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13 Congruence Subgroups, Cuspidal Cohomology and Hecke Operators
13.1 Eichler-Shimura isomorphism
13.2 Generators for SL_2( Z) and the cubic tree
13.3 One-dimensional fundamental domains and generators for congruence subgroups
13.4 Cohomology of congruence subgroups
13.4-1 Cohomology with rational coefficients
13.5 Cuspidal cohomology
13.6 Hecke operators on forms of weight 2
13.7 Hecke operators on forms of weight ≥ 2
13.8 Reconstructing modular forms from cohomology computations
13.9 The Picard group
13.10 Bianchi groups
13.11 Some other infinite matrix groups
13.12 Ideals and finite quotient groups
13.13 Congruence subgroups for ideals
13.14 First homology

### 13 Congruence Subgroups, Cuspidal Cohomology and Hecke Operators

In this chapter we explain how HAP can be used to make computions about modular forms associated to congruence subgroups Γ of SL_2( Z). Also, in Subsection 10.8 onwards, we demonstrate cohomology computations for the Picard group SL_2( Z[i]), some Bianchi groups PSL_2(cal O_-d) where cal O_d is the ring of integers of Q(sqrt-d) for square free positive integer d, and some other groups of the form SL_m(cal O), GL_m(cal O), PSL_m(cal O), PGL_m(cal O), for m=2,3,4 and certain cal O= Z, cal O_-d.

#### 13.1 Eichler-Shimura isomorphism

We begin by recalling the Eichler-Shimura isomorphism [Eic57][Shi59]

S_k(\Gamma) \oplus \overline{S_k(\Gamma)} \oplus E_k(\Gamma) \cong_{\sf Hecke} H^1(\Gamma,P_{\mathbb C}(k-2))

which relates the cohomology of groups to the theory of modular forms associated to a finite index subgroup Γ of SL_2( Z). In subsequent sections we explain how to compute with the right-hand side of the isomorphism. But first, for completeness, let us define the terms on the left-hand side.

Let N be a positive integer. A subgroup Γ of SL_2( Z) is said to be a congruence subgroup of level N if it contains the kernel of the canonical homomorphism π_N: SL_2( Z) → SL_2( Z/N Z). So any congruence subgroup is of finite index in SL_2( Z), but the converse is not true.

One congruence subgroup of particular interest is the group Γ_1(N)=ker(π_N), known as the principal congruence subgroup of level N. Another congruence subgroup of particular interest is the group Γ_0(N) of those matrices that project to upper triangular matrices in SL_2( Z/N Z).

A modular form of weight k for a congruence subgroup Γ is a complex valued function on the upper-half plane, f: frakh}={z∈ C : Re(z)>0} → C, satisfying:

- displaystyle f(fracaz+bcz+d) = (cz+d)^k f(z) for (beginarraylla&b c &d endarray) ∈ Γ,

- f is `holomorphic' on the extended upper-half plane frakh^∗ = frakh ∪ Q ∪ {∞} obtained from the upper-half plane by `adjoining a point at each cusp'.

The collection of all weight k modular forms for Γ form a vector space M_k(Γ) over C.

A modular form f is said to be a cusp form if f(∞)=0. The collection of all weight k cusp forms for Γ form a vector subspace S_k(Γ). There is a decomposition

M_k(\Gamma) \cong S_k(\Gamma) \oplus E_k(\Gamma)

involving a summand E_k(Γ) known as the Eisenstein space. See [Ste07] for further introductory details on modular forms.

The Eichler-Shimura isomorphism is more than an isomorphism of vector spaces. It is an isomorphism of Hecke modules: both sides admit notions of Hecke operators, and the isomorphism preserves these operators. The bar on the left-hand side of the isomorphism denotes complex conjugation, or anti-holomorphic forms. See [Wie78] for a full account of the isomorphism.

On the right-hand side of the isomorphism, the ZΓ-module P_ C(k-2)⊂ C[x,y] denotes the space of homogeneous degree k-2 polynomials with action of Γ given by

\left(\begin{array}{ll}a&b\\ c &d \end{array}\right)\cdot p(x,y) = p(dx-by,-cx+ay)\ .

In particular P_ C(0)= C is the trivial module. Below we shall compute with the integral analogue P_ Z(k-2) ⊂ Z[x,y].

In the following sections we explain how to use the right-hand side of the Eichler-Shimura isomorphism to compute eigenvalues of the Hecke operators restricted to the subspace S_k(Γ) of cusp forms.

#### 13.2 Generators for SL_2( Z) and the cubic tree

The matrices S=(beginarrayrr0&-1 1 &0 endarray) and T=(beginarrayrr1&1 0 &1 endarray) generate SL_2( Z) and it is not difficult to devise an algorithm for expressing an arbitrary integer matrix A of determinant 1 as a word in S, T and their inverses. The following illustrates such an algorithm.

    gap> A:=[[4,9],[7,16]];;
    gap> word:=AsWordInSL2Z(A);
    [ [ [ 1, 0 ], [ 0, 1 ] ], [ [ 0, 1 ], [ -1, 0 ] ], [ [ 1, -1 ], [ 0, 1 ] ],
      [ [ 0, 1 ], [ -1, 0 ] ], [ [ 1, 1 ], [ 0, 1 ] ], [ [ 0, 1 ], [ -1, 0 ] ],
      [ [ 1, -1 ], [ 0, 1 ] ], [ [ 1, -1 ], [ 0, 1 ] ], [ [ 1, -1 ], [ 0, 1 ] ],
      [ [ 0, 1 ], [ -1, 0 ] ], [ [ 1, 1 ], [ 0, 1 ] ], [ [ 1, 1 ], [ 0, 1 ] ] ]
    gap> Product(word);
    [ [ 4, 9 ], [ 7, 16 ] ]

It is convenient to introduce the matrix U=ST = (beginarrayrr0&-1 1 &1 endarray). The matrices S and U also generate SL_2( Z). In fact we have a free presentation SL_2( Z)= ⟨ S,U | S^4=U^6=1, S^2=U^3 ⟩.

The cubic tree cal T is a tree (i.e. a 1-dimensional contractible regular CW-complex) with countably infinitely many edges in which each vertex has degree 3. We can realize the cubic tree cal T by taking the left cosets of cal U=⟨ U⟩ in SL_2( Z) as vertices, and joining cosets xcal U and ycal U by an edge if, and only if, x^-1y ∈ cal U Scal U. Thus the vertex cal U is joined to Scal U, UScal U and U^2Scal U. The vertices of this tree are in one-to-one correspondence with all reduced words in S, U and U^2 that, apart from the identity, end in S.

From our realization of the cubic tree cal T we see that SL_2( Z) acts on cal T in such a way that each vertex is stabilized by a cyclic subgroup conjugate to cal U=⟨ U⟩ and each edge is stabilized by a cyclic subgroup conjugate to cal S =⟨ S ⟩.

In order to store this action of SL_2( Z) on the cubic tree cal T we just need to record the following finite amount of information.

