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<p id="mathjaxlink" class="pcenter"><a href="chap14.html">[MathJax off]</a></p>
<p><a id="X805848868005D528" name="X805848868005D528"></a></p>
<div class="ChapSects"><a href="chap14_mj.html#X805848868005D528">14 <span class="Heading">Fundamental domains for Bianchi groups</span></a>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X858B1B5D8506FE81">14.1 <span class="Heading">Bianchi groups</span></a>
</span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X872D22507F797001">14.2 <span class="Heading">Swan's description of a fundamental domain</span></a>
</span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X7B9DE54F7ECB7E44">14.3 <span class="Heading">Computing a fundamental domain</span></a>
<span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X7A489A5D79DA9E5C">14.4 <span class="Heading">Examples</span></a>
</span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X86CD59CB7A04EE5A">14.5 <span class="Heading">Establishing correctness of a fundamental domain</span></a>
</span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X78476F127B73BBD1">14.6 <span class="Heading">Computing a free resolution for <span class="SimpleMath">\(SL_2({\mathcal O}_{-d})\)</span></span></a>
</span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X784B2156823AEB15">14.7 <span class="Heading">Some sanity checks</span></a>
</span>
<div class="ContSSBlock">
<span class="ContSS"><br /><span class="nocss">  </span><a href="chap14_mj.html#X7E5A36D47F9D4A47">14.7-1 <span class="Heading">Equivariant Euler characteristic</span></a>
</span>
<span class="ContSS"><br /><span class="nocss">  </span><a href="chap14_mj.html#X852CDAFF84C5DF01">14.7-2 <span class="Heading">Boundary squares to zero</span></a>
</span>
<span class="ContSS"><br /><span class="nocss">  </span><a href="chap14_mj.html#X7E64819A7C058EDD">14.7-3 <span class="Heading">Compare different algorithms or implementations</span></a>
</span>
<span class="ContSS"><br /><span class="nocss">  </span><a href="chap14_mj.html#X8223864085412705">14.7-4 <span class="Heading">Compare geometry to algebra</span></a>
</span>
</div></div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X78BC9D077956089A">14.8 <span class="Heading">Group presentations</span></a>
</span>
</div>
<div class="ContSect"><span class="tocline"><span class="nocss"> </span><a href="chap14_mj.html#X786CFAA17C0A6E7A">14.9 <span class="Heading">Finite index subgroups</span></a>
</span>
</div>
</div>

<h3>14 <span class="Heading">Fundamental domains for Bianchi groups</span></h3>

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

<h4>14.1 <span class="Heading">Bianchi groups</span></h4>

<p>The <em>Bianchi groups</em> are the groups <span class="SimpleMath">\(G_{-d}=PSL_2({\cal O}_{-d})\)</span> where <span class="SimpleMath">\(d\)</span> is a square free positive integer and <span class="SimpleMath">\({\cal O}_{-d}\)</span> is the ring of integers of the imaginary quadratic field <span class="SimpleMath">\(\mathbb Q(\sqrt{-d})\)</span>. These groups act on <em>upper-half space</em></p>

<p class="center">\[{\frak h}^3 =\{(z,t) \in \mathbb C\times \mathbb R\ |\ t > 0\}  \]</p>

<p>by the formula</p>

<p class="center">\[\left(\begin{array}{ll}a&b\\ c &d \end{array}\right)\cdot (z+tj) \ = \ \left(a(z+tj)+b\right)\left(c(z+tj)+d\right)^{-1}\ \]</p>

<p>where we use the symbol <span class="SimpleMath">\(j\)</span> satisfying <span class="SimpleMath">\(j^2=-1\)</span>, <span class="SimpleMath">\(ij=-ji\)</span> and write <span class="SimpleMath">\(z+tj\)</span> instead of <span class="SimpleMath">\((z,t)\)</span>. Alternatively, the action is given by</p>

