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<h1 ><strong class="pkg" >Congruence</strong ></h1 >
<h2>Congruence subgroups of <span class="SimpleMath" >\(SL_2(ℤ)\)</span ></h2>
<p >Version 1 .2 .7 </p >
<p >28 August 2024 </p >
</div >
<p ><b >Ann Dooms
</b >
<br />Email: <span class="URL" ><a href="mailto:andooms@vub.ac.be" >andooms@vub.ac.be</a ></span >
<br />Homepage: <span class="URL" ><a href="http://homepages.vub.ac.be/~andooms " >http://homepages.vub.ac.be/~andooms</a ></span >
<br />Address : <br />Department of Mathematics, Vrije Universiteit Brussel<br /> Pleinlaan 2 , Brussels, B -1050 Belgium
</p ><p ><b >Eric Jespers
</b >
<br />Email: <span class="URL" ><a href="mailto:efjesper@vub.ac.be" >efjesper@vub.ac.be</a ></span >
<br />Homepage: <span class="URL" ><a href="http://homepages.vub.ac.be/~efjesper " >http://homepages.vub.ac.be/~efjesper</a ></span >
<br />Address : <br />Department of Mathematics, Vrije Universiteit Brussel<br /> Pleinlaan 2 , Brussels, B -1050 Belgium
</p ><p ><b >Olexandr Konovalov
</b >
<br />Email: <span class="URL" ><a href="mailto:obk1@st-andrews.ac.uk" >obk1@st-andrews.ac.uk</a ></span >
<br />Homepage: <span class="URL" ><a href="https://olexandr-konovalov.github.io/ " >https://olexandr-konovalov.github.io/</a ></span >
<br />Address : <br />School of Computer Science<br /> University of St Andrews<br /> Jack Cole Building, North Haugh,<br /> St Andrews, Fife, KY16 9 SX, Scotland
</p ><p ><b >Helena Verrill
</b >
<br />Email: <span class="URL" ><a href="mailto:verrill@math.lsu.edu" >verrill@math.lsu.edu</a ></span >
<br />Homepage: <span class="URL" ><a href="http://www.math.lsu.edu/~verrill/ " >http://www.math.lsu.edu/~verrill/</a ></span >
<br />Address : <br />Department of Mathematics<br /> Louisiana State University<br /> Baton Rouge, Louisiana, 70803 -4918 <br /> USA
</p >
<p ><a id="X7AA6C5737B711C89" name="X7AA6C5737B711C89" ></a ></p >
<h3>Abstract</h3>
<p >The <strong class="pkg" >GAP</strong > package <strong class="pkg" >Congruence</strong > provides functionality to work with congruence subgroups of <span class="SimpleMath" >\(SL_2(ℤ)\)</span >.</p >
<p ><a id="X81488B807F2A1CF1" name="X81488B807F2A1CF1" ></a ></p >
<h3>Copyright</h3>
<p >© 2006 -2024 by Ann Dooms, Eric Jespers, Olexandr Konovalov and Helena Verrill.</p >
<p ><strong class="pkg" >Congruence</strong > is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option ) any later version. For details , see the FSF's own site <span class="URL"><a href="https://www.gnu.org/licenses/gpl.html ">https://www.gnu.org/licenses/gpl.html</a></span>.</p>
<p >If you obtained <strong class="pkg" >Congruence</strong >, we would be grateful for a short notification sent to one of the authors.</p >
<p >If you publish a result which was partially obtained with the usage of <strong class="pkg" >Congruence</strong >, please cite it in the following form :</p >
<p >A . Dooms, E. Jespers, O. Konovalov and H. Verrill. <em >Congruence --- Congruence subgroups of <span class="SimpleMath" >\(SL_2(ℤ)\)</span >, Version 1 .2 .7 ;</em > 2024 (<span class="URL" ><a href="https://gap-packages.github.io/congruence/ " >https://gap-packages.github.io/congruence/</a ></span >).</p >
<p ><a id="X82A988D47DFAFCFA" name="X82A988D47DFAFCFA" ></a ></p >
<h3>Acknowledgements</h3>
<p >We are very grateful to Mong-Lung Lang, Chong-Hai Lim and Ser Peow Tan for their comments provided while implementing algorithms from <a href="chapBib_mj.html#biBLLT-Algorithm" >[LLT95a]</a > and <a href="chapBib_mj.html#biBLLT-Hecke" >[LLT95b]</a >, and to Francqui Stichting (Belgium) for the support of the third author.</p >
<p ><a id="X8537FEB07AF2BEC8" name="X8537FEB07AF2BEC8" ></a ></p >
<div class="contents" >
<h3>Contents<a id="contents" name="contents" ></a ></h3>
<div class="ContChap" ><a href="chap1_mj.html#X7DFB63A97E67C0A1" >1 <span class="Heading" >Introduction</span ></a >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap1_mj.html#X80AE633F82C4D9BF" >1 .1 <span class="Heading" >General aims of <strong class="pkg" >Congruence</strong > package</span ></a >
</span >
</div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap1_mj.html#X7DB566D5785B7DBC" >1 .2 <span class="Heading" >Installation and system requirements</span ></a >
</span >
</div >
</div >
<div class="ContChap" ><a href="chap2_mj.html#X7B010EE67FACF45E" >2 <span class="Heading" >Construction of congruence subgroups</span ></a >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap2_mj.html#X7B010EE67FACF45E" >2 .1 <span class="Heading" >Construction of congruence subgroups</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7A61F693873F7136" >2 .1 -1 PrincipalCongruenceSubgroup</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7B8DB77B81BE58D7" >2 .1 -2 CongruenceSubgroupGamma0</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7B4FBED17ECE2A7F" >2 .1 -3 CongruenceSubgroupGammaUpper0</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7CFDC47279AC0E85" >2 .1 -4 CongruenceSubgroupGamma1</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7C3FCDD878FE57ED" >2 .1 -5 CongruenceSubgroupGammaUpper1</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7FE839377D7F45EB" >2 .1 -6 IntersectionOfCongruenceSubgroups</a ></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap2_mj.html#X8267F261874959E5" >2 .2 <span class="Heading" >Properties of congruence subgroups</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X828F7E08787650DC" >2 .2 -1 IsPrincipalCongruenceSubgroup</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X85124A697E826AB4" >2 .2 -2 IsCongruenceSubgroupGamma0</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7A03633C83A286F5" >2 .2 -3 IsCongruenceSubgroupGammaUpper0</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X8262396080F3B0DD" >2 .2 -4 IsCongruenceSubgroupGamma1</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7D731035834CF878" >2 .2 -5 IsCongruenceSubgroupGammaUpper1</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X83B4E4FA7F4DFB97" >2 .2 -6 IsIntersectionOfCongruenceSubgroups</a ></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap2_mj.html#X8664A60E875EA5DE" >2 .3 <span class="Heading" >Attributes of congruence subgroups</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7D5696F584970D21" >2 .3 -1 LevelOfCongruenceSubgroup</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X87302F8A7E44D67B" >2 .3 -2 IndexInSL2Z</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7BF57D157824FFC8" >2 .3 -3 DefiningCongruenceSubgroups</a ></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap2_mj.html#X7B15B49583DC9EF5" >2 .4 <span class="Heading" >Operations for congruence subgroups</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X8146AC8587C65DEE" >2 .4 -1 Random</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X87BDB89B7AAFE8AD" ><code >2 .4 -2 \in</code ></a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X7FC5BF527931FF4C" >2 .4 -3 CanEasilyCompareCongruenceSubgroups</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X79CA175481F8105F" >2 .4 -4 IsSubset</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2_mj.html#X83A0356F839C696F" >2 .4 -5 Index</a ></span >
</div ></div >
</div >
<div class="ContChap" ><a href="chap3_mj.html#X85CABB30818CD99C" >3 <span class="Heading" >Farey symbols and their properties</span ></a >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap3_mj.html#X7B7B81E584CCA30C" >3 .1 <span class="Heading" >Construction of Farey symbols</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3_mj.html#X7F8F5919870A46FE" >3 .1 -1 FareySymbolByData</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3_mj.html#X845F9BA182F4E73B" >3 .1 -2 IsValidFareySymbol</a ></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap3_mj.html#X8016C45082AEC784" >3 .2 <span class="Heading" >Properties of Farey symbols</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3_mj.html#X8245766978F02751" >3 .2 -1 GeneralizedFareySequence</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3_mj.html#X80BB58E58492D103" >3 .2 -2 NumeratorOfGFSElement</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3_mj.html#X87477604878BCD42" >3 .2 -3 DenominatorOfGFSElement</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3_mj.html#X83C941047D486000" >3 .2 -4 LabelsOfFareySymbol</a ></span >
</div ></div >
</div >
<div class="ContChap" ><a href="chap4_mj.html#X831C60277F7D80B2" >4 <span class="Heading" >Farey symbols for congruence subgroups</span ></a >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4_mj.html#X7F43DB8B803F313F" >4 .1 <span class="Heading" >Computation of the Farey symbol for a finite index subgroup</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4_mj.html#X8594896287DCFE8D" >4 .1 -1 FareySymbol</a ></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4_mj.html#X80AE179D869BEE90" >4 .2 <span class="Heading" >Computation of generators of a finite index subgroup from its Farey symbol</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4_mj.html#X8790C1498107A39A" >4 .2 -1 MatrixByEvenInterval</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4_mj.html#X78779BDF7A1DB4AE" >4 .2 -2 MatrixByOddInterval</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4_mj.html#X7F792846795E3A63" >4 .2 -3 MatrixByFreePairOfIntervals</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4_mj.html#X7905B050800E4416" >4 .2 -4 GeneratorsByFareySymbol</a ></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4_mj.html#X79C44528864044C5" >4 .2 -5 GeneratorsOfGroup</a ></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4_mj.html#X7C5AB1D786207745" >4 .3 <span class="Heading" >Other properties derived from Farey symbols</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4_mj.html#X80EED34183408106" >4 .3 -1 IndexInPSL2ZByFareySymbol</a ></span >
</div ></div >
</div >
<div class="ContChap" ><a href="chap5_mj.html#X82C56A367A418E7C" >5 <span class="Heading" >Service functions of the <strong class="pkg" >Congruence</strong > package</span ></a >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap5_mj.html#X86D04EE08437C320" >5 .1 <span class="Heading" >Additional information displayed by <strong class="pkg" >Congruence</strong > algorithms</span ></a >
</span >
<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5_mj.html#X83B2A8607C2E6A38" >5 .1 -1 InfoCongruence</a ></span >
</div ></div >
</div >
<div class="ContChap" ><a href="chapBib_mj.html" ><span class="Heading" >References</span ></a ></div >
<div class="ContChap" ><a href="chapInd_mj.html" ><span class="Heading" >Index</span ></a ></div >
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