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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Alexander Hulpke.
##
## Copyright of GAP belongs to its developers, whose names are too numerous
## to list here. Please refer to the COPYRIGHT file for details.
##
## SPDX-License-Identifier: GPL-2.0-or-later
##
## This file contains the declarations for the Dixon-Schneider algorithm
##

#############################################################################
##
## <#GAPDoc Label="[1]{ctblgrp}">
## <Index>Dixon-Schneider algorithm</Index>
## The &GAP; library implementation of the Dixon-Schneider algorithm
## first computes the linear characters, using the commutator factor group.
## If irreducible characters are missing afterwards,
## they are computed using the techniques described in <Cite Key="Dix67"/>,
## <Cite Key="Sch90"/> and <Cite Key="Hulpke93"/>.
## 
## Called with a group <M>G</M>, the function
## <Ref Oper="CharacterTable" Label="for a group"/> returns a character
## table object that stores already information such as class lengths,
## but not the irreducible characters.
## The routines that compute the irreducibles may use the information that
## is already contained in this table object.
## In particular the ordering of classes in the computed characters
## coincides with the ordering of classes in the character table of <A>G</A>
## (see <Ref Sect="The Interface between Character Tables and Groups"/>).
## Thus it is possible to combine computations using the group
## with character theoretic computations
## (see <Ref Sect="Advanced Methods for Dixon-Schneider Calculations"/>
## for details),
## for example one can enter known characters.
## Note that the user is responsible for the correctness of the characters.
## (There is little use in providing the trivial character to the routine.)
## 
## The computation of irreducible characters from the group needs to
## identify the classes of group elements very often,
## so it can be helpful to store a class list of all group elements.
## Since this is obviously limited by the group order,
## it is controlled by the global function <Ref Func="IsDxLargeGroup"/>.
## 
## The routines compute in a prime field of size <M>p</M>,
## such that the exponent of the group divides <M>(p-1)</M> and such that
## <M>2 \sqrt{{|G|}} < p</M>.
## Currently prime fields of size smaller than <M>65\,536</M> are handled more
## efficiently than larger prime fields,
## so the runtime of the character calculation depends on how large the
## chosen prime is.
## 
## The routine stores a Dixon record (see <Ref Attr="DixonRecord"/>)
## in the group that helps routines that identify classes,
## for example <Ref Oper="FusionConjugacyClasses" Label="for two groups"/>,
## to work much faster.
## Note that interrupting Dixon-Schneider calculations will prevent &GAP;
## from cleaning up the Dixon record;
## when the computation by <Ref Attr="IrrDixonSchneider"/> is complete,
## the possibly large record is shrunk to an acceptable size.
## <#/GAPDoc>
##

#############################################################################
##
#F IsDxLargeGroup( <G> )
##
## <#GAPDoc Label="IsDxLargeGroup">
## <ManSection>
## <Func Name="IsDxLargeGroup" Arg='G'/>
##
## <Description>
## returns <K>true</K> if the order of the group <A>G</A> is smaller than
## the current value of the global variable <C>DXLARGEGROUPORDER</C>,
## and <K>false</K> otherwise.
## In Dixon-Schneider calculations, for small groups in the above sense a
## class map is stored, whereas for large groups,
## each occurring element is identified individually.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "IsDxLargeGroup" );

#############################################################################
##
#F DxModularValuePol
#F DxDegreeCandidates
##
## <ManSection>
## <Func Name="DxModularValuePol" Arg='...'/>
## <Func Name="DxDegreeCandidates" Arg='...'/>
##
## <Description>
## </Description>
## </ManSection>
##
DeclareGlobalFunction("DxModularValuePol");
DeclareGlobalFunction("DxDegreeCandidates");

DeclareGlobalFunction("DxGaloisOrbits");
DeclareGlobalFunction("DoubleCentralizerOrbit");