#### 13.3 One-dimensional fundamental domains and generators for congruence subgroups

The modular group cal M=PSL_2( Z) is isomorphic, as an abstract group, to the free product Z_2∗ Z_3. By the Kurosh subgroup theorem, any finite index subgroup M ⊂ cal M is isomorphic to the free product of finitely many copies of Z_2s, Z_3s and Zs. A subset underline x ⊂ M is an independent set of subgroup generators if M is the free product of the cyclic subgroups <x > as x runs over underline x. Let us say that a set of elements in SL_2( Z) is projectively independent if it maps injectively onto an independent set of subgroup generators underline x⊂ cal M. The generating set {S,U} for SL_2( Z) given in the preceding section is projectively independent.

We are interested in constructing a set of generators for a given congruence subgroup Γ. If a small generating set for Γ is required then we should aim to construct one which is close to being projectively independent.

It is useful to invoke the following general result which follows from a perturbation result about free ZG-resolutons in [EHS06, Theorem 2] and an old observation of John Milnor that a free ZG-resolution can be realized as the cellular chain complex of a CW-complex if it can be so realized in low dimensions.

Theorem. Let X be a contractible CW-complex on which a group G acts by permuting cells. The cellular chain complex C_∗ X is a ZG-resolution of Z which typically is not free. Let [e^n] denote the orbit of the n-cell e^n under the action. Let G^e^n ≤ G denote the stabilizer subgroup of e^n, in which group elements are not required to stabilize e^n point-wise. Let Y_e^n denote a contractible CW-complex on which G^e^n acts cellularly and freely. Then there exists a contractible CW-complex W on which G acts cellularly and freely, and in which the orbits of n-cells are labelled by [e^p]⊗ [f^q] where p+q=n and [e^p] ranges over the G-orbits of p-cells in X, [f^q] ranges over the G^e^p-orbits of q-cells in Y_e^p.

Let W be as in the theorem. Then the quotient CW-complex B_G=W/G is a classifying space for G. Let T denote a maximal tree in the 1-skeleton B^1_G. Basic geometric group theory tells us that the 1-cells in B^1_G∖ T correspond to a generating set for G.

Suppose we wish to compute a set of generators for a principal congruence subgroup Γ=Γ_1(N). In the above theorem take X=cal T to be the cubic tree, and note that Γ acts freely on cal T and thus that W=cal T. To determine the 1-cells of B_Γ∖ T we need to determine a cellular subspace D_Γ ⊂ cal T whose images under the action of Γ cover cal T and are pairwise either disjoint or identical. The subspace D_Γ will not be a CW-complex as it won't be closed, but it can be chosen to be connected, and hence contractible. We call D_Γ a fundamental region for Γ. We denote by mathring D_Γ the largest CW-subcomplex of D_Γ. The vertices of mathring D_Γ are the same as the vertices of D_Γ. Thus mathring D_Γ is a subtree of the cubic tree with |Γ|/6 vertices. For each vertex v in the tree mathring D_Γ define η(v)=3 - degree(v). Then the number of generators for Γ will be (1/2)∑_v∈ mathring D_Γ η(v).

The following commands determine projectively independent generators for Γ_1(6) and display mathring D_Γ_1(6). The subgroup Γ_1(6) is free on 13 generators.

    gap> G:=HAP_PrincipalCongruenceSubgroup(6);;
    gap> HAP_SL2TreeDisplay(G);

    gap> gens:=GeneratorsOfGroup(G);
    [ [ [ -83, -18 ], [ 60, 13 ] ], [ [ -77, -18 ], [ 30, 7 ] ],
      [ [ -65, -12 ], [ 168, 31 ] ], [ [ -53, -12 ], [ 84, 19 ] ],
      [ [ -47, -18 ], [ 222, 85 ] ], [ [ -41, -12 ], [ 24, 7 ] ],
      [ [ -35, -6 ], [ 6, 1 ] ], [ [ -11, -18 ], [ 30, 49 ] ],
      [ [ -11, -6 ], [ 24, 13 ] ], [ [ -5, -18 ], [ 12, 43 ] ],
      [ [ -5, -12 ], [ 18, 43 ] ], [ [ -5, -6 ], [ 6, 7 ] ],
      [ [ 1, 0 ], [ -6, 1 ] ] ]

An alternative but very related approach to computing generators of congruence subgroups of SL_2( Z) is described in [Kul91].

The congruence subgroup Γ_0(N) does not act freely on the vertices of cal T, and so one needs to incorporate a generator for the cyclic stabilizer group according to the above theorem. Alternatively, we can replace the cubic tree by a six-fold cover cal T' on whose vertex set Γ_0(N) acts freely. This alternative approach will produce a redundant set of generators. The following commands display mathring D_Γ_0(39) for a fundamental region in cal T'. They also use the corresponding generating set for Γ_0(39), involving 18 generators, to compute the abelianization Γ_0(39)^ab= Z_2 ⊕ Z_3^2 ⊕ Z^9. The abelianization shows that any generating set has at least 11 generators.

    gap> G:=HAP_CongruenceSubgroupGamma0(39);;
    gap> HAP_SL2TreeDisplay(G);
    gap> Length(GeneratorsOfGroup(G));
    18
    gap> AbelianInvariants(G);
    [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 3, 3 ]

Note that to compute D_Γ one only needs to be able to test whether a given matrix lies in Γ or not. Given an inclusion Γ'⊂ Γ of congruence subgroups, it is straightforward to use the trees mathring D_Γ' and mathring D_Γ to compute a system of coset representative for Γ'∖ Γ.

#### 13.4 Cohomology of congruence subgroups

To compute the cohomology H^n(Γ,A) of a congruence subgroup Γ with coefficients in a ZΓ-module A we need to construct n+1 terms of a free ZΓ-resolution of Z. We can do this by first using perturbation techniques (as described in [BE14]) to combine the cubic tree with resolutions for the cyclic groups of order 4 and 6 in order to produce a free ZG-resolution R_∗ for G=SL_2( Z). This resolution is also a free ZΓ-resolution with each term of rank

{\rm rank}_{\mathbb Z\Gamma} R_k = |G:\Gamma|\times {\rm rank}_{\mathbb ZG} R_k\ .

For congruence subgroups of lowish index in G this resolution suffices to make computations.

The following commands compute

H^1(\Gamma_0(39),\mathbb Z) = \mathbb Z^9\ .

    gap> R:=ResolutionSL2Z_alt(2);
    Resolution of length 2 in characteristic 0 for SL(2,Integers) .

    gap> gamma:=HAP_CongruenceSubgroupGamma0(39);;
    gap> S:=ResolutionFiniteSubgroup(R,gamma);
    Resolution of length 2 in characteristic 0 for
    CongruenceSubgroupGamma0( 39)  .

    gap> Cohomology(HomToIntegers(S),1);
    [ 0, 0, 0, 0, 0, 0, 0, 0, 0 ]

This computation establishes that the space M_2(Γ_0(39)) of weight 2 modular forms is of dimension 9.