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

<p>We take the boundary <span class="SimpleMath">\(\partial {\frak h}^3\)</span> to be the Riemann sphere <span class="SimpleMath">\(\mathbb C \cup \infty\)</span> and let <span class="SimpleMath">\(\overline{\frak h}^3\)</span> denote the union of <span class="SimpleMath">\({\frak h}^3\)</span> and its boundary. The action of <span class="SimpleMath">\(G_{-d}\)</span> extends to the boundary. The element <span class="SimpleMath">\(\infty\)</span> and each element of the number field <span class="SimpleMath">\(\mathbb Q(\sqrt{-d})\)</span> are thought of as lying in the boundary <span class="SimpleMath">\(\partial {\frak h}^3\)</span> and are referred to as <em>cusps</em>. Let <span class="SimpleMath">\(X\)</span> denote the union of <span class="SimpleMath">\({\frak h}^3\)</span> with the set of cusps, <span class="SimpleMath">\(X={\frak h}^3 \cup \{\infty\} \cup \mathbb Q(\sqrt{-d})\)</span>. It follows from work of Bianchi and Humbert that the space <span class="SimpleMath">\(X\)</span> admits the structure of a regular CW-complex (depending on <span class="SimpleMath">\(d\)</span>) for which the action of <span class="SimpleMath">\(G_{-d}\)</span> on <span class="SimpleMath">\({\frak h}^3\)</span> extends to a cellular action on <span class="SimpleMath">\(X\)</span> which permutes cells. Moreover, <span class="SimpleMath">\(G_{-d}\)</span> acts transitively on the <span class="SimpleMath">\(3\)</span>-cells of <span class="SimpleMath">\(X\)</span> and each <span class="SimpleMath">\(3\)</span>-cell has trivial stabilizer in <span class="SimpleMath">\(G_{-d}\)</span>. Details are provided in Richard Swan's paper <a href="chapBib_mj.html#biBswanB">[Swa71b]</a>.</p>

<p>We refer to the closure in <span class="SimpleMath">\(X\)</span> of any one of these <span class="SimpleMath">\(3\)</span>-cells as a <em>fundamental domain</em> for the action <span class="SimpleMath">\(G_{-d}\)</span>. Cohomology of <span class="SimpleMath">\(G_{-d}\)</span> can be computed from a knowledge of the combinatorial structure of this fundamental domain together with a knowledge of the stabilizer groups of the cells of dimension <span class="SimpleMath">\(\le 2\)</span>.</p>

<p><a id="java.lang.StringIndexOutOfBoundsException: Index 15 out of bounds for length 0

<h4>14.2 <span class="Heading">Swan's description of a fundamental domain</span></h4>

<p>A pair <span class="SimpleMath">\((a,b)\)</span> of elements in <span class="SimpleMath">\({\cal O}_{-d}\)</span> is said to be <em>unimodular</em> if the 

<p>When  span =""\D=overlineF \ap B})<span> where  closurehas span "impleMath>\\ D\<> four   <span class=S"\2\<>-    <=>(\/>ofjava.lang.StringIndexOutOfBoundsException: Range [371, 370) out of bounds for length 1069

<p>It is explained in <a href="chapBib_mj.html#biBswanB">[Swa71b]</a> that <span class="SimpleMath">\(F\cap B\)</span> is a <span class="SimpleMath">\(3\)</span>-cell in the above mentioned regular CW-complex structure on <span class="SimpleMath">\(X\)</span>.</p>

<p><a id="X7B9DE54F7ECB7E44" name="X7B9DE54F7ECB7E44"></a></p>

<h4>14.3 <span class="Heading">Computing a fundamental domain</span></h4>

<p>Explicit fundamental domains for certain values of <span class="SimpleMath

<p>More recently a <strong class="button">GAP</strong> implementation of Swan's algorithm has been included in <strong class="button">HAP</strong>. The implementation java.lang.StringIndexOutOfBoundsException: Index 169 out of bounds for length 0

<p><a id="X7A489A5D79DA9E5C" name="X7A489A5D79DA9E5C">involving  of minimum radius 1/9java.lang.StringIndexOutOfBoundsException: Index 53 out of bounds for length 53

