the Fourier coefficients of f are an(y)=cnyKν(2π|n|y)cn∈ℂn≠0a0(y)=c0y12−ν+d0y12+νc0,d0∈ℂn=0

Proof: We have

Δ(f)=(14−ν2)f.

By the definition of the Fourier coefficients we get

an(y)=∫01f(x+iy)e−2πinxdx

for n∈ℤ.

Together it follows that

(14−ν2)an(y)=∫01(14−ν2)f(x+iy)e−2πinxdx=∫01(Δf)(x+iy)e−2πinxdx=−y2(∫01∂2f∂x2(x+iy)e−2πinxdx+∫01∂2f∂y2(x+iy)e−2πinxdx)=(1)−y2(2πin)2an(y)−y2∂2∂y2∫01f(x+iy)e−2πinxdx=−y2(2πin)2an(y)−y2∂2∂y2an(y)=4π2n2y2an(y)−y2∂2∂y2an(y)

for n∈ℤ.

In (1) we used that the nth Fourier coefficient of ∂2f∂x2 is (2πin)2an(y) for the first summation term. In the second term we changed the order of integration and differentiation, which is allowed since f is smooth in y . We get a linear differential equation of second degree:

y2∂2∂y2an(y)+(14−ν2−4πn2y2)an(y)=0

For n=0 one can show, that for every solution f there exist unique coefficients c0,d0∈ℂ with the property a0(y)=c0y12−ν+d0y12+ν.

For n≠0 every solution f has coefficients of the form

an(y)=cnyKv(2π|n|y)+dnyIv(2π|n|y)

for unique cn,dn∈ℂ. Here Kv(s) and Iv(s) are Bessel functions.

The Bessel functions Iv grow exponentially, while the Bessel functions Kv decrease exponentially. Together with the polynomial growth condition 3) we get f:an(y)=cnyKv(2π|n|y) (also dn=0) for a unique cn∈ℂ. Q.E.D.

Even and odd Maass forms: Let i(z):=−z‾. Then i operates on all functions f:ℋ->ℂ by i(f):=f(i(z)) and commutes with the hyperbolic Laplacian. A Maass form f is called even, if i(f)=f and odd if i(f)=−f. If f is a Maass form, then 12(f+i(f)) is an even Maass form and 12(f−i(f)) an odd Maass form and it holds that f=12(f+i(f))+12(f−i(f)).

#### Theorem: The L-function of a Maass form

Let

f(x+iy)=∑n≠0cnyKν(2π|n|y)e2πinx

be a Maass cusp form. We define the L-function of f as

L(s,f)=∑n=1∞cnn−s.

Then the series L(s,f) converges for ℜ(s)>32 and we can continue it to a whole function on ℂ.

If f is even or odd we get

Λ(s,f):=π−sΓ(s+ε+ν2)Γ(s+ε−ν2)L(s,f).

Here ε=0 if f is even and ε=−1 if f is odd. Then Λ satisfies the functional equation

Λ(s,f)=(−1)εΛ(1−s,f).

## Example: The non-holomorphic Eisenstein-series E

The non-holomorphic Eisenstein-series is defined for z=x+iy∈ℋ and s∈ℂ as

E(z,s):=π−sΓ(s)12∑(m,n)≠(0,0)ys|mz+n|2s

where Γ(s) is the Gamma function.

The series converges absolutely in z∈ℋ for ℜ(s)>1 and locally uniformly in ℋ×{ℜ(s)>1}, since one can show, that the series

S(z,s):=∑(m,n)≠(0,0)1|mz+n|s

converges absolutely in z∈ℋ, if ℜ(s)>2. More precisely it converges uniformly on every set K×{ℜ(s)≥α}, for every compact set K⊂ℋ and every α>2.

### E is a Maass form

We only show SL2(ℤ)-invariance and the differential equation. A proof of the smoothness can be found in Deitmar or Bump. The growth condition follows from the Fourier expansion of the Eisenstein series.

We will first show the SL2(ℤ)-invariance. Let

Γ∞:=±(1ℤ01)

be the stabilizer group ∞ corresponding to the operation of SL2(ℤ) on ℋ∪{∞}.

