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<a id="X86D5DB887ACB1661" name="X86D5DB887ACB1661"></a>
<div class="ChapSects"><a href="chap13_mj.html#X86D5DB887ACB1661">13 Congruence Subgroups, Cuspidal Cohomology and Hecke Operators</a>
<div class="ContSect"> <a href="chap13_mj.html#X79A1974B7B4987DE">13.1 Eichler-Shimura isomorphism</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X7BFA2C91868255D9">13.2 Generators for \(SL_2(\mathbb Z)\) and the cubic tree</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X7D1A56967A073A8B">13.3 One-dimensional fundamental domains and
generators for congruence subgroups</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X818BFA9A826C0DB3">13.4 Cohomology of congruence subgroups</a>

<div class="ContSSBlock">
 <a href="chap13_mj.html#X7F55F8EA82FE9122">13.4-1 Cohomology with rational coefficients</a>

</div></div>
<div class="ContSect"> <a href="chap13_mj.html#X84D30F1580CD42D1">13.5 Cuspidal cohomology</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X80861D3F87C29C43">13.6 Hecke operators on forms of weight 2</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X831BB0897B988DA3">13.7 Hecke operators on forms of weight \( \ge 2\)</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X84CC51EE8525E0D9">13.8 Reconstructing modular forms from cohomology computations</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X8180E53C834301EF">13.9 The Picard group</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X858B1B5D8506FE81">13.10 Bianchi groups</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X851390E07C3B3BB1">13.11 (Co)homology of Bianchi groups and \(SL_2({\cal O}_{-d})\)</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X86A6858884B9C05B">13.12 Some other infinite matrix groups</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X7EF5D97281EB66DA">13.13 Ideals and finite quotient groups</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X7D1F72287F14C5E1">13.14 Congruence subgroups for ideals</a>

</div>
<div class="ContSect"> <a href="chap13_mj.html#X85E912617AFE03F4">13.15 First homology</a>

</div>
</div>

<h3>13 Congruence Subgroups, Cuspidal Cohomology and Hecke Operators</h3>

In this chapter we explain how HAP can be used to make computions about modular forms associated to congruence subgroups \(\Gamma\) of \(SL_2(\mathbb Z)\). Also, in Subsection 10.8 onwards, we demonstrate cohomology computations for the Picard group \(SL_2(\mathbb Z[i])\), some Bianchi groups \(PSL_2({\cal O}_{-d}) \) where \({\cal O}_{d}\) is the ring of integers of \(\mathbb Q(\sqrt{-d})\) 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 \({\cal O}=\mathbb Z, {\cal O}_{-d}\).

<a id="X79A1974B7B4987DE" name="X79A1974B7B4987DE"></a>

<h4>13.1 Eichler-Shimura isomorphism</h4>

We begin by recalling the Eichler-Shimura isomorphism <a href="chapBib_mj.html#biBeichler">[Eic57]</a><a href="chapBib_mj.html#biBshimura">[Shi59]</a>

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

which relates the cohomology of groups to the theory of modular forms associated to a finite index subgroup \(\Gamma\) of \(SL_2(\mathbb Z)\). 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 \(\Gamma\) of \(SL_2(\mathbb Z)\) is said to be a congruence subgroup of level \(N \) if it contains the kernel of the canonical homomorphism \(\pi_N\colon SL_2(\mathbb Z) \rightarrow SL_2(\mathbb Z/N\mathbb Z)\). So any congruence subgroup is of finite index in \(SL_2(\mathbb Z)\), but the converse is not true.

One congruence subgroup of particular interest is the group \(\Gamma_1(N)=\ker(\pi_N)\), known as the principal congruence subgroup of level \(N\). Another congruence subgroup of particular interest is the group \(\Gamma_0(N)\) of those matrices that project to upper triangular matrices in \(SL_2(\mathbb Z/N\mathbb Z)\).

A modular form of weight \(k\) for a congruence subgroup \(\Gamma\) is a complex valued function on the upper-half plane, \(f\colon {\frak{h}}=\{z\in \mathbb C : Re(z)>0\} \rightarrow \mathbb C\), satisfying:

<ul>
<li>\(\displaystyle f(\frac{az+b}{cz+d}) = (cz+d)^k f(z)\) for \(\left(\begin{array}{ll}a&b\\ c &d \end{array}\right) \in \Gamma\),

</li>
<li>\(f\) is `holomorphic' on the extended upper-half plane \(\frak{h}^\ast = \frak{h} \cup \mathbb Q \cup \{\infty\}\) obtained from the upper-half plane by `adjoining a point at each cusp'.