<h4>14.4 <span java.lang.StringIndexOutOfBoundsException: Index 17 out of bounds for length 0

<p>The fundamental domainjava.lang.StringIndexOutOfBoundsException: Range [6, 5) out of bounds for length 83

<java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0


<div java.lang.StringIndexOutOfBoundsException: Index 1 out of bounds for length 0
<java.lang.StringIndexOutOfBoundsException: Range [81, 5) out of bounds for length 95
3-dimensional
p em>usp vertex</em> of<span ="SimpleMath"\D)/>   of  =">\\)/s>  :  2  they   byred dots.Computer calculations show that these cusps lie in precisely three orbits under the action of <span class="SimpleMath">\(G_{-d}\)</span>. Thus, together with the orbit of <span class="SimpleMath">\(\infty\)</span> there are four distinct orbits of cusps. By the well-known correspondence between cusp orbits and elements of the class group it follows that the class group of <span class="SimpleMath">\(\mathbb Q(\sqrt{-39})\)</span> is of order <span class="SimpleMath">\(4\)</span>.</p>
and non-cuspidal vertices of minimum squared height 1/49 . 

<span class="GAPprompt">gap<pan class="APprompt"gap&<span span=":=(D;/span>
<span class="GAPprompt">gap&java.lang.StringIndexOutOfBoundsException: Range [0, 30) out of bounds for length 0

/>/>

<p><img

<p>A 

<p>The following additional commands comvert the Bianchi polyhedron <span class="SimpleMath">\(D\)</span> to a regular CW-java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0


<div class="example"><pre>
<span class="div class="example<re>
<panclass"APprompt">gap&;/ span ">:=D;<java.lang.StringIndexOutOfBoundsException: Index 92 out of bounds for length 92
Regular CW-complex of dimension 2

<span p><img src="images/bianchi3D22.png" align="center" height="550" alt="Fundamental domain for a Bianchi group"/> <img src=

</pre></div>

<p><img src="images/bianchi1skeleton.gif" align="center" java.lang.StringIndexOutOfBoundsException: Index 60 out of bounds for length 0

<p>A span class="GAP<><class"OQ,D<span


<div class="example"><pre>
 ="java.lang.StringIndexOutOfBoundsException: Range [28, 27) out of bounds for length 96
< java.lang.StringIndexOutOfBoundsException: Range [12, 11) out of bounds for length 86
<span class="GAPprompt">gap></span> <span class="GAPinput">Display2D(OQ,D);;<java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0

</pre>h4>145 spanclass"

<p

<p>Two cusps are visible in

<p>A<pan class"APprompt>gap></pan><span class="GAPinput">P:=BianchiPolyhedron(-46);</span>


<div class="example"><pre>
<span class="P:=ianchiPolyhedron(OQ,N);
<span class="GAPprompt">gap></span> <span class="GAPinput">Display3D(OQ,D);;</span>
<an class="GAPinput">Display2D(OQ,D);;/span>

</pre></divSwanBianchiCriterion);

<p><img src="images/bianchi3D163.png" align="center" height="550" alt="Fundamental domain for a Bianchi group"/> <img src=t testif theof waslarge.If testreturns thenyou\

<> is  a orbit ofcuspsin this ,  orbit containing <span class="SimpleMath">\(\infty\)</span>, since <span class="SimpleMath">\(\mathbb Q(\sqrt{-163})\)</ich canbe edited inthe hap//bianchig .

<p>A fundamental domain for <span class="SimpleMath">\(G_{-33}\)</span> is visualizeddimensionalBianchi  overOQ (


<div class="example"><pre
<java.lang.StringIndexOutOfBoundsException: Range [22, 5) out of bounds for length 96
<span class="GAPprompt">java.lang.StringIndexOutOfBoundsException: Index 26 out of bounds for length 0
<>a="78476F127B73BBD1"name"78476F127B73BBD1"<a<p

</>/>

<java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0

<p><a id=java.lang.StringIndexOutOfBoundsException: Index 9 out of bounds for length 0

<h4>14.5 <span =GAPprompt"gap&<span> <span class=>:=BianchiGcomplex(-43);;</span>