Proposition. E is Γ(1)-invariant.

Proof. Define:

E~(z,s):=∑γ∈Γ∞∖Γℑ(γz)s.

(a) E~ converges absolutely in z∈ℋ for ℜ(s)>1 and E(z,s)=π−sΓ(s)ζ(2s)E~(z,s).

Since

γ=(abcd)∈Γ(1)⟹ℑ(γz)=ℑ(z)|cz+d|2,

we obtain

E~(z,s)=∑γ∈Γ∞∖Γℑ(γz)s=∑(c,d)=1mod±1ys|cz+d|2s.

That proves the absolute convergence in z∈ℋ for Re⁡(s)>1.

Furthermore, it follows that

ζ(2s)E~(z,s)=∑n=1∞n−s∑(c,d)=1mod±1ys|cz+d|2s=∑n=1∞∑(c,d)=1mod±1ys|ncz+nd|2s=∑(m,n)≠(0,0)ys|mz+n|2s,

since the map

{ℕ×{(x,y)∈ℤ2−{(0,0)}:(x,y)=1}->ℤ2−{(0,0)}(n,(x,y))↦(nx,ny)

is a bijection (a) follows.

(b) We have E(γz,s)=E(z,s) for all γ∈Γ(1).

For γ~∈Γ(1) we get

E~(γ~z,s)=∑γ∈Γ∞∖Γℑ(γ~γz)s=∑γ∈Γ∞∖Γℑ(γz)s=E~(z,s).

Together with (a), E is also invariant under Γ(1). Q.E.D.

Proposition. E is an eigenform of the hyperbolic Laplace operator

We need the following Lemma:

Lemma: Δ commutes with the operation of G on C∞(ℋ). More precisely for all g∈G we have: LgΔ=ΔLg.

Proof: The group SL2(ℝ) is generated by the elements of the form

(a001a),a∈ℝ×;(1x01),x∈ℝ;S=(0−110).

One calculates the claim for these generators and obtains the claim for all g∈SL2(ℝ). Q.E.D.

Since E(z,s)=π−sΓ(s)ζ(2s)E~(z,s) it is sufficient to show the differential equation for E~. We have:

ΔE~(z,s):=Δ∑γ∈Γ∞∖Γℑ(γz)s=∑γ∈Γ∞∖ΓΔ(ℑ(γz)s)

Furthermore, one has

Δ(ℑ(z)s)=Δ(ys)=−y2(∂2ys∂x2+∂2ys∂y2)=s(1−s)ys.

Since the Laplace Operator commutes with the Operation of Γ(1), we get

∀γ∈Γ(1):Δ(ℑ(γz)s)=s(1−s)ℑ(γz)s

and so

ΔE~(z,s)=s(1−s)E~(z,s).

Therefore, the differential equation holds for E in ℜ(s)>3. In order to obtain the claim for all s∈ℂ, consider the function ΔE(z,s)−s(1−s)E(z,s). By explicitly calculating the Fourier expansion of this function, we get that it is meromorphic. Since it vanishes for ℜ(s)>3, it must be the zero function by the identity theorem.

#### The Fourier-expansion of E

The nonholomorphic Eisenstein series has a Fourier expansion

E(z,s)=∑n=−∞∞an(y,s)e2πinx

where

a0(y,s)=π−sΓ(s)ζ(2s)ys+πs−1Γ(1−s)ζ(2(1−s))y1−san(y,s)=2|n|s−12σ1−2s(|n|)yKs−12(2π|n|y)n≠0

If z∈ℋ, E(z,s) has a meromorphic continuation on ℂ. It is holomorphic except for simple poles at s=0,1.

The Eisenstein series satisfies the functional equation

E(z,s)=E(z,1−s)

for all z∈ℋ.

Locally uniformly in x∈ℝ the growth condition

E(x+iy,s)=𝟎(yσ)

holds, where σ=max⁡(Re⁡(s),1−Re⁡(s)).

The meromorphic continuation of E is very important in the spectral theory of the hyperbolic Laplace operator.

## Maass forms of weight k

### Congruence subgroups

For N∈ℕ let Γ(N) be the kernel of the canonical projection

SL2(ℤ)->SL2(ℤ/Nℤ).