#############################################################################
##
#A DixonRecord( <G> )
##
## <#GAPDoc Label="DixonRecord">
## <ManSection>
## <Attr Name="DixonRecord" Arg='G'/>
##
## <Description>
## The <Ref Attr="DixonRecord"/> of a group contains information used by the
## routines to compute the irreducible characters and related information
## via the Dixon-Schneider algorithm such as class arrangement and character
## spaces split obtained so far.
## Usually this record is passed as argument to all subfunctions to avoid a
## long argument list.
## It has a component <C>conjugacyClasses</C> which contains the classes of
## <A>G</A> <E>ordered as the algorithm needs them</E>.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute("DixonRecord",IsGroup,"mutable");

#############################################################################
##
#O DxPreparation(<G>,<D>)
##
## <ManSection>
## <Oper Name="DxPreparation" Arg='G,D'/>
##
## <Description>
## Creates entries in the dixon record <A>D</A> of the group <A>G</A>
## which are representation dependent,
## like functions to identify the class of elements.
## </Description>
## </ManSection>
##
DeclareOperation("DxPreparation",[IsGroup,IsRecord]);

#############################################################################
##
#F ClassComparison(<c>,<d>) . . . . . . . . . . . . compare classes c and d
##
## <ManSection>
## <Func Name="ClassComparison" Arg='c,d'/>
##
## <Description>
## Comparison function for conjugacy classes,
## used by <Ref Func="Sort"/>.
## Comparison is based first on the size of the class and then on the
## order of the representatives.
## Thus the class containing the identity element is in the first position,
## as required. Since sorting is primary by the class sizes,smaller
## classes are in earlier positions, making the active columns those to
## smaller classes, thus reducing the work for calculating class matrices.
## Additionally, galois conjugated classes are kept together, thus increasing
## the chance,that with one columns of them active to be several active,
## again reducing computation time.
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "ClassComparison");

#############################################################################
##
#F DxIncludeIrreducibles( <D>, <new>[, <newmod>] )
##
## <#GAPDoc Label="DxIncludeIrreducibles">
## <ManSection>
## <Func Name="DxIncludeIrreducibles" Arg='D, new[, newmod]'/>
##
## <Description>
## This function takes a list of irreducible characters <A>new</A>,
## each given as a list of values (corresponding to the class arrangement in
## <A>D</A>), and adds these to a partial computed list of irreducibles as
## maintained by the Dixon record <A>D</A>.
## This permits one to add characters in interactive use obtained from other
## sources and to continue the Dixon-Schneider calculation afterwards.
## If the optional argument <A>newmod</A> is given, it must be a
## list of reduced characters, corresponding to <A>new</A>.
## (Otherwise the function has to reduce the characters itself.)
## 
## The function closes the new characters under the action of Galois
## automorphisms and tensor products with linear characters.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "DxIncludeIrreducibles" );

#############################################################################
##
#F SplitCharacters( <D>, <list> ) split characters according to the spaces
##
## <#GAPDoc Label="SplitCharacters">
## <ManSection>
## <Func Name="SplitCharacters" Arg='D, list'/>
##
## <Description>
## This routine decomposes the characters given in <A>list</A> according to
## the character spaces found up to this point. By applying this routine to
## tensor products etc., it may result in characters with smaller norm,
## even irreducible ones. Since the recalculation of characters is only
## possible if the degree is small enough, the splitting process is
## applied only to characters of sufficiently small degree.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "SplitCharacters" );

#############################################################################
##
#F OrbitSplit(<D>) . . . . . . . . . . . . . . try to split two-orbit-spaces
##
## <ManSection>
## <Func Name="OrbitSplit" Arg='D'/>
##
## <Description>
## Tries to split two-orbit character spaces.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("OrbitSplit");

#############################################################################
##
#F DxSplitDegree(<D>,<space>,<r>) local
##
## <ManSection>
## <Func Name="DxSplitDegree" Arg='D,space,r'/>
##
## <Description>
## estimates the number of parts obtained when splitting the character space
## <A>space</A> with matrix number <A>r</A>.
## This estimate is obtained using character morphisms.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("DxSplitDegree");

DeclareGlobalFunction("DxOnedimCleanout");