<java.lang.StringIndexOutOfBoundsException: Range [6, 2) out of bounds for length 611

<p>For a few values of <span class="SimpleMath">\ >apg;/ span =GAPinputC(R;<spanjava.lang.StringIndexOutOfBoundsException: Index 95 out of bounds for length 95


<div class="example"><pre>
<span  class"GAPprompt">gap></span> <span class="GAPinput">P:=BianchiPolyhedron(-46);</span>
Try 
  :=BianchiPolyhedronO,N)
for java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0
  SwanBianchiCriterion(P);
to test if the value of N was large enough. If the test returns false then you\
'll need to try a larger value of N.

A successful value<span class"">ap&;span>spanclass"APinput">[.0],-HomologyR,);/
ichcan edited manually  the file /libCongruence/ianchi. .


<span class="  [ 2, 4, 12 ],,4,12],[ 2 ,2  ,[2,2 2, 12 ,[2 2 ,24 , 
3-dimensional Bianchi polyhedron over OQ( Sqrt(
-))involving  of  squared 1/
441 and non-cuspidal vertices of minimum squared

<span class="GAPprompt">gap></span> <span class="GAPinput">SwanBianchiCriterion(P);</span>
true

<pre></iv>

<p><a id[[0],[6 00,00 0 ] [2,22, ,12 00,0,0 ] 

<h4>14.6 <span class="Heading">Computing a free resolution for <span class="SimpleMath">\(SL_2({\mathcal 

<p>The above fundamental domains can be


<div class="example"><pre>
<span class="GAPprompt">gap></java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0
<span class="GAPprompt">gap></span> <span class=p>  commands the of orbits  cusps i addition to the orbitofspan class"SimpleMath">\(\infty\)</span>). They determine that there is precisely one 
<span>&;/><spanclass">([1.K.dimension()]-&tOrder(K!.stabilizer(0,k)));<span>
<span class="GAPprompt">gap></span> <span class="GAPinput">List([0..10],n->Homology(C,n));</span>
[ [ 0 ], [ 00 ], [ 22120 ], [ 2224 ], [ 22 ],
  [ 2, <classjava.lang.StringIndexOutOfBoundsException: Range [23, 22) out of bounds for length 128

<span class=[6 ,4 ,6,]
<span class="GAPprompt">gap></span> <span class="GAPinput">R:=FreeGResolution(K,11);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">ist0.,gHomologyTensorWithIntegers)));<span
[, 00],[22 2,12 , 0], [2,2224 ],
   ,4 12 , [2 , 2,6]  222 12 ,[2 2,2,24
  [ 2412 ], [ 2226 ], [ 22212 ] ]

<span class="GAPprompt">gap></span> <
<span class="GAPprompt">gap></span> <span class="GAPinput">R:=FreeGResolution(K,11);;</span>
<span class="GAPprompt">gap></span> <span class=<>a id"X7E5A36D47F9D4A47" name="X7E5A36D47F9D4A47"></a></p>
[ [0], [ 6 , 0,0,0,0] [2,22 4,12 0,0,0,0 ] 
  [ 222224 ], [ 222412 ], [ 222224 ], 
  [ 2,22,4,12 ], [22,, 2 224] ,4 12 ], 
  [ 222224 ], java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0

</pre></div>

<span class"APprompt"gap>/ spanclass=GAPinputchi=;<spanjava.lang.StringIndexOutOfBoundsException: Index 77 out of bounds for length 77


<div class="example"><pre>
<<spanclass="GAPprompt">></span> <span class="GAPinput">for k in [1..K!.dimension(n)] do</span>
<span class="GAPprompt">gap<span class="GAPprompt">></span> <span class="GAPinput">g:=Order(K!.stabilizer(n,k));</span>
 , , ,4 4 ,12 ]

<span class="GAPprompt">gap></span> <span class="GAPinput">K:=BianchiGcomplex(-10);;</span>
<span class=GAPprompt>apgt;/span> <span class=GAPinput"List([1..K!.dimension(0)],k->Order(K!.stabilizer(0,k)));</span>
66446, infinity ]