We call Γ(N) principal congruence subgroup of level N. A subgroup Γ⊆SL2(ℤ) is called congruence subgroup, if there exists N∈ℕ, so that Γ(N)⊆Γ. All congruence subgroups are discrete.

Let

Γ(1)‾:=Γ(1)/{±1}.

For a congruence subgroup Γ, let Γ‾ be the image of Γ in Γ(1)‾. If S is a system of representatives of Γ‾∖Γ(1)‾, then

SD=⋃γ∈SγD

is a fundamental domain for Γ. The set S is uniquely determined by the fundamental domain SD. Furthermore, S is finite.

The points γ∞ for γ∈S are called cusps of the fundamental domain SD. They are a subset of ℚ∪{∞}.

For every cusp c there exists σ∈Γ(1) with σ∞=c.

### Maass forms of weight k

Let Γ be a congruence subgroup and k∈ℤ.

We define the hyperbolic Laplace operator Δk of weight k as

Δk:C∞(ℋ)->C∞(ℋ),
Δk=−y2(∂2∂x2+∂2∂y2)+iky∂∂x.

This is a generalization of the hyperbolic Laplace operator Δ0=Δ.

We define an operation of SL2(ℝ) on C∞(ℋ) by

f||kg(z):=(cz+d|cz+d|)−kf(gz)

where

z∈ℋ,g=(∗∗cd)∈SL2(ℝ),f∈C∞(ℋ).

It can be shown that

(Δkf)||kg=Δk(f||kg)

holds for all f∈C∞(ℋ),k∈ℤ and every g∈SL2(ℝ).

Therefore, Δk operates on the vector space

C∞(Γ∖ℋ,k):={f∈C∞(ℋ):f||kγ=f∀γ∈Γ}.

Definition. A Maass form of weight k∈ℤ for Γ is a function f∈C∞(Γ∖ℋ,k) that is an eigenfunction of Δk and is of moderate growth at the cusps.

The term moderate growth at cusps needs clarification. Infinity is a cusp for Γ, a function f∈C∞(Γ∖ℋ,k) is of moderate growth at ∞ if f(x+iy) is bounded by a polynomial in y as y->∞. Let c∈ℚ be another cusp. Then there exists θ∈SL2(ℤ) with θ(∞)=c. Let f′:=f||kθ. Then f′∈C∞(Γ′∖ℋ,k), where Γ′ is the congruence subgroup θ−1Γθ. We say f is of moderate growth at the cusp c, if f′ is of moderate growth at ∞.

Definition. If Γ contains a principal congruence subgroup of level N, we say that f is cuspidal at infinity, if

∀z∈ℋ:∫0Nf(z+u)du=0.

We say that f is cuspidal at the cusp c if f′ is cuspidal at infinity. If f is cuspidal at every cusp, we call f a cusp form.

We give a simple example of a Maass form of weight k>1 for the modular group:

Example. Let g:ℋ->ℂ be a modular form of even weight k for Γ(1). Then f(z):=yk2g(z) is a Maass form of weight k for the group Γ(1).

### The spectral problem

Let Γ be a congruence subgroup of SL2(ℝ) and let L2(Γ∖ℋ,k) be the vector space of all measurable functions f:ℋ->ℂ with f||kγ=f for all γ∈Γ satisfying

‖f‖2:=∫Γ∖ℋ|f(z)|2dμ(z)<∞

modulo functions with ‖f‖=0. The integral is well defined, since the function |f(z)|2 is Γ-invariant. This is a Hilbert space with inner product

⟨f,g⟩=∫Γ∖ℋf(z)g(z)‾dμ(z).

The operator Δk can be defined in a vector space B⊂L2(Γ∖ℋ,k)∩C∞(Γ∖ℋ,k) which is dense in L2(Γ∖ℋ,k). There Δk is a positive semidefinite symmetric operator. It can be shown, that there exists a unique self-adjoint continuation on L2(Γ∖ℋ,k).

