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<h1 >RepnDecomp</h1 >
<h2>Decompose representations of finite groups into irreducibles</h2>
<p>
1 .3 .1 </p>
<p>
10 September 2025
</p>
</div >
<p><b>
Kaashif Hymabaccus
</b>
<br />Email: <span class="URL" ><a href="mailto:kaashif@kaashif.co.uk" >kaashif@kaashif.co.uk</a></span >
<br />Homepage: <span class="URL" ><a href="https://kaashif.co.uk " >https://kaashif.co.uk</a></span >
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<div class="contents" >
<h3>Contents<a id="contents" name="contents" ></a></h3>
<div class="ContChap" ><a href="chap1.html#X7DFB63A97E67C0A1" >1 <span class="Heading" >Introduction</span ></a>
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap1.html#X789233A47A277072" >1 .1 <span class="Heading" >Getting started with RepnDecomp</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap1.html#X8360C04082558A12" >1 .1 -1 <span class="Heading" >Installation</span ></a>
</span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap1.html#X792C0F507B4A3B89" >1 .1 -2 <span class="Heading" >Note on what is meant by a representation</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap1.html#X8315479878D25E37" >1 .1 -3 <span class="Heading" >API Overview</span ></a>
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</div ></div >
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<div class="ContChap" ><a href="chap2.html#X7D9B253E794EF912" >2 <span class="Heading" >Isomorphisms between representations</span ></a>
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap2.html#X7AEE81C2809E0B98" >2 .1 <span class="Heading" >Finding explicit isomorphisms</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2.html#X7F0D3CFB7800149A" >2 .1 -1 LinearRepresentationIsomorphism</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2.html#X841DE7D08491325F" >2 .1 -2 LinearRepresentationIsomorphismSlow</a></span >
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<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap2.html#X85DAC9E583D8EFB9" >2 .2 <span class="Heading" >Testing isomorphisms</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2.html#X86EB9DD586958473" >2 .2 -1 AreRepsIsomorphic</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap2.html#X81080E1B7917B361" >2 .2 -2 IsLinearRepresentationIsomorphism</a></span >
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</div >
<div class="ContChap" ><a href="chap3.html#X83B4D1DB7F92BD3A" >3 <span class="Heading" >Algorithms for unitary representations</span ></a>
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap3.html#X870B3D0D80CFADB1" >3 .1 <span class="Heading" >Unitarising representations</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3.html#X86B2367A79BE5B9F" >3 .1 -1 UnitaryRepresentation</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3.html#X87D121227C027253" >3 .1 -2 IsUnitaryRepresentation</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3.html#X78F7DFD186A4E7CA" >3 .1 -3 LDLDecomposition</a></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap3.html#X7974D0C580C833D1" >3 .2 <span class="Heading" >Decomposing unitary representations</span ></a>
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<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap3.html#X8175C1167A31C3D6" >3 .2 -1 IrreducibleDecompositionDixon</a></span >
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<div class="ContChap" ><a href="chap4.html#X8346542B8387968B" >4 <span class="Heading" >Miscellaneous useful functions</span ></a>
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4.html#X87196FDB78749ECA" >4 .1 <span class="Heading" >Predicates for representations</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X8631A1417C3C1D88" >4 .1 -1 IsFiniteGroupLinearRepresentation</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X826D5ADF7FA87782" >4 .1 -2 IsFiniteGroupPermutationRepresentation</a></span >
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<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4.html#X8271F7A386CFEA63" >4 .2 <span class="Heading" >Efficient summing over groups</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X85E8A5FC844DC09A" >4 .2 -1 GroupSumBSGS</a></span >
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<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4.html#X86F42D257CFB192D" >4 .3 <span class="Heading" >Space-efficient representation of tensors of matrices</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X84335C447DE377B0" >4 .3 -1 IsTensorProductOfMatricesObj</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X868B9CB1873C93AC" >4 .3 -2 IsTensorProductPairRep</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X85AD76DB7D5F8B12" >4 .3 -3 IsTensorProductKroneckerRep</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X86671AD582FF77E2" >4 .3 -4 TensorProductOfMatrices</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X79D0379D79F5DF9B" >4 .3 -5 CharacterOfTensorProductOfRepresentations</a></span >
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<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4.html#X83F160967ED7EE14" >4 .4 <span class="Heading" >Matrices and homomorphisms</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X7D17785482F143B0" >4 .4 -1 ComposeHomFunction</a></span >
</div ></div >
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap4.html#X7C3EDA5E7A24196C" >4 .5 <span class="Heading" >Representation theoretic functions</span ></a>
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<div class="ContSSBlock" >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X841424DF824E258B" >4 .5 -1 TensorProductRepLists</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X84EAE3DB7FA8102C" >4 .5 -2 DirectSumOfRepresentations</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X85147CF97B912CC3" >4 .5 -3 DegreeOfRepresentation</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X7B14287E7BFC548D" >4 .5 -4 PermToLinearRep</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap4.html#X7E67A4817A5E4879" >4 .5 -5 IsOrthonormalSet</a></span >
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<div class="ContChap" ><a href="chap5.html#X7F968DF987DE4A6E" >5 <span class="Heading" >Computing decompositions of representations</span ></a>
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap5.html#X7E29883984400D2C" >5 .1 <span class="Heading" >Block diagonalizing</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X8361AD057AD282AC" >5 .1 -1 BlockDiagonalBasisOfRepresentation</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X86EB837579C1416D" >5 .1 -2 BlockDiagonalRepresentation</a></span >
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<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap5.html#X863A16A179A7486B" >5 .2 <span class="Heading" >Algorithms due to the authors</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X831574AD864C94A8" >5 .2 -1 REPN_ComputeUsingMyMethod</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X7DB659DB7E48D502" >5 .2 -2 REPN_ComputeUsingMyMethodCanonical</a></span >
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<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap5.html#X7C22F13E80A74438" >5 .3 <span class="Heading" >Algorithms due to Serre</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X7E95B0367992BEC4" >5 .3 -1 CanonicalDecomposition</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X795C63F386C45308" >5 .3 -2 IrreducibleDecomposition</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X87E91CBE7992D126" >5 .3 -3 IrreducibleDecompositionCollected</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap5.html#X7C1CF0547D72D354" >5 .3 -4 REPN_ComputeUsingSerre</a></span >
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<div class="ContChap" ><a href="chap6.html#X7A0EF2C67E2DB726" >6 <span class="Heading" >Centralizer (commutant) rings</span ></a>
<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap6.html#X7E70A3D881CD5FFA" >6 .1 <span class="Heading" >Finding a basis for the centralizer</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap6.html#X7901B6A7860D35C3" >6 .1 -1 CentralizerBlocksOfRepresentation</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap6.html#X86B19E2B877121E9" >6 .1 -2 CentralizerOfRepresentation</a></span >
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<div class="ContSect" ><span class="tocline" ><span class="nocss" > </span ><a href="chap6.html#X83C4F8C17DA976EE" >6 .2 <span class="Heading" >Using the centralizer for computations</span ></a>
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<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap6.html#X87E5BAEB82DC00C3" >6 .2 -1 ClassSumCentralizer</a></span >
<span class="ContSS" ><br /><span class="nocss" > </span ><a href="chap6.html#X78719DC8868B0744" >6 .2 -2 ClassSumCentralizerNC</a></span >
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