<span class="GAPprompt">gap></span> <span class="GAPinput0
<span class="GAPprompt">gap></span> <span class="GAPinput">List([1..K!.dimension(0)],k->Order(K!.stabilizer(0,k)));</span>
66222, infinity, infinity, 2, infinity, 64 ]

</pre></div>

<p><a id="X784B2156823AEB15" name="X784B2156823AEB15"></a></p>

<h4>14.7 <span class="Heading">Some sanity checks</span></h4>

<p>There is ample scope for bugs in the implementation of the above method for computing resolutions of Bianchi groups. The following sanity checks lend confidence to the implementation.</p>

<p><a id="X7E5A36D47F9D4A47" name="X7E5A36D47F9D4A47"></a></p>

<h5>14.7-1 <span class="Heading">Equivariant Euler characteristic</span></h5>

<p>Let <span java.lang.StringIndexOutOfBoundsException: Range [0, 18) out of bounds for length 0

<p>One easy test to make in our computations is to check that the equivariant Euler characteristic of the <span class="SimpleMath">\(2\)</span>-complex is indeed java.lang.StringIndexOutOfBoundsException: Index 164 out of bounds for length 0


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

<span GAPprompt"gap>/>< =GAPinput">:0;/span>
<span class="span class="GAPprompt">gap></span> <span class="GAPinput">K:=BianchiGcomplex(-23);;</span>
<span class""&;/span < class"">fork [.K.(n)do/>
<span class="GAPprompt">></span> <span class="GAPinput">g:=Order(K!.stabilizer(n,k));</span>
<class=GAPprompt>gt;/span><class"APinput"ifg lt  thenchi= +-)n/;;/>
<span class="GAPprompt">></span> <span class="GAPinput">od;od;</span>
<">ap&;<span> <pan class="APinputchi</span>
0

</pre></div>  [ ],[  ,[] []  ] []   ,[]   ]   ]  ]  ] 

<p><a id< =">&t</> <pan =:=;List([.R.dimension(n)]k-&t;ResolutionBoundaryOfWord(R,n-1,R!.boundary(n,k)));</span>

<h5>14.7-2 <span class="Heading">Boundary , [  ], [  ] [  ] [ ] [ ],java.lang.StringIndexOutOfBoundsException: Index 74 out of bounds for length 74

<p>The signs in the   ]  ]  ]]


<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="[ [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ],
<span class="GAPprompt">gap></span   ] [ ], [  java.lang.StringIndexOutOfBoundsException: Index 20 out of bounds for length 20
<span class="APprompt>ap&;<span>< class=GAPinput"n:2;([.Rdimension),-gtR(R,n,R!.(nk));span>
[ [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], 
  [  ], [  ], [  ], [  ], [  ], [  ], [  ], [[  ],[  ],[ ], [ ] [ ] [ ], [  ] [ ] [ ] [  ] [ ] [ ],
   ],[  ] [ ] ]
<span class="GAPprompt">gap></span> <span class="GAPinput">n:=3;;List([1..R!.dimension(n)],k->ResolutionBoundaryOfWord(R,n-1,R!.boundary(n,k)));</span>
[ [  ], [  ], [  ], </pre></iv
  [  ], [  ], [  ], [  ], <><aid="7E64819A7C058EDD" nameX7"></a</>
  [  ], [  ], [  ] ]
<span 
[ [  ],[  ],[]   ,[  ,[ ,[ ] [] [] [  ,[ ]  ] 
  [  ], [  ], [  ], [  ], [  ], java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0
  [] [ ,[ ] ]
<span class="GAPprompt">gap></span> <span class="GAPinput">java.lang.StringIndexOutOfBoundsException: Range [0, 63) out of bounds for length 0
[[  ], [  ] []   ]]  ]   ] [] [ ,[] [  ]  ] 
  [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  ], [  
  [  , [  ], ]]