Define C(Γ∖ℋ,k) as the space of all cusp forms L2(Γ∖ℋ,k)∩C∞(Γ∖ℋ,k). Then Δk operates on C(Γ∖ℋ,k) and has a discrete spectrum. The spectrum belonging to the orthogonal complement has a continuous part and can be described with the help of (modified) non-holomorphic Eisenstein series, their meromorphic continuations and their residues. (See Bump or Iwaniec).

If Γ is a discrete (torsion free) subgroup of SL2(ℝ), so that the quotient Γ∖ℋ is compact, the spectral problem simplifies. This is because a discrete cocompact subgroup has no cusps. Here all of the space L2(Γ∖ℋ,k) is a sum of eigenspaces.

### Embedding into the space L2(Γ \ G)

G=SL2(ℝ) is a locally compact unimodular group with the topology of ℝ4. Let Γ be a congruence subgroup. Since Γ is discrete in G, it is closed in G as well. The group G is unimodular and since the counting measure is a Haar-measure on the discrete group Γ, Γ is also unimodular. By the Quotient Integral Formula there exists a G-right-invariant Radon measure dx on the locally compact space Γ∖G. Let L2(Γ∖G) be the corresponding L2-space. This space decomposes into a Hilbert space direct sum:

L2(Γ∖G)=⨁k∈ℤL2(Γ∖G,k)

where

L2(Γ∖G,k):={ϕ∈L2(Γ∖G)∣ϕ(xkθ)=eikθF(x)∀x∈Γ∖G∀θ∈ℝ}

and

kθ=(cos⁡(θ)−sin⁡(θ)sin⁡(θ)cos⁡(θ))∈SO(2),θ∈ℝ.

The Hilbert-space L2(Γ∖ℋ,k) can be embedded isometrically into the Hilbert space L2(Γ∖G,k). The isometry is given by the map

{ψk:L2(Γ∖ℋ,k)->L2(Γ∖G,k)ψk(f)(g):=f||kγ(i)

Therefore, all Maass cusp forms for the congruence group Γ can be thought of as elements of L2(Γ∖G).

L2(Γ∖G) is a Hilbert space carrying an operation of the group G, the so-called right regular representation:

Rgϕ:=ϕ(xg), where x∈Γ∖G and ϕ∈L2(Γ∖G).

One can easily show, that R is a unitary representation of G on the Hilbert space L2(Γ∖G). One is interested in a decomposition into irreducible subrepresentations. This is only possible if Γ is cocompact. If not, there is also a continuous Hilbert-integral part. The interesting part is, that the solution of this problem also solves the spectral problem of Maass forms. (see Bump, C. 2.3)

## Maass cusp form

A Maass cusp form, a subset of Maass forms, is a function on the upper half-plane that transforms like a modular form but need not be holomorphic. They were first studied by Hans Maass in (Maass 1949).

### Definition

Let k be an integer, s be a complex number, and Γ be a discrete subgroup of SL2(R). A Maass form of weight k for Γ with Laplace eigenvalue s is a smooth function from the upper half-plane to the complex numbers satisfying the following conditions:

* For all γ=(abcd)∈Γ and all z∈ℋ, we have f(az+bcz+d)=(cz+d|cz+d|)kf(z).
* We have Δkf=sf, where Δk is the weight k hyperbolic Laplacian defined as Δk=−y2(∂2∂x2+∂2∂y2)+iky∂∂x.
* The function f is of at most polynomial growth at cusps.

A weak Maass form is defined similarly but with the third condition replaced by "The function f has at most linear exponential growth at cusps". Moreover, f is said to be harmonic if it is annihilated by the Laplacian operator.

### Major results

Let f be a weight 0 Maass cusp form. Its normalized Fourier coefficient at a prime p is bounded by p7/64 \+ p−7/64. This theorem is due to Henry Kim and Peter Sarnak. It is an approximation toward Ramanujan-Petersson conjecture.

### Higher dimensions

Maass cusp forms can be regarded as automorphic forms on GL(2). It is natural to define Maass cusp forms on GL(n) as spherical automorphic forms on GL(n) over the rational number field. Their existence is proved by Miller, Mueller, etc.

## Automorphic representations of the adele group

### The group GL2(A)

Let R be a commutative ring with unit and let GR:=GL2(R) be the group of 2×2 matrices with entries in R and invertible determinant. Let 𝔸=𝔸ℚ be the ring of rational adeles, 𝔸fin the ring of the finite (rational) adeles and for a prime number p∈ℕ let ℚp be the field of p-adic numbers. Furthermore, let ℤp be the ring of the p-adic integers (see Adele ring). Define Gp:=Gℚp. Both Gp and Gℝ are locally compact unimodular groups if one equips them with the subspace topologies of ℚp4 respectively ℝ4. Then:

Gfin:=G𝔸fin≅∏p<∞Kp^Gp.

The right side is the restricted product, concerning the compact, open subgroups Kp:=Gℤp of Gp. Then Gfin locally compact group, if we equip it with the restricted product topology.

The group G𝔸 is isomorphic to

Gfin×Gℝ

and is a locally compact group with the product topology, since Gfin and Gℝ are both locally compact.

Let

ℤ^=∏p<∞ℤp.

The subgroup

Gℤ^:=∏p<∞Kp

is a maximal compact, open subgroup of Gfin and can be thought of as a subgroup of G𝔸, when we consider the embedding xfin↦(xfin,1∞).

We define Zℝ as the center of G∞, that means Zℝ is the group of all diagonal matrices of the form (λλ), where λ∈ℝ×. We think of Zℝ as a subgroup of G𝔸 since we can embed the group by z↦(1Gfin,z).

The group Gℚ is embedded diagonally in G𝔸, which is possible, since all four entries of a x∈Gℚ can only have finite amount of prime divisors and therefore x∈Kp for all but finitely many prime numbers p∈ℕ.

Let G𝔸1 be the group of all x∈G𝔸 with |det⁡(x)|=1. (see Adele Ring for a definition of the absolute value of an Idele). One can easily calculate, that Gℚ is a subgroup of G𝔸1.

With the one-to-one map G𝔸1↪G𝔸 we can identify the groups Gℚ∖G𝔸1 and GℚZℝ∖G𝔸 with each other.

The group Gℚ is dense in Gfin and discrete in G𝔸. The quotient GℚZℝ∖G𝔸=Gℚ∖G𝔸1 is not compact but has finite Haar-measure.

Therefore, Gℚ is a lattice of G𝔸1, similar to the classical case of the modular group and SL2(ℝ). By harmonic analysis one also gets that G𝔸1 is unimodular.

### Adelisation of cuspforms

We now want to embed the classical Maass cusp forms of weight 0 for the modular group into ZℝGℚ∖G𝔸. This can be achieved with the "strong approximation theorem", which states that the map

ψ:Gℤx∞↦Gℚ(1,x∞)Gℤ^

is a Gℝ-equivariant homeomorphism. So we get

Gℤ∖Gℝ->∼Gℚ∖G𝔸/Gℤ^

and furthermore

GℤZℝ∖Gℝ->∼GℚZℝ∖G𝔸/Gℤ^.

Maass cuspforms of weight 0 for modular group can be embedded into

L2(SL2(ℤ)∖SL2(ℝ))≅L2(GL2(ℤ)Zℝ∖GL2(ℝ)).

By the strong approximation theorem this space is unitary isomorphic to

L2(GℚZℝ∖G𝔸/Gℤ^)≅L2(GℚZℝ∖G𝔸)Gℤ^

which is a subspace of L2(GℚZℝ∖G𝔸).

In the same way one can embed the classical holomorphic cusp forms. With a small generalization of the approximation theorem, one can embed all Maass cusp forms (as well as the holomorphic cuspforms) of any weight for any congruence subgroup Γ in L2(GℚZℝ∖G𝔸).

We call L2(GℚZℝ∖G𝔸) the space of automorphic forms of the adele group.

### Cusp forms of the adele group

Let R be a Ring and let NR be the group of all (1r1), where r∈R. This group is isomorphic to the additive group of R.

We call a function f∈L2(Gℚ∖G𝔸1) cusp form, if

∫Nℚ∖N𝔸f(nx)dn=0

holds for almost allx∈Gℚ∖G𝔸1. Let Lcusp2(Gℚ∖G𝔸1) (or just Lcusp2) be the vector space of these cusp forms. Lcusp2 is a closed subspace of L2(GℚZℝ∖G𝔸) and it is invariant under the right regular representation of G𝔸1.

One is again interested in a decomposition of Lcusp2 into irreducible closed subspaces.

We have the following theorem:

The space Lcusp2 decomposes in a direct sum of irreducible Hilbert-spaces with finite multiplicities Ncusp(π)∈ℕ0:

Lcusp2=⨁π∈G^𝔸^Ncusp(π)π

The calculation of these multiplicities Ncusp(π) is one of the most important and most difficult problems in the theory of automorphic forms.

### Cuspidal representations of the adele group

An irreducible representation π of the group G𝔸 is called cuspidal, if it is isomorphic to a subrepresentation of Lcusp2.

An irreducible representation π of the group G𝔸 is called admissible if there exists a compact subgroup K of K⊂G𝔸, so that dimK(Vπ,Vτ)<∞ for all τ∈G^𝔸.

One can show, that every cuspidal representation is admissible.

The admissibility is needed to proof the so-called Tensorprodukt-Theorem anzuwenden, which says, that every irreducible, unitary and admissible representation of the group G𝔸 is isomorphic to an infinite tensor product

⨂p≤∞πp.

The πp are irreducible representations of the group Gp. Almost all of them need to be umramified.

(A representation πp of the group Gp (p<∞) is called unramified, if the vector space

VπpKp={v∈Vπp∣πp(k)v=v∀k∈Kp}

is not the zero space.)

A construction of an infinite tensor product can be found in Deitmar,C.7.

### Automorphic L-functions

Let π be an irreducible, admissible unitary representation of G𝔸. By the tensor product theorem, π is of the form π=⨂p≤∞πp (see cuspidal representations of the adele group)

Let S be a finite set of places containing ∞ and all ramified places . One defines the global Hecke - function of π as

LS(s,π):=∏p∉SL(s,πp)

where L(s,πp) is a so-called local L-function of the local representation πp. A construction of local L-functions can be found in Deitmar C. 8.2.

If π is a cuspidal representation, the L-function LS(s,π) has a meromorphic continuation on ℂ. This is possible, since LS(s,π), satisfies certain functional equations.

## See also

* Harmonic Maass form
* Mock modular form
* Real analytic Eisenstein series
* Voronoi formula

## References

* Bringmann, Kathrin; Folsom, Amanda (2014), "Almost harmonic Maass forms and Kac–Wakimoto characters", Journal für die Reine und Angewandte Mathematik 2014 (694): 179–202, doi:10.1515/crelle-2012-0102
* Bump, Daniel (1997), Automorphic forms and representations, Cambridge Studies in Advanced Mathematics, 55, Cambridge University Press, doi:10.1017/CBO9780511609572, ISBN 978-0-521-55098-7
* Anton Deitmar: Automorphe Formen. Springer, Berlin/ Heidelberg u. a. 2010, ISBN 978-3-642-12389-4.
* Duke, W.; Friedlander, J. B.; Iwaniec, H. (2002), "The subconvexity problem for Artin L-functions", Inventiones Mathematicae 149 (3): 489–577, doi:10.1007/s002220200223, Bibcode: 2002InMat.149..489DL-functions&rft.jtitle=[[Inventiones+Mathematicae]]&rft.aulast=Duke&rft.aufirst=W.&rft.au=Duke, W.&rft.au=Friedlander, J.+B.&rft.au=Iwaniec, H.&rft.date=2002&rft.volume=149&rft.issue=3&rft.pages=489–577&rft_id=info:doi/10.1007/s002220200223&rft_id=info:bibcode/2002InMat.149..489D&rfr_id=info:sid/en.wikibooks.org:Maass_wave_form">
* Henryk Iwaniec : Spectral Methods of Automorphic Forms (Graduate Studies in Mathematics). American Mathematical Society; Auflage: 2. (November 2002), ISBN 978-0821831601.
* Maass, Hans (1949), "Über eine neue Art von nichtanalytischen automorphen Funktionen und die Bestimmung Dirichletscher Reihen durch Funktionalgleichungen", Mathematische Annalen 121: 141–183, doi:10.1007/BF01329622

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