The following commands show that rank_ ZΓ_0(39) R_1 = 112 but that it is possible to apply `Tietze like' simplifications to R_∗ to obtain a free ZΓ_0(39)-resolution T_∗ with rank_ ZΓ_0(39) T_1 = 11. It is more efficient to work with T_∗ when making cohomology computations with coefficients in a module A of large rank.

    gap> S!.dimension(1);
    112
    gap> T:=TietzeReducedResolution(S);
    Resolution of length 2 in characteristic 0 for CongruenceSubgroupGamma0(
    39)  .

    gap> T!.dimension(1);
    11

The following commands compute

H^1(\Gamma_0(39),P_{\mathbb Z}(8)) = \mathbb Z_3 \oplus \mathbb Z_6 \oplus \mathbb Z_{168} \oplus \mathbb Z^{84}\ ,

H^1(\Gamma_0(39),P_{\mathbb Z}(9)) = \mathbb Z_2 \oplus \mathbb Z_2 .

    gap> P:=HomogeneousPolynomials(gamma,8);;
    gap> c:=Cohomology(HomToIntegralModule(T,P),1);
    [ 3, 6, 168, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ]
    gap> Length(c);
    87

    gap> P:=HomogeneousPolynomials(gamma,9);;
    gap> c:=Cohomology(HomToIntegralModule(T,P),1);
    [ 2, 2 ]

This computation establishes that the space M_10(Γ_0(39)) of weight 10 modular forms is of dimension 84, and M_11(Γ_0(39)) is of dimension 0. (There are never any modular forms of odd weight, and so M_k(Γ)=0 for all odd k and any congruence subgroup Γ.)

##### 13.4-1 Cohomology with rational coefficients

To calculate cohomology H^n(Γ,A) with coefficients in a QΓ-module A it suffices to construct a resolution of Z by non-free ZΓ-modules where Γ acts with finite stabilizer groups on each module in the resolution. Computing over Q is computationally less expensive than computing over Z. The following commands first compute H^1(Γ_0(39), Q) = H_1(Γ_0(39), Q)= Q^9. As a larger example, they then compute H^1(Γ_0(2^13-1), Q) = Q^1365 where Γ_0(2^13-1) has index 8192 in SL_2( Z).

    gap> K:=ContractibleGcomplex("SL(2,Z)");
    Non-free resolution in characteristic 0 for SL(2,Integers) .

    gap> gamma:=HAP_CongruenceSubgroupGamma0(39);;
    gap> KK:=NonFreeResolutionFiniteSubgroup(K,gamma);
    Non-free resolution in characteristic 0 for <matrix group with
    18 generators> .

    gap> C:=TensorWithRationals(KK);
    gap> Homology(C,1);
    9

    gap> G:=HAP_CongruenceSubgroupGamma0(2^13-1);;
    gap> IndexInSL2Z(G);
    8192
    gap> KK:=NonFreeResolutionFiniteSubgroup(K,G);;
    gap> C:=TensorWithRationals(KK);;
    gap> Homology(C,1);
    1365

#### 13.5 Cuspidal cohomology

To define and compute cuspidal cohomology we consider the action of SL_2( Z) on the upper-half plane frak h given by

\left(\begin{array}{ll}a&b\\ c &d \end{array}\right) z = \frac{az +b}{cz+d}\ .

A standard 'fundamental domain' for this action is the region

\begin{array}{ll} D=&\{z\in {\frak h}\ :\ |z| > 1, |{\rm Re}(z)| < \frac{1}{2}\} \\ & \cup\ \{z\in {\frak h} \ :\ |z| \ge 1, {\rm Re}(z)=-\frac{1}{2}\}\\ & \cup\ \{z \in {\frak h}\ :\ |z|=1, -\frac{1}{2} \le {\rm Re}(z) \le 0\} \end{array}

illustrated below.

The action factors through an action of PSL_2( Z) =SL_2( Z)/⟨ (beginarrayrr-1&0 0 &-1 endarray)⟩. The images of D under the action of PSL_2( Z) cover the upper-half plane, and any two images have at most a single point in common. The possible common points are the bottom left-hand corner point which is stabilized by ⟨ U⟩, and the bottom middle point which is stabilized by ⟨ S⟩.

A congruence subgroup Γ has a `fundamental domain' D_Γ equal to a union of finitely many copies of D, one copy for each coset in Γ∖ SL_2( Z). The quotient space X=Γ∖ frak h is not compact, and can be compactified in several ways. We are interested in the Borel-Serre compactification. This is a space X^BS for which there is an inclusion X↪ X^BS and this inclusion is a homotopy equivalence. One defines the boundary ∂ X^BS = X^BS - X and uses the inclusion ∂ X^BS ↪ X^BS ≃ X to define the cuspidal cohomology group, over the ground ring C, as

H_{cusp}^n(\Gamma,P_{\mathbb C}(k-2)) = \ker (\ H^n(X,P_{\mathbb C}(k-2)) \rightarrow H^n(\partial X^{BS},P_{\mathbb C}(k-2)) \ ).

Strictly speaking, this is the definition of interior cohomology H_!^n(Γ,P_ C(k-2)) which in general contains the cuspidal cohomology as a subgroup. However, for congruence subgroups of SL_2( Z) there is equality H_!^n(Γ,P_ C(k-2)) = H_cusp^n(Γ,P_ C(k-2)).

Working over C has the advantage of avoiding the technical issue that Γ does not necessarily act freely on frak h since there are points with finite cyclic stabilizer groups in SL_2( Z). But it has the disadvantage of losing information about torsion in cohomology. So HAP confronts the issue by working with a contractible CW-complex tilde X^BS on which Γ acts freely, and Γ-equivariant inclusion ∂ tilde X^BS ↪ tilde X^BS. The definition of cuspidal cohomology that we use, which coincides with the above definition when working over C, is

H_{cusp}^n(\Gamma,A) = \ker (\ H^n({\rm Hom}_{\, \mathbb Z\Gamma}(C_\ast(\tilde X^{BS}), A)\, ) \rightarrow H^n(\ {\rm Hom}_{\, \mathbb Z\Gamma}(C_\ast(\tilde \partial X^{BS}), A)\, \ ).

The following data is recorded and, using perturbation theory, is combined with free resolutions for C_4 and C_6 to constuct tilde X^BS.

The following commands calculate

H^1_{cusp}(\Gamma_0(39),\mathbb Z) = \mathbb Z^6\ .

    gap> gamma:=HAP_CongruenceSubgroupGamma0(39);;
    gap> k:=2;; deg:=1;; c:=CuspidalCohomologyHomomorphism(gamma,deg,k);
    [ g1, g2, g3, g4, g5, g6, g7, g8, g9 ] -> [ g1^-1*g3, g1^-1*g3, g1^-1*g3,
      g1^-1*g3, g1^-1*g2, g1^-1*g3, g1^-1*g4, g1^-1*g4, g1^-1*g4 ]
    gap> AbelianInvariants(Kernel(c));
    [ 0, 0, 0, 0, 0, 0 ]

From the Eichler-Shimura isomorphism and the already calculated dimension of M_2(Γ_0(39))≅ C^9, we deduce from this cuspidal cohomology that the space S_2(Γ_0(39)) of cuspidal weight 2 forms is of dimension 3, and the Eisenstein space E_2(Γ_0(39))≅ C^3 is of dimension 3.

The following commands show that the space S_4(Γ_0(39)) of cuspidal weight 4 forms is of dimension 12.

    gap> gamma:=HAP_CongruenceSubgroupGamma0(39);;
    gap> k:=4;; deg:=1;; c:=CuspidalCohomologyHomomorphism(gamma,deg,k);;
    gap> AbelianInvariants(Kernel(c));
    [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ]

#### 13.6 Hecke operators on forms of weight 2

A congruence subgroup Γ ≤ SL_2( Z) and element g∈ SL_2( Q) determine the subgroup Γ' = Γ ∩ gΓ g^-1 and homomorphisms

\Gamma\ \hookleftarrow\ \Gamma'\ \ \stackrel{\gamma \mapsto g^{-1}\gamma g}{\longrightarrow}\ \ g^{-1}\Gamma' g\ \hookrightarrow \Gamma\ .

These homomorphisms give rise to homomorphisms of cohomology groups

H^n(\Gamma,\mathbb Z)\ \ \stackrel{tr}{\leftarrow} \ \ H^n(\Gamma',\mathbb Z) \ \ \stackrel{\alpha}{\leftarrow} \ \ H^n(g^{-1}\Gamma' g,\mathbb Z) \ \ \stackrel{\beta}{\leftarrow} H^n(\Gamma, \mathbb Z)

with α, β functorial maps, and tr the transfer map. We define the composite T_g=tr ∘ α ∘ β: H^n(Γ, Z) → H^n(Γ, Z) to be the Hecke operator determined by g. Further details on this description of Hecke operators can be found in [Ste07, Appendix by P. Gunnells].

For each prime integer p≥ 1 we set T_p =T_g with for g=(beginarraycc1&00&pendarray).

The following commands compute T_2 and T_5 for n=1 and Γ=Γ_0(39). The commands also compute the eigenvalues of these two Hecke operators. The final command confirms that T_2 and T_5 commute. (It is a fact that T_pT_q=T_qT_p for all primes p,q.)

    gap> gamma:=HAP_CongruenceSubgroupGamma0(39);;
    gap> p:=2;;N:=1;;h:=HeckeOperatorWeight2(gamma,p,N);;
    gap> AbelianInvariants(Source(h));
    [ 0, 0, 0, 0, 0, 0, 0, 0, 0 ]
    gap> T2:=HomomorphismAsMatrix(h);;
    gap> Display(T2);
    [ [  -2,  -2,   2,   2,   1,   2,   0,   0,   0 ],
      [  -2,   0,   1,   2,  -2,   2,   2,   2,  -2 ],
      [  -2,  -1,   2,   2,  -1,   2,   1,   1,  -1 ],
      [  -2,  -1,   2,   2,   1,   1,   0,   0,   0 ],
      [  -1,   0,   0,   2,  -3,   2,   3,   3,  -3 ],
      [   0,   1,   1,   1,  -1,   0,   1,   1,  -1 ],
      [  -1,   1,   1,  -1,   0,   1,   2,  -1,   1 ],
      [  -1,  -1,   0,   2,  -3,   2,   1,   4,  -1 ],
      [   0,   1,   0,  -1,  -2,   1,   1,   1,   2 ] ]
    gap> Eigenvalues(Rationals,T2);
    [ 3, 1 ]

    gap> p:=5;;N:=1;;h:=HeckeOperator(gamma,p,N);;
    gap> T5:=HomomorphismAsMatrix(h);;
    gap> Display(T5);
    [ [  -1,  -1,   3,   4,   0,   0,   1,   1,  -1 ],
      [  -5,  -1,   5,   4,   0,   0,   3,   3,  -3 ],
      [  -2,   0,   4,   4,   1,   0,  -1,  -1,   1 ],
      [  -2,   0,   3,   2,  -3,   2,   4,   4,  -4 ],
      [  -4,  -2,   4,   4,   3,   0,   1,   1,  -1 ],
      [  -6,  -4,   5,   6,   1,   2,   2,   2,  -2 ],
      [   1,   5,   0,  -4,  -3,   2,   5,  -1,   1 ],
      [  -2,  -2,   2,   4,   0,   0,  -2,   4,   2 ],
      [   1,   3,   0,  -4,  -4,   2,   2,   2,   4 ] ]
    gap> Eigenvalues(Rationals,T5);
    [ 6, 2 ]

    gap>T2*T5=T5*T2;
    true

#### 13.7 Hecke operators on forms of weight ≥ 2

The above definition of Hecke operator T_g for g∈ SL_2( Q) extends to a Hecke operator T_g: H^1(Γ,P_ Q(k-2) ) → H^1(Γ,P_ Q(k-2) ) for k≥ 2. We work over the rationals rather than the integers in order to ensure that the action of Γ on homogeneous polynomials extends to an action of g. The following commands compute the matrix of T_2: H^1(Γ,P_ Q(k-2) ) → H^1(Γ,P_ Q(k-2) ) for Γ=SL_2( Z) and k=4;

    gap> H:=HAP_CongruenceSubgroupGamma0(1);;
    gap> h:=HeckeOperator(H,2,12);;Display(h);
    [ [   2049,  -7560,      0 ],
      [      0,    -24,      0 ],
      [      0,      0,    -24 ] ]

#### 13.8 Reconstructing modular forms from cohomology computations

Given a modular form f: frak h → C associated to a congruence subgroup Γ, and given a compact edge e in the tessellation of frak h (i.e. an edge in the cubic tree cal T) arising from the above fundamental domain for SL_2( Z), we can evaluate

\int_e f(z)\,dz \ .

In this way we obtain a cochain f_1: C_1(cal T) → C in Hom_ ZΓ(C_1(cal T), C) representing a cohomology class c(f) ∈ H^1( Hom_ ZΓ(C_∗(cal T), C) ) = H^1(Γ, C). The correspondence f↦ c(f) underlies the Eichler-Shimura isomorphism. Hecke operators can be used to recover modular forms from cohomology classes.

Hecke operators restrict to operators on cuspidal cohomology. On the left-hand side of the Eichler-Shimura isomorphism Hecke operators restrict to operators T_s: S_2(Γ) → S_2(Γ) for s≥ 1.

Let us now introduce the function q=q(z)=e^2π i z which is holomorphic on C. For any modular form f(z) there are numbers a_s such that

f(z) = \sum_{s=0}^\infty a_sq^s

for all z∈ frak h. The form f is a cusp form if a_0=0.

A non-zero cusp form f∈ S_2(Γ) is an eigenform if it is simultaneously an eigenvector for the Hecke operators T_s for all s =1,2,3,⋯. An eigenform is said to be normalized if its coefficient a_1=1. It turns out that if f is a normalized eigenform then the coefficient a_s is an eigenvalue for T_s (see for instance [Ste07] for details). It can be shown [AL70] that f∈ S_2(Γ_0(N)) admits a basis of eigenforms.

This all implies that, in principle, we can construct an approximation to an explicit basis for the space S_2(Γ) of cusp forms by computing eigenvalues for Hecke operators.

Suppose that we would like a basis for S_2(Γ_0(11)). The following commands first show that H^1_cusp(Γ_0(11), Z)= Z⊕ Z from which we deduce that S_2(Γ_0(11)) = C is 1-dimensional. Then eigenvalues of Hecke operators are calculated to establish that the modular form

f = q -2q^2 -q^3 +2q^4 +q^5 +2q^6 -2q^7 + -2q^9 -2q^{10} + \cdots

constitutes a basis for S_2(Γ_0(11)).

    gap> gamma:=HAP_CongruenceSubgroupGamma0(11);;
    gap> AbelianInvariants(Kernel(CuspidalCohomologyHomomorphism(gamma,1,2)));
    [ 0, 0 ]

    gap> T1:=HomomorphismAsMatrix(HeckeOperator(gamma,1,1));; Display(T1);
    [ [  1,  0,  0 ],
      [  0,  1,  0 ],
      [  0,  0,  1 ] ]
    gap> T2:=HomomorphismAsMatrix(HeckeOperator(gamma,2,1));; Display(T2);
    [ [   3,  -4,   4 ],
      [   0,  -2,   0 ],
      [   0,   0,  -2 ] ]
    gap> T3:=HomomorphismAsMatrix(HeckeOperator(gamma,3,1));; Display(T3);
    [ [   4,  -4,   4 ],
      [   0,  -1,   0 ],
      [   0,   0,  -1 ] ]
    gap> T5:=HomomorphismAsMatrix(HeckeOperator(gamma,5,1));; Display(T5);
    [ [   6,  -4,   4 ],
      [   0,   1,   0 ],
      [   0,   0,   1 ] ]
    gap> T7:=HomomorphismAsMatrix(HeckeOperator(gamma,7,1));; Display(T7);
    [ [   8,  -8,   8 ],
      [   0,  -2,   0 ],
      [   0,   0,  -2 ] ]

For a normalized eigenform f=1 + ∑_s=2^∞ a_sq^s the coefficients a_s with s a composite integer can be expressed in terms of the coefficients a_p for prime p. If r,s are coprime then T_rs =T_rT_s. If p is a prime that is not a divisor of the level N of Γ then a_p^m =a_p^m-1}a_p - p a_p^m-2}. If the prime p divides N then a_p^m = (a_p)^m. It thus suffices to compute the coefficients a_p for prime integers p only.

The following commands establish that S_12(SL_2( Z)) has a basis consisting of one cusp eigenform

q - 24q^2 + 252q^3 - 1472q^4 + 4830q^5 - 6048q^6 - 16744q^7 + 84480q^8 - 113643q^9

- 115920q^10 + 534612q^11 - 370944q^12 - 577738q^13 + 401856q^14 + 1217160q^15 + 987136q^16

- 6905934q^17 + 2727432q^18 + 10661420q^19 + ...

    gap> H:=HAP_CongruenceSubgroupGamma0(1);;
    gap> for p in [2,3,5,7,11,13,17,19] do
    > T:=HeckeOperator(H,p,1,12);;
    > Print("eigenvalues= ",Eigenvalues(Rationals,T), " and eigenvectors = ", Eigenvectors(Rationals,T)," for p= ",p,"\n");
    > od;
    eigenvalues= [ 2049, -24 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 2
    eigenvalues= [ 177148, 252 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 3
    eigenvalues= [ 48828126, 4830 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 5
    eigenvalues= [ 1977326744, -16744 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 7
    eigenvalues= [ 285311670612, 534612 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 11
    eigenvalues= [ 1792160394038, -577738 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 13
    eigenvalues= [ 34271896307634, -6905934 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 17
    eigenvalues= [ 116490258898220, 10661420 ] and eigenvectors = [ [ 1, -2520/691, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] for p= 19

#### 13.9 The Picard group

Let us now consider the Picard group G=SL_2( Z[ i]) and its action on upper-half space

{\frak h}^3 =\{(z,t) \in \mathbb C\times \mathbb R\ |\ t > 0\} \ .

To describe the action we introduce the symbol j satisfying j^2=-1, ij=-ji and write z+tj instead of (z,t). The action is given by

\left(\begin{array}{ll}a&b\\ c &d \end{array}\right)\cdot (z+tj) \ = \ \left(a(z+tj)+b\right)\left(c(z+tj)+d\right)^{-1}\ .

Alternatively, and more explicitly, the action is given by

\left(\begin{array}{ll}a&b\\ c &d \end{array}\right)\cdot (z+tj) \ = \ \frac{(az+b)\overline{(cz+d) } + a\overline c y^2}{|cz +d|^2 + |c|^2y^2} \ +\ \frac{y}{|cz+d|^2+|c|^2y^2}\, j \ .

A standard 'fundamental domain' D for this action is the following region (with some of the boundary points removed).

\{z+tj\in {\frak h}^3\ |\ 0 \le |{\rm Re}(z)| \le \frac{1}{2}, 0\le {\rm Im}(z) \le \frac{1}{2}, z\overline z +t^2 \ge 1\}

The four bottom vertices of D are a = -frac12 +frac12i +fracsqrt2}2j, b = -frac12 +fracsqrt3}2j, c = frac12 +fracsqrt3}2j, d = frac12 +frac12i +fracsqrt2}2j.

The upper-half space frak h^3 can be retracted onto a 2-dimensional subspace cal T ⊂ frak h^3. The space cal T is a contractible 2-dimensional regular CW-complex, and the action of the Picard group G restricts to a cellular action of G on cal T. Under this action there is one orbit of 2-cells, represented by the curvilinear square with vertices a, b, c and d in the picture. This 2-cell has stabilizer group isomorphic to the quaternion group Q_4 of order 8. There are two orbits of 1-cells, both with stabilizer group isomorphic to a semi-direct product C_3:C_4. There is one orbit of 0-cells, with stabilizer group isomorphic to SL(2,3).

Using perturbation techniques, the 2-complex cal T can be combined with free resolutions for the cell stabilizer groups to contruct a regular CW-complex X on which the Picard group G acts freely. The following commands compute the first few terms of the free ZG-resolution R_∗ =C_∗ X. Then R_∗ is used to compute

H^1(G,\mathbb Z) =0\ ,

H^2(G,\mathbb Z) =\mathbb Z_2\oplus \mathbb Z_2\ ,

H^3(G,\mathbb Z) =\mathbb Z_6\ ,

H^4(G,\mathbb Z) =\mathbb Z_4\oplus \mathbb Z_{24}\ ,

and compute a free presentation for G involving four generators and seven relators.

    gap> K:=ContractibleGcomplex("SL(2,O-1)");;
    gap> R:=FreeGResolution(K,5);;
    gap> Cohomology(HomToIntegers(R),1);
    [  ]
    gap> Cohomology(HomToIntegers(R),2);
    [ 2, 2 ]
    gap> Cohomology(HomToIntegers(R),3);
    [ 6 ]
    gap> Cohomology(HomToIntegers(R),4);
    [ 4, 24 ]
    gap> P:=PresentationOfResolution(R);
    rec( freeGroup := <free group on the generators [ f1, f2, f3, f4 ]>,
      gens := [ 184, 185, 186, 187 ],
      relators := [ f1^2*f2^-1*f1^-1*f2^-1, f1*f2*f1*f2^-2,
          f3*f2^2*f1*(f2*f1^-1)^2*f3^-1*f1^2*f2^-2,
          f1*(f2*f1^-1)^2*f3^-1*f1^2*f2^-1*f3^-1,
          f4*f2*f1*(f2*f1^-1)^2*f4^-1*f1*f2^-1, f1*f4^-1*f1^-2*f4^-1,
          f3*f2*f1*(f2*f1^-1)^2*f4^-1*f1*f2^-1*f3^-1*f4*f2 ] )

We can also compute the cohomology of G=SL_2( Z[i]) with coefficients in a module such as the module P_ Z[i](k) of degree k homogeneous polynomials with coefficients in Z[i] and with the action described above. For instance, the following commands compute

H^1(G,P_{\mathbb Z[i]}(24)) = (\mathbb Z_2)^4 \oplus \mathbb Z_4 \oplus \mathbb Z_8 \oplus \mathbb Z_{40} \oplus \mathbb Z_{80}\, ,

H^2(G,P_{\mathbb Z[i]}(24)) = (\mathbb Z_2)^{24} \oplus \mathbb Z_{520030}\oplus \mathbb Z_{1040060} \oplus \mathbb Z^2\, ,

H^3(G,P_{\mathbb Z[i]}(24)) = (\mathbb Z_2)^{22} \oplus \mathbb Z_{4}\oplus (\mathbb Z_{12})^2 \, .

    gap> G:=R!.group;;
    gap> M:=HomogeneousPolynomials(G,24);;
    gap> C:=HomToIntegralModule(R,M);;
    gap> Cohomology(C,1);
    [ 2, 2, 2, 2, 4, 8, 40, 80 ]
    gap> Cohomology(C,2);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
      520030, 1040060, 0, 0 ]
    gap> Cohomology(C,3);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 4, 12, 12
     ]

#### 13.10 Bianchi groups

The Bianchi groups are the groups G=PSL_2(cal O_-d) where d is a square free positive integer and cal O_-d is the ring of integers of the imaginary quadratic field Q(sqrt-d). More explicitly,

{\cal O}_{-d} = \mathbb Z\left[\sqrt{-d}\right]~~~~~~~~ {\rm if~} d \equiv 1 {\rm ~mod~} 4\, ,

{\cal O}_{-d} = \mathbb Z\left[\frac{1+\sqrt{-d}}{2}\right]~~~~~ {\rm if~} d \equiv 2,3 {\rm ~mod~} 4\, .

These groups act on upper-half space frak h^3 in the same way as the Picard group. Upper-half space can be tessellated by a 'fundamental domain' for this action. Moreover, as with the Picard group, this tessellation contains a 2-dimensional cellular subspace cal T⊂ frak h^3 where cal T is a contractible CW-complex on which G acts cellularly. It should be mentioned that the fundamental domain and the contractible 2-complex cal T are not uniquely determined by G. Various algorithms exist for computing cal T and its cell stabilizers. One algorithm due to Swan [Swa71] has been implemented by Alexander Rahm [Rah10] and the output for various values of d are stored in HAP. Another approach is to use Voronoi's theory of perfect forms. This approach has been implemented by Sebastian Schoennenbeck [BCNS15] and, again, its output for various values of d are stored in HAP. The following commands combine data from Schoennenbeck's algorithm with free resolutions for cell stabiliers to compute

H^1(PSL_2({\cal O}_{-6}),P_{{\cal O}_{-6}}(24)) = (\mathbb Z_2)^4 \oplus \mathbb Z_{12} \oplus \mathbb Z_{24} \oplus \mathbb Z_{9240} \oplus \mathbb Z_{55440} \oplus \mathbb Z^4\,,

H^2(PSL_2({\cal O}_{-6}),P_{{\cal O}_{-6}}(24)) = \begin{array}{l} (\mathbb Z_2)^{26} \oplus \mathbb (Z_{6})^8 \oplus \mathbb (Z_{12})^{9} \oplus \mathbb Z_{24} \oplus (\mathbb Z_{120})^2 \oplus (\mathbb Z_{840})^3\\ \oplus \mathbb Z_{2520} \oplus (\mathbb Z_{27720})^2 \oplus (\mathbb Z_{24227280})^2 \oplus (\mathbb Z_{411863760})^2\\ \oplus \mathbb Z_{2454438243748928651877425142836664498129840}\\ \oplus \mathbb Z_{14726629462493571911264550857019986988779040}\\ \oplus \mathbb Z^4\end{array}\ ,

H^3(PSL_2({\cal O}_{-6}),P_{{\cal O}_{-6}}(24)) = (\mathbb Z_2)^{23} \oplus \mathbb Z_{4} \oplus (\mathbb Z_{12})^2\ .

Note that the action of SL_2(cal O_-d) on P_{cal O_-d}(k) induces an action of PSL_2(cal O_-d) provided k is even.

    gap> R:=ResolutionPSL2QuadraticIntegers(-6,4);
    Resolution of length 4 in characteristic 0 for PSL(2,O-6) .
    No contracting homotopy available.

    gap> G:=R!.group;;
    gap> M:=HomogeneousPolynomials(G,24);;
    gap> C:=HomToIntegralModule(R,M);;
    gap> Cohomology(C,1);
    [ 2, 2, 2, 2, 12, 24, 9240, 55440, 0, 0, 0, 0 ]
    gap> Cohomology(C,2);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
      2, 6, 6, 6, 6, 6, 6, 6, 6, 12, 12, 12, 12, 12, 12, 12, 12, 12, 24, 120, 120,
      840, 840, 840, 2520, 27720, 27720, 24227280, 24227280, 411863760, 411863760,
      2454438243748928651877425142836664498129840,
      14726629462493571911264550857019986988779040, 0, 0, 0, 0 ]
    gap> Cohomology(C,3);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 4, 12,
      12 ]

We can also consider the coefficient module

P_{{\cal O}_{-d}}(k,\ell) = P_{{\cal O}_{-d}}(k) \otimes_{{\cal O}_{-d}} \overline{P_{{\cal O}_{-d}}(\ell)}

where the bar denotes a twist in the action obtained from complex conjugation. For an action of the projective linear group we must insist that k+ℓ is even. The following commands compute

H^2(PSL_2({\cal O}_{-11}),P_{{\cal O}_{-11}}(5,5)) = (\mathbb Z_2)^8 \oplus \mathbb Z_{60} \oplus (\mathbb Z_{660})^3 \oplus \mathbb Z^6\,,

a computation which was first made, along with many other cohomology computationsfor Bianchi groups, by Mehmet Haluk Sengun [Sen11].

    gap> R:=ResolutionPSL2QuadraticIntegers(-11,3);;
    gap> M:=HomogeneousPolynomials(R!.group,5,5);;
    gap> C:=HomToIntegralModule(R,M);;
    gap> Cohomology(C,2);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 60, 660, 660, 660, 0, 0, 0, 0, 0, 0 ]

The function `ResolutionPSL2QuadraticIntegers(-d,n)` relies on a limited data base produced by the algorithms implemented by Schoennenbeck and Rahm. The function also covers some cases covered by entering a sring "-d+I" as first variable. These cases correspond to projective special groups of module automorphisms of lattices of rank 2 over the integers of the imaginary quadratic number field Q(sqrt-d) with non-trivial Steinitz-class. In the case of a larger class group there are cases labelled "-d+I2",...,"-d+Ik" and the Ij together with O-d form a system of representatives of elements of the class group modulo squares and Galois action. For instance, the following commands compute

H_2(PSL({\cal O}_{-21+I2}),\mathbb Z) = \mathbb Z_2\oplus \mathbb Z^6\, .

    gap> R:=ResolutionPSL2QuadraticIntegers("-21+I2",3);
    Resolution of length 3 in characteristic 0 for PSL(2,O-21+I2)) .
    No contracting homotopy available.

    gap> Homology(TensorWithIntegers(R),2);
    [ 2, 0, 0, 0, 0, 0, 0 ]

#### 13.11 Some other infinite matrix groups

Analogous to the functions for Bianchi groups, HAP has functions

- `ResolutionSL2QuadraticIntegers(-d,n)`

- `ResolutionSL2ZInvertedInteger(m,n)`

- `ResolutionGL2QuadraticIntegers(-d,n)`

- `ResolutionPGL2QuadraticIntegers(-d,n)`

- `ResolutionGL3QuadraticIntegers(-d,n)`

- `ResolutionPGL3QuadraticIntegers(-d,n)`

for computing free resolutions for certain values of SL_2(cal O_-d), SL_2( Z[frac1m]), GL_2(cal O_-d) and PGL_2(cal O_-d). Additionally, the function

- `ResolutionArithmeticGroup("string",n)`

can be used to compute resolutions for groups whose data (provided by Sebastian Schoennenbeck, Alexander Rahm and Mathieu Dutour) is stored in the directory `gap/pkg/Hap/lib/Perturbations/Gcomplexes` .

For instance, the following commands compute

H^1(SL_2({\cal O}_{-6}),P_{{\cal O}_{-6}}(24)) = (\mathbb Z_2)^4 \oplus \mathbb Z_{12} \oplus \mathbb Z_{24} \oplus \mathbb Z_{9240} \oplus \mathbb Z_{55440} \oplus \mathbb Z^4\,,

H^2(SL_2({\cal O}_{-6}),P_{{\cal O}_{-6}}(24)) = \begin{array}{l} (\mathbb Z_2)^{26} \oplus \mathbb (Z_{6})^7 \oplus \mathbb (Z_{12})^{10} \oplus \mathbb Z_{24} \oplus (\mathbb Z_{120})^2 \oplus (\mathbb Z_{840})^3\\ \oplus \mathbb Z_{2520} \oplus (\mathbb Z_{27720})^2 \oplus (\mathbb Z_{24227280})^2 \oplus (\mathbb Z_{411863760})^2\\ \oplus \mathbb Z_{2454438243748928651877425142836664498129840}\\ \oplus \mathbb Z_{14726629462493571911264550857019986988779040}\\ \oplus \mathbb Z^4\end{array}\ ,

H^3(SL_2({\cal O}_{-6}),P_{{\cal O}_{-6}}(24)) = (\mathbb Z_2)^{58} \oplus (\mathbb Z_{4})^4 \oplus (\mathbb Z_{12})\ .

    gap> R:=ResolutionSL2QuadraticIntegers(-6,4);
    Resolution of length 4 in characteristic 0 for PSL(2,O-6) .
    No contracting homotopy available.

    gap> G:=R!.group;;
    gap> M:=HomogeneousPolynomials(G,24);;
    gap> C:=HomToIntegralModule(R,M);;
    gap> Cohomology(C,1);
    [ 2, 2, 2, 2, 12, 24, 9240, 55440, 0, 0, 0, 0 ]
    gap> Cohomology(C,2);
    gap> Cohomology(C,2);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
      2, 6, 6, 6, 6, 6, 6, 6, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 24, 120,
      120, 840, 840, 840, 2520, 27720, 27720, 24227280, 24227280, 411863760,
      411863760, 2454438243748928651877425142836664498129840,
      14726629462493571911264550857019986988779040, 0, 0, 0, 0 ]
    gap> Cohomology(C,3);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
      2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
      2, 2, 2, 2, 2, 2, 2, 2, 4, 4, 4, 4, 12, 12 ]

The following commands construct free resolutions up to degree 5 for the groups SL_2( Z[frac12]), GL_2(cal O_-2), GL_2(cal O_2), PGL_2(cal O_2), GL_3(cal O_-2), PGL_3(cal O_-2). The final command constructs a free resolution up to degree 3 for PSL_4( Z).

    gap> R1:=ResolutionSL2ZInvertedInteger(2,5);
    Resolution of length 5 in characteristic 0 for SL(2,Z[1/2]) .

    gap> R2:=ResolutionGL2QuadraticIntegers(-2,5);
    Resolution of length 5 in characteristic 0 for GL(2,O-2) .
    No contracting homotopy available.

    gap> R3:=ResolutionGL2QuadraticIntegers(2,5);
    Resolution of length 5 in characteristic 0 for GL(2,O2) .
    No contracting homotopy available.

    gap> R4:=ResolutionPGL2QuadraticIntegers(2,5);
    Resolution of length 5 in characteristic 0 for PGL(2,O2) .
    No contracting homotopy available.

    gap> R5:=ResolutionGL3QuadraticIntegers(-2,5);
    Resolution of length 5 in characteristic 0 for GL(3,O-2) .
    No contracting homotopy available.

    gap> R6:=ResolutionPGL3QuadraticIntegers(-2,5);
    Resolution of length 5 in characteristic 0 for PGL(3,O-2) .
    No contracting homotopy available.

    gap> R7:=ResolutionArithmeticGroup("PSL(4,Z)",3);
    Resolution of length 3 in characteristic 0 for <matrix group with 655 generators> .
    No contracting homotopy available.

#### 13.12 Ideals and finite quotient groups

The following commands first construct the number field Q(sqrt-7), its ring of integers cal O_-7=cal O( Q(sqrt-7)), and the principal ideal I=⟨ 5 + 2sqrt-7⟩ ◃ cal O( Q(sqrt-7)) of norm cal N(I)=53. The ring I is prime since its norm is a prime number. The primality of I is also demonstrated by observing that the quotient ring R=cal O_-7/I is an integral domain and hence isomorphic to the unique finite field of order 53, R≅ Z/53 Z . (In a ring of quadratic integers prime ideal is the same as maximal ideal).

The finite group G=SL_2(cal O_-7/I) is then constructed and confirmed to be isomorphic to SL_2( Z/53 Z). The group G is shown to admit a periodic ZG-resolution of Z of period dividing 52.

Finally the integral homology

H_n(G,\mathbb Z) = \left\{\begin{array}{ll} 0 & n\ne 3,7, {\rm~for~} 0\le n \le 8,\\ \mathbb Z_{2808} & n=3,7, \end{array}\right.

is computed.

    gap> Q:=QuadraticNumberField(-7);
    Q(Sqrt(-7))

    gap> OQ:=RingOfIntegers(Q);
    O(Q(Sqrt(-7)))

    gap> I:=QuadraticIdeal(OQ,5+2*Sqrt(-7));
    ideal of norm 53 in O(Q(Sqrt(-7)))

    gap> R:=OQ mod I;
    ring mod ideal of norm 53

    gap> IsIntegralRing(R);
    true

    gap> gens:=GeneratorsOfGroup( SL2QuadraticIntegers(-7) );;
    gap> G:=Group(gens*One(R));;G:=Image(IsomorphismPermGroup(G));;
    gap> StructureDescription(G);
    "SL(2,53)"

    gap> IsPeriodic(G);
    true
    gap> CohomologicalPeriod(G);
    52

    gap> GroupHomology(G,1);
    [  ]
    gap> GroupHomology(G,2);
    [  ]
    gap> GroupHomology(G,3);
    [ 8, 27, 13 ]
    gap> GroupHomology(G,4);
    [  ]
    gap> GroupHomology(G,5);
    [  ]
    gap> GroupHomology(G,6);
    [  ]
    gap> GroupHomology(G,7);
    [ 8, 27, 13 ]
    gap> GroupHomology(G,8);
    [  ]

The following commands show that the rational prime 7 is not prime in cal O_-5=cal O( Q(sqrt-5)). Moreover, 7 totally splits in cal O_-5 since the final command shows that only the rational primes 2 and 5 ramify in cal O_-5.

    gap> Q:=QuadraticNumberField(-5);;
    gap> OQ:=RingOfIntegers(Q);;
    gap> I:=QuadraticIdeal(OQ,7);;
    gap> IsPrime(I);
    false

    gap> Factors(Discriminant(OQ));
    [ -2, 2, 5 ]

For d < 0 the rings cal O_d=cal O( Q(sqrtd)) are unique factorization domains for precisely

d = -1, -2, -3, -7, -11, -19, -43, -67, -163.

This result was conjectured by Gauss, and essentially proved by Kurt Heegner, and then later proved by Harold Stark.

The following commands construct the classic example of a prime ideal I that is not principal. They then illustrate reduction modulo I.

    gap> Q:=QuadraticNumberField(-5);;
    gap> OQ:=RingOfIntegers(Q);;
    gap> I:=QuadraticIdeal(OQ,[2,1+Sqrt(-5)]);
    ideal of norm 2 in O(Q(Sqrt(-5)))

    gap> 6 mod I;
    0

#### 13.13 Congruence subgroups for ideals

Given a ring of integers cal O and ideal I ◃ cal O there is a canonical homomorphism π_I: SL_2(cal O) → SL_2(cal O/I). A subgroup Γ ≤ SL_2(cal O) is said to be a congruence subgroup if it contains ker π_I. Thus congruence subgroups are of finite index. Generalizing the definition in 13.1 above, we define the principal congruence subgroup Γ_1(I)=ker π_I, and the congruence subgroup Γ_0(I) consisting of preimages of the upper triangular matrices in SL_2(cal O/I).

The following commands construct Γ=Γ_0(I) for the ideal I◃ cal O Q(sqrt-5) generated by 12 and 36sqrt-5. The group Γ has index 385 in SL_2(cal O Q(sqrt-5)). The final command displays a tree in a Cayley graph for SL_2(cal O Q(sqrt-5)) whose nodes represent a transversal for Γ.

    gap> Q:=QuadraticNumberField(-5);;
    gap> OQ:=RingOfIntegers(Q);;
    gap> I:=QuadraticIdeal(OQ,[36*Sqrt(-5), 12]);;
    gap> G:=HAP_CongruenceSubgroupGamma0(I);
    CongruenceSubgroupGamma0(ideal of norm 144 in O(Q(Sqrt(-5))))

    gap> IndexInSL2O(G);
    385

    gap> HAP_SL2TreeDisplay(G);

The next commands first construct the congruence subgroup Γ_0(I) of index 144 in SL_2(cal O Q(sqrt-2)) for the ideal I in cal O Q(sqrt-2) generated by 4+5sqrt-2. The commands then compute

H_1(\Gamma_0(I),\mathbb Z) = \mathbb Z_3 \oplus \mathbb Z_6 \oplus \mathbb Z_{30} \oplus \mathbb Z^8\, ,

H_2(\Gamma_0(I), \mathbb Z) = (\mathbb Z_2)^9 \oplus \mathbb Z^7\, ,

H_3(\Gamma_0(I), \mathbb Z) = (\mathbb Z_2)^9 \, .

    gap> Q:=QuadraticNumberField(-2);;
    gap> OQ:=RingOfIntegers(Q);;
    gap> I:=QuadraticIdeal(OQ,4+5*Sqrt(-2));;
    gap> G:=HAP_CongruenceSubgroupGamma0(I);
    CongruenceSubgroupGamma0(ideal of norm 66 in O(Q(Sqrt(-2))))

    gap> IndexInSL2O(G);
    144

    gap> R:=ResolutionSL2QuadraticIntegers(-2,4,true);;
    gap> S:=ResolutionFiniteSubgroup(R,G);;

    gap> Homology(TensorWithIntegers(S),1);
    [ 3, 6, 30, 0, 0, 0, 0, 0, 0, 0, 0 ]
    gap> Homology(TensorWithIntegers(S),2);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0 ]
    gap> Homology(TensorWithIntegers(S),3);
    [ 2, 2, 2, 2, 2, 2, 2, 2, 2 ]

#### 13.14 First homology

The isomorphism H_1(G, Z) ≅ G_ab allows for the computation of first integral homology using computational methods for finitely presented groups. Such methods underly the following computation of

H_1( \Gamma_0(I),\mathbb Z) \cong \mathbb Z_2 \oplus \cdots \oplus \mathbb Z_{4078793513671}

where I is the prime ideal in the Gaussian integers generated by 41+56sqrt-1.

    gap> Q:=QuadraticNumberField(-1);;
    gap> OQ:=RingOfIntegers(Q);;
    gap> I:=QuadraticIdeal(OQ,41+56*Sqrt(-1));
    ideal of norm 4817 in O(GaussianRationals)
    gap> G:=HAP_CongruenceSubgroupGamma0(I);;
    gap> AbelianInvariants(G);
    [ 2, 2, 4, 5, 7, 16, 29, 43, 157, 179, 1877, 7741, 22037, 292306033,
      4078793513671 ]

We write G^ab_tors to denote the maximal finite summand of the first homology group of G and refer to this as the torsion subgroup. Nicholas Bergeron and Akshay Venkatesh [Ber16] have conjectured relationships between the torsion in congruence subgroups Γ and the volume of their quotient manifold frak h^3/Γ. For instance, for the Gaussian integers they conjecture

\frac{\log |\Gamma_0(I)_{tors}^{ab}|}{{\rm Norm}(I)} \rightarrow \frac{\lambda}{18\pi},\ \lambda =L(2,\chi_{\mathbb Q(\sqrt{-1})}) = 1 -\frac{1}{9} + \frac{1}{25} - \frac{1}{49} + \cdots

as the norm of the prime ideal I tends to ∞. The following approximates λ/18π = 0.0161957 and fraclog |Γ_0(I)_tors^ab|{ Norm(I) = 0.00913432 for the above example.

    gap> Q:=QuadraticNumberField(-1);;
    gap> Lfunction(Q,2)/(18*3.142);
    0.0161957

    gap> 1.0*Log(Product(AbelianInvariants(F)),10)/Norm(I);
    0.00913432

The link with volume is given by the Humbert volume formula

{\rm Vol} ( {\frak h}^3 / PSL_2( {\cal O}_{d} ) ) = \frac{|D|^{3/2}}{24} \zeta_{ \mathbb Q( \sqrt{d} ) }(2)/\zeta_{\mathbb Q}(2)

valid for square-free d<0, where D is the discriminant of Q(sqrtd). The volume of a finite index subgroup Γis obtained by multiplying the right-hand side by the index |PSL_2(cal O_d): Γ|.

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