</pre></div>

<p><span class"APprompt>></span span class=GAPinput":FreeGResolutionK,0;<span>

<h5>14.7-3 <span class="Heading">Compare different span class="GAPprompt">gap></span> <span class="GAPinput">List([0..9],n->Homology(TensorWithIntegers(R),n));<   , ,,00] [, ,2 ,12, ,2  ,java.lang.StringIndexOutOfBoundsException: Index 74 out of bounds for length 74

<p>Sebastian Schoennenbeck in his thesis work computed some contractible <span class="SimpleMath">\(2\)</span>-complexes on which Bianchi groups act java.lang.StringIndexOutOfBoundsException: Index 152 out of bounds for length 0

<p>The following commands test span class="GAPprompt">gap>< G>ist[.9]n&;omologyTensorWithIntegers()n);<span>


<div ,  , ,212,  , ,[,    ]
<span class="GAPprompt">gap></span> <span class="GAPinput">K
<java.lang.StringIndexOutOfBoundsException: Index 3 out of bounds for length 0
<span h5>14.7span "eading"Compareto algebra/>/>
[ [ 0
  [ 2212 ], [ 2212 ], [ 2212 ], [ 2212 ], [ 2212 ] ]

<span class="GAPprompt">gap></span> <span class="GAPinput">K:=BianchiGcomplex(-23);;</span>
<span class
<span class="GAPprompt<div class"example>pre
00 ], [2 2,12]  2 2 12 , 
  [ 22,spanclass"APprompt>gt;/ span class=GAPinput"!d(0]k&tO(!stabilizer,))<span>

</pre></div>

<p><a id="X8223864085412705" name="X8223864085412705"></a></

<h5>14.7-4 <span class="Heading">Compare geometry to algebra</span></h5>

<java.lang.StringIndexOutOfBoundsException: Range [3, 2) out of bounds for length 502


<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">K:=BianchiGcomplex(-23);;</span>
<span
 622,4 infinity  java.lang.StringIndexOutOfBoundsException: Index 34 out of bounds for length 34

</pre></div>

<p>A visualization of the fundamental domain tells us a certain amount about the algebra. In the case of <span class="SimpleMath">\(SL_2({\mathcal O}_{-23})\)</span></p>

<p><img src="images/sl2O-23.png" align=" x*k^-2*r^-1*x*r^-1*s^-*k^-1*s^-,x^-1**k^3*s*r*x^-1*s*r ]

<p>a span classjava.lang.StringIndexOutOfBoundsException: Range [23, 22) out of bounds for length 101


<div class="example"><>gapgt</java.lang.StringIndexOutOfBoundsException: Range [38, 37) out of bounds for length 77
< class"GAPprompt"gapgt;<span span "APinput"K=-23;<span
<span class="GAPprompt">gap></span> <span class="GAPinput">List([1..K
[, 4 ]

</pre></div>

<p><a id="X78BC9D077956089A" name="X78BC9D077956089A

<h4>14.8 <span class="Heading">Group presentations</span></h4>

<p>Swan's reason for studying fundamental domains was to obtain explicit group presentations for <span class="SimpleMath">\(SL_2({\mathcal O}_{-d})\)</span> for various values of <span class="SimpleMath">\(d\)</span>. The following commands obtain a presentation for <span class="SimpleMath">\(SL_2({\mathcal O}_{-23})\)</span>.</p>


<div class="example"><pre>
<span class="GAPprompt">gap></span> <span <pan class="APprompt">gapgt;/span spanclass">:R!.elts[22]</pan
<span> <span class="GAPinput"R=FreeGResolution(,2);;/>
<span class="GAPprompt">gap></span> <span class="GAPinput">P:=PresentationOfResolution(R);</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">G:=SimplifiedFpGroupspan class""ap&;<span><spanclass=GAPinput">w:=R!.elts[2];</span>
<fp group on the generators [ k, r, s, w, x ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">RelatorsOfFpGroup(G);</span>
[ w3/2 ++ 1/ Sqrt(-23, -3/ +12 Sqrt(-23)], 
  ^1*w*x^-*sr*w^-1**s^-1, ^1k^-**^-, 
  x*k^-2*r^-1*x*r^-1*s^-1*k^-1*s^-1, x^-1*k^3*s*r*x^-1*s*r ]