</li>
</ul>
The collection of all weight \(k\) modular forms for \(\Gamma\) form a vector space \(M_k(\Gamma)\) over \(\mathbb C\).

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

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

involving a summand \(E_k(\Gamma)\) known as the Eisenstein space. See <a href="chapBib_mj.html#biBstein">[Ste07]</a> 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 <a href="chapBib_mj.html#biBwieser">[Wie78]</a> for a full account of the isomorphism.

On the right-hand side of the isomorphism, the \(\mathbb Z\Gamma\)-module \(P_{\mathbb C}(k-2)\subset \mathbb C[x,y]\) denotes the space of homogeneous degree \(k-2\) polynomials with action of \(\Gamma\) given by

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

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

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 \(S_k(\Gamma)\) of cusp forms.

<a id="X7BFA2C91868255D9" name="X7BFA2C91868255D9"></a>

<h4>13.2 Generators for \(SL_2(\mathbb Z)\) and the cubic tree</h4>

The matrices \(S=\left(\begin{array}{rr}0&-1\\ 1 &0 \end{array}\right)\) and \(T=\left(\begin{array}{rr}1&1\\ 0 &1 \end{array}\right)\) generate \(SL_2(\mathbb Z)\) 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.

<div class="example"><pre>
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 ] ]

</pre></div>

It is convenient to introduce the matrix \(U=ST = \left(\begin{array}{rr}0&-1\\ 1 &1 \end{array}\right)\). The matrices \(S\) and \(U\) also generate \(SL_2(\mathbb Z)\). In fact we have a free presentation \(SL_2(\mathbb Z)= \langle S,U\, |\, S^4=U^6=1, S^2=U^3 \rangle \).

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 \({\cal U}=\langle U\rangle\) in \(SL_2(\mathbb Z)\) as vertices, and joining cosets \(x\,{\cal U} \) and \(y\,{\cal U}\) by an edge if, and only if, \(x^{-1}y \in {\cal U}\, S\,{\cal U}\). Thus the vertex \(\cal U \) is joined to \(S\,{\cal U} \), \(US\,{\cal U}\) and \(U^2S\,{\cal U}\). 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 \(SL_2(\mathbb Z)\) acts on \(\cal T\) in such a way that each vertex is stabilized by a cyclic subgroup conjugate to \({\cal U}=\langle U\rangle\) and each edge is stabilized by a cyclic subgroup conjugate to \({\cal S} =\langle S \rangle\).

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

<img src="images/fdsl2.png" align="center" width="350" alt="Information for the cubic tree"/>

<a id="X7D1A56967A073A8B" name="X7D1A56967A073A8B"></a>

<h4>13.3 One-dimensional fundamental domains and
generators for congruence subgroups</h4>

The modular group \({\cal M}=PSL_2(\mathbb Z)\) is isomorphic, as an abstract group, to the free product \(\mathbb Z_2\ast \mathbb Z_3\). By the Kurosh subgroup theorem, any finite index subgroup \(M \subset {\cal M}\) is isomorphic to the free product of finitely many copies of \(\mathbb Z_2\)s, \(\mathbb Z_3\)s and \(\mathbb Z\)s. A subset \(\underline x \subset M\) 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 \(SL_2(\mathbb Z)\) is projectively independent if it maps injectively onto an independent set of subgroup generators \(\underline x\subset {\cal M}\). The generating set \(\{S,U\}\) for \(SL_2(\mathbb Z)\) given in the preceding section is projectively independent.

We are interested in constructing a set of generators for a given congruence subgroup \(\Gamma\). If a small generating set for \(\Gamma\) 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 \(\mathbb ZG\)-resolutons in <a href="chapBib_mj.html#biBellisharrisskoldberg">[EHS06, Theorem 2]</a> and an old observation of John Milnor that a free \(\mathbb ZG\)-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 \(C_\ast X\) is a \(\mathbb ZG\)-resolution of \(\mathbb Z\) which typically is not free. Let \([e^n]\) denote the orbit of the n-cell \(e^n\) under the action. Let \(G^{e^n} \le G\) 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 \([e^p]\otimes [f^q]\) 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 \(B^1_G\setminus T\) correspond to a generating set for \(G\).

Suppose we wish to compute a set of generators for a principal congruence subgroup \(\Gamma=\Gamma_1(N)\). In the above theorem take \(X={\cal T}\) to be the cubic tree, and note that \(\Gamma\) acts freely on \(\cal T\) and thus that \(W={\cal T}\). To determine the \(1\)-cells of \(B_{\Gamma}\setminus T\) we need to determine a cellular subspace \(D_\Gamma \subset \cal T\) whose images under the action of \(\Gamma\) cover \(\cal T\) and are pairwise either disjoint or identical. The subspace \(D_\Gamma\) 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 \(D_\Gamma\) a fundamental region for \(\Gamma\). We denote by \(\mathring D_\Gamma\) the largest CW-subcomplex of \(D_\Gamma\). The vertices of \(\mathring D_\Gamma\) are the same as the vertices of \(D_\Gamma\). Thus \(\mathring D_\Gamma\) is a subtree of the cubic tree with \(|\Gamma|/6\) vertices. For each vertex \(v\) in the tree \(\mathring D_\Gamma\) define \(\eta(v)=3 -{\rm degree}(v)\). Then the number of generators for \( \Gamma \) will be \((1/2)\sum_{v\in \mathring D_\Gamma} \eta(v)\).

The following commands determine projectively independent generators for \(\Gamma_1(6)\) and display \(\mathring D_{\Gamma_1(6)}\). The subgroup \(\Gamma_1(6)\) is free on \(13\) generators.

<div class="example"><pre>
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 ] ] ]

</pre></div>

<img src="images/pctree6.gif" align="center" width="350" alt="Fundamental region in the cubic tree"/>

An alternative but very related approach to computing generators of congruence subgroups of \(SL_2(\mathbb Z)\) is described in <a href="chapBib_mj.html#biBkulkarni">[Kul91]</a>.

The congruence subgroup \(\Gamma_0(N)\) 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 \(\Gamma_0(N)\) acts freely. This alternative approach will produce a redundant set of generators. The following commands display \(\mathring D_{\Gamma_0(39)}\) for a fundamental region in \({\cal T}'\). They also use the corresponding generating set for \(\Gamma_0(39)\), involving \(18\) generators, to compute the abelianization \(\Gamma_0(39)^{ab}= \mathbb Z_2 \oplus \mathbb Z_3^2 \oplus \mathbb Z^9\). The abelianization shows that any generating set has at least \(11\) generators.

<div class="example"><pre>
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 ]

</pre></div>

<img src="images/g0tree39.gif" align="center" width="350" alt="Fundamental region in the cubic tree"/>

Note that to compute \(D_\Gamma\) one only needs to be able to test whether a given matrix lies in \(\Gamma\) or not. Given an inclusion \(\Gamma'\subset \Gamma\) of congruence subgroups, it is straightforward to use the trees \(\mathring D_{\Gamma'}\) and \(\mathring D_{\Gamma}\) to compute a system of coset representative for \(\Gamma'\setminus \Gamma\).

<a id="X818BFA9A826C0DB3" name="X818BFA9A826C0DB3"></a>

<h4>13.4 Cohomology of congruence subgroups</h4>

To compute the cohomology \(H^n(\Gamma,A)\) of a congruence subgroup \(\Gamma\) with coefficients in a \(\mathbb Z\Gamma\)-module \(A\) we need to construct \(n+1\) terms of a free \(\mathbb Z\Gamma\)-resolution of \(\mathbb Z\). We can do this by first using perturbation techniques (as described in <a href="chapBib_mj.html#biBbuiellis">[BE14]</a>) to combine the cubic tree with resolutions for the cyclic groups of order \(4\) and \(6\) in order to produce a free \(\mathbb ZG\)-resolution \(R_\ast\) for \(G=SL_2(\mathbb Z)\). This resolution is also a free \(\mathbb Z\Gamma\)-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

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

<div class="example"><pre>
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 ]

</pre></div>

This computation establishes that the space \(M_2(\Gamma_0(39))\) of weight \(2\) modular forms is of dimension \(9\).

The following commands show that \({\rm rank}_{\mathbb Z\Gamma_0(39)} R_1 = 112\) but that it is possible to apply `Tietze like' simplifications to \(R_\ast\) to obtain a free \(\mathbb Z\Gamma_0(39)\)-resolution \(T_\ast\) with \({\rm rank}_{\mathbb Z\Gamma_0(39)} T_1 = 11\). It is more efficient to work with \(T_\ast\) when making cohomology computations with coefficients in a module \(A\) of large rank.

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

gap> T!.dimension(1);
11

</pre></div>

The following commands compute

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

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

<div class="example"><pre>
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 ]

</pre></div>

This computation establishes that the space \(M_{10}(\Gamma_0(39))\) of weight \(10\) modular forms is of dimension \(84\), and \(M_{11}(\Gamma_0(39))\) is of dimension \(0\). (There are never any modular forms of odd weight, and so \(M_k(\Gamma)=0\) for all odd \(k\) and any congruence subgroup \(\Gamma\).)

<a id="X7F55F8EA82FE9122" name="X7F55F8EA82FE9122"></a>

<h5>13.4-1 Cohomology with rational coefficients</h5>

To calculate cohomology \(H^n(\Gamma,A)\) with coefficients in a \(\mathbb Q\Gamma\)-module \(A\) it suffices to construct a resolution of \(\mathbb Z\) by non-free \(\mathbb Z\Gamma\)-modules where \(\Gamma\) acts with finite stabilizer groups on each module in the resolution. Computing over \(\mathbb Q\) is computationally less expensive than computing over \(\mathbb Z\). The following commands first compute \(H^1(\Gamma_0(39),\mathbb Q) = H_1(\Gamma_0(39),\mathbb Q)= \mathbb Q^9\). As a larger example, they then compute \(H^1(\Gamma_0(2^{13}-1),\mathbb Q) =\mathbb Q^{1365}\) where \(\Gamma_0(2^{13}-1)\) has index \(8192\) in \(SL_2(\mathbb Z)\).

<div class="example"><pre>
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

</pre></div>

<a id="X84D30F1580CD42D1" name="X84D30F1580CD42D1"></a>

<h4>13.5 Cuspidal cohomology</h4>

To define and compute cuspidal cohomology we consider the action of \(SL_2(\mathbb Z)\) 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.

<img src="images/filename-1.png" align="center" width="450" alt="Fundamental domain in the upper-half plane"/>

The action factors through an action of \(PSL_2(\mathbb Z) =SL_2(\mathbb Z)/\langle \left(\begin{array}{rr}-1&0\\ 0 &-1 \end{array}\right)\rangle\). The images of \(D\) under the action of \(PSL_2(\mathbb Z)\) 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 \(\langle U\rangle\), and the bottom middle point which is stabilized by \(\langle S\rangle\).

A congruence subgroup \(\Gamma\) has a `fundamental domain' \(D_\Gamma\) equal to a union of finitely many copies of \(D\), one copy for each coset in \(\Gamma\setminus SL_2(\mathbb Z)\). The quotient space \(X=\Gamma\setminus {\frak h}\) 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 \(X\hookrightarrow X^{BS}\) and this inclusion is a homotopy equivalence. One defines the boundary \(\partial X^{BS} = X^{BS} - X\) and uses the inclusion \(\partial X^{BS} \hookrightarrow X^{BS} \simeq X\) to define the cuspidal cohomology group, over the ground ring \(\mathbb C\), 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 \(H_!^n(\Gamma,P_{\mathbb C}(k-2))\) which in general contains the cuspidal cohomology as a subgroup. However, for congruence subgroups of \(SL_2(\mathbb Z)\) there is equality \(H_!^n(\Gamma,P_{\mathbb C}(k-2)) = H_{cusp}^n(\Gamma,P_{\mathbb C}(k-2))\).

Working over \(\mathbb C\) has the advantage of avoiding the technical issue that \(\Gamma \) does not necessarily act freely on \({\frak h}\) since there are points with finite cyclic stabilizer groups in \(SL_2(\mathbb Z)\). 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 \(\Gamma\) acts freely, and \(\Gamma\)-equivariant inclusion \(\partial \tilde X^{BS} \hookrightarrow \tilde X^{BS}\). The definition of cuspidal cohomology that we use, which coincides with the above definition when working over \(\mathbb C\), 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 \(\tilde X^{BS}\).

<img src="images/filename-2.png" align="center" width="450" alt="Borel-Serre compactified fundamental domain in the upper-half plane"/>

The following commands calculate

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

<div class="example"><pre>
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 ]

</pre></div>

From the Eichler-Shimura isomorphism and the already calculated dimension of \(M_2(\Gamma_0(39))\cong \mathbb C^9\), we deduce from this cuspidal cohomology that the space \(S_2(\Gamma_0(39))\) of cuspidal weight \(2\) forms is of dimension \(3\), and the Eisenstein space \(E_2(\Gamma_0(39))\cong \mathbb C^3\) is of dimension \(3\).

The following commands show that the space \(S_4(\Gamma_0(39))\) of cuspidal weight \(4\) forms is of dimension \(12\).

<div class="example"><pre>
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 ]

</pre></div>

<a id="X80861D3F87C29C43" name="X80861D3F87C29C43"></a>

<h4>13.6 Hecke operators on forms of weight 2</h4>

A congruence subgroup \(\Gamma \le SL_2(\mathbb Z)\) and element \(g\in SL_2(\mathbb Q)\) determine the subgroup \(\Gamma' = \Gamma \cap g\Gamma g^{-1} \) 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 \(\alpha\), \(\beta\) functorial maps, and \(tr\) the transfer map. We define the composite \(T_g=tr \circ \alpha \circ \beta\colon H^n(\Gamma, \mathbb Z) \rightarrow H^n(\Gamma, \mathbb Z)\) to be the Hecke component determined by \(g\).

For \(\Gamma=\Gamma_0(N)\), prime integer \(p\) coprime to \(N\), and cohomology degree \(n=1\) we define the Hecke operator \(T_p =T_g\) where \(g=\left(\begin{array}{cc}1&0\\0&p\end{array}\right)\). Further details on this description of Hecke operators can be found in <a href="chapBib_mj.html#biBstein">[Ste07, Appendix by P. Gunnells]</a>.

The following commands compute \(T_2\) and \(T_5\) and \(\Gamma=\Gamma_0(39)\). 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 \(p,q\).)

<div class="example"><pre>
gap> gamma:=HAP_CongruenceSubgroupGamma0(39);;
gap> p:=2;;k:=2;;T2:=HeckeOperator(gamma,p,k);;
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;;k:=2;;h:=HeckeOperator(gamma,p,k);;
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

</pre></div>

<a id="X831BB0897B988DA3" name="X831BB0897B988DA3"></a>

<h4>13.7 Hecke operators on forms of weight \( \ge 2\)</h4>

The above definition of Hecke operator \(T_p\) for \(\Gamma=\Gamma_0(N)\) extends to a Hecke operator \(T_p\colon H^1(\Gamma,P_{\mathbb Q}(k-2) ) \rightarrow H^1(\Gamma,P_{\mathbb Q}(k-2) )\) for \(k\ge 2\). We work over the rationals since that is a setting of much interest. The following commands compute the matrix of \(T_2\colon H^1(\Gamma,P_{\mathbb Q}(k-2) ) \rightarrow H^1(\Gamma,P_{\mathbb Q}(k-2) )\) for \(\Gamma=SL_2(\mathbb Z)\) and \(k=4\);

<div class="example"><pre>
gap> H:=HAP_CongruenceSubgroupGamma0(1);;
gap> h:=HeckeOperator(H,2,12);;Display(h);
[ [ 2049, -7560, 0 ],
 [ 0, -24, 0 ],
 [ 0, 0, -24 ] ]

</pre></div>

<a id="X84CC51EE8525E0D9" name="X84CC51EE8525E0D9"></a>

<h4>13.8 Reconstructing modular forms from cohomology computations</h4>

Given a modular form \(f\colon {\frak h} \rightarrow \mathbb C\) associated to a congruence subgroup \(\Gamma\), 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 \(SL_2(\mathbb Z)\), we can evaluate

\[\int_e f(z)\,dz \ .\]

In this way we obtain a cochain \(f_1\colon C_1({\cal T}) \rightarrow \mathbb C\) in \(Hom_{\mathbb Z\Gamma}(C_1({\cal T}), \mathbb C)\) representing a cohomology class \(c(f) \in H^1(\, Hom_{\mathbb Z\Gamma}(C_\ast({\cal T}), \mathbb C) \,) = H^1(\Gamma,\mathbb C)\). The correspondence \(f\mapsto c(f)\) underlies the Eichler-Shimura isomorphism. Hecke operators can be used to recover modular forms from cohomology classes.

Let \(\Gamma=\Gamma_0(N)\). The above defined Hecke operators restrict to operators on cuspidal cohomology. On the left-hand side of the Eichler-Shimura isomorphism Hecke operators restrict to operators \(T_s\colon S_2(\Gamma) \rightarrow S_2(\Gamma)\) for \(s\ge 1\).

Consider the function \(q=q(z)=e^{2\pi i z}\) which is holomorphic on \(\mathbb C\). For any modular form \(f(z) \in M_k(\Gamma)\) there are numbers \(a_s\) such that

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

for all \(z\in {\frak h}\). The form \(f\) is a cusp form if \(a_0=0\).

A non-zero cusp form \(f\in S_2(\Gamma)\) is a cusp eigenform if it is simultaneously an eigenvector for the Hecke operators \(T_s\) for all \(s =1,2,3,\cdots\) coprime to the level \(N\). A cusp eigenform is said to be normalized if its coefficient \(a_1=1\). It turns out that if \(f\) is normalized then the coefficient \(a_s\) is an eigenvalue for \(T_s\) (see for instance <a href="chapBib_mj.html#biBstein">[Ste07]</a> for details). It can be shown <a href="chapBib_mj.html#biBatkinlehner">[AL70]</a> that \(S_2(\Gamma_0(N))\) admits a "basis constructed from eigenforms".

This all implies that, in principle, we can construct an approximation to an explicit basis for the space \(S_2(\Gamma_0(N))\) of cusp forms by computing eigenvalues for Hecke operators.

Suppose that we would like a basis for \(S_2(\Gamma_0(11))\). The following commands first show that \(H^1_{cusp}(\Gamma_0(11),\mathbb Z)=\mathbb Z\oplus \mathbb Z\) from which we deduce that \(S_2(\Gamma_0(11)) =\mathbb C\) is \(1\)-dimensional and thus admits a basis of eigenforms. 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 \(S_2(\Gamma_0(11))\).

<div class="example"><pre>
gap> gamma:=HAP_CongruenceSubgroupGamma0(11);;
gap> AbelianInvariants(Kernel(CuspidalCohomologyHomomorphism(gamma,1,2)));
[ 0, 0 ]

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

</pre></div>

For a normalized eigenform \(f=1 + \sum_{s=2}^\infty a_sq^s\) 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 \(\Gamma\) 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 \(S_{12}(SL_2(\mathbb Z))\) has a basis consisting of one cusp eigenform

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

\(- 115920q^{10} + 534612q^{11} - 370944q^{12} - 577738q^{13} + 401856q^{14} + 1217160q^{15} + 987136q^{16}\)

\( - 6905934q^{17} + 2727432q^{18} + 10661420q^{19} + ...\)

<div class="example"><pre>
gap> R:=ResolutionSL2Z_alt(2);;
gap> G:=R!.group;;
gap> P:=HomogeneousPolynomials(G,14);
MappingByFunction( SL(2,Integers), <matrix group with
2 generators>, function( x ) ... end )
gap> Cohomology(HomToIntegralModule(R,P),1);
[ 2, 2, 156, 0, 0, 0 ]
gap> #Thus the space S_12 of cusp forms is of dimension 1

gap> G:=HAP_CongruenceSubgroupGamma0(1);;
gap> for p in [2,3,5,7,11,13,17,19] do
> T:=HeckeOperator(G,p,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

</pre></div>

<a id="X8180E53C834301EF" name="X8180E53C834301EF"></a>

<h4>13.9 The Picard group</h4>

Let us now consider the Picard group \(G=SL_2(\mathbb Z[ i])\) 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

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

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

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

<img src="images/picarddomain.png" align="center" width="350" alt="Fundamental domain for the Picard group"/>

The four bottom vertices of \(D\) are \(a = -\frac{1}{2} +\frac{1}{2}i +\frac{\sqrt{2}}{2}j\), \(b = -\frac{1}{2} +\frac{\sqrt{3}}{2}j\), \(c = \frac{1}{2} +\frac{\sqrt{3}}{2}j\), \(d = \frac{1}{2} +\frac{1}{2}i +\frac{\sqrt{2}}{2}j\).

The upper-half space \({\frak h}^3\) can be retracted onto a \(2\)-dimensional subspace \({\cal T} \subset {\frak h}^3\). 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 \({\cal T}\).

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 \(\mathbb ZG\)-resolution \(R_\ast =C_\ast X\). Then \(R_\ast\) 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.

<div class="example"><pre>
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 ] )

</pre></div>

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

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

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

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

<div class="example"><pre>
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
]

</pre></div>

<a id="X858B1B5D8506FE81" name="X858B1B5D8506FE81"></a>

<h4>13.10 Bianchi groups</h4>

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 \(\mathbb Q(\sqrt{-d})\). More explicitly,

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

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

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 \({\cal T}\subset {\frak h}^3\) 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 <a href="chapBib_mj.html#biBswan">[Swa71a]</a> has been implemented by Alexander Rahm <a href="chapBib_mj.html#biBrahmthesis">[Rah10]</a> 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.

<div class="example"><pre>
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 ]

</pre></div>

We can also consider the coefficient module

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