<span class="GAPprompt">gap></span> <span class="GAPinput">#Next we identify the generators as matrices</span>
spanclass=GAPprompt">ap&t;/pan><span class=GAPinput">GeneratorsOfGroup(P!.freeGroup);</span>
[ k, m, n, pq, r, s, t, u, v, w, x, y, z ]
<panclass"APprompt"gap></span> <span class="GAPinput">P!.gens;</span>
196620621226525325014 ]

<pan class=GAPprompt"gap>/span<span class=">=!elts[19]<span>
[ [ 11 ], 
  [ -10 ] ]
<span class="GAPprompt"gap&;/span>span "GAPinput"r:R.[21;/span
[ [ 33 + -1 Sqrt(-23) ], 
  [ -3/2 + -1/2 Sqrt(-23), -5 ] ]
<span class="GAPprompt">gap></span> <span class="GAPinput">s:=R!.elts[22];</span>
[ [ 2 + 1 Sqrt(-23), 13/2 + 
   52 + 1/2 Sqrt(-23), -1 Sqrt(-23) ] ]
<span class="GAPprompt">gap></span> <span class="GAPinput">w:=R!.elts[2];</span>
[ [ 3/2 + 1/2 Sqrt(-23), -3/2 + 1/2 Sqrt(-23) ], 
  [ 3/2 + -1/2 Sqrt(-23), 3 ] ]
<java.lang.StringIndexOutOfBoundsException: Range [30, 5) out of bounds for length 84
[ [ 11/2 + 1/2 Sqrt(-23), 15/2 + -1/2 Sqrt(-23) ], 
  [ -1 Sqrt(-23), -4 + -1 Sqrt(-23) ] ]

</pre></div>

<p><a id="X786CFAA17C0A6E7A" name="X786CFAA17C0A6E7A"></a></p>

<h4>14.9 <span class="Heading">Finite index <span class="GAPprompt>gapgt;</span>span class">:BianchiGcomplex-);<>

<p>The following commands compute the span class="GAPprompt">gap></span> <span="GAPinput">R:=QuadraticToCyclotomicCoefficients(R);;</span>


<div class="example"><pre>
<span class="GAPprompt">gap></span> <span class="GAPinput">OQ:=RingOfIntegers(QuadraticNumberField(-23));;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">I:=QuadraticIdeal(OQ,[Sqrt(-23)]);</span>
ideal of norm 23 in O(Q(Sqrt(-23)))
<span class="GAPprompt">gap></span> <span class="GAPinput">G:=HAP_CongruenceSubgroupGamma0(I);</span>
<group of 2x2 matrices in characteristic 0>
<span class="GAPprompt">gap></span> <span class="GAPinput">IndexInSL2O(G);</span>
24

<span class="GAPprompt">gap></span> <span class="GAPinput">K:=BianchiGcomplex(-23);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">R:=FreeGResolution(K,11);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">R:=QuadraticToCyclotomicCoefficients(R);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">S:=ResolutionFiniteSubgroup(R,G);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">List([0..10],n->Homology(TensorWithIntegers(S),n));</span>
[ [ 0 ], [ 200000000 ], 
  [ 222222220000000 ], [ 22222222 ],
  [ 22222222 ], [ 22222222 ], 
  [ 22222222 ], [ 22222222 ], 
  [ 22222222 ], [ 22222222 ], 
  [ 22222222 ] ]

<span class="GAPprompt">gap></span> <span class="GAPinput">P:=PresentationOfResolution(S);;</span>
<span class="GAPprompt">gap></span> <span class="GAPinput">H:=SimplifiedFpGroup(P!.freeGroup/P!.relators);</span>
<fp group on the generators [ f8, f10, f15, f70, f86, f125, f132, f138, f182, 
  f187, f191, f273, f279 ]>
<span class="GAPprompt">gap></span> <span class="GAPinput">Length(RelatorsOfFpGroup(H));</span>
24

</pre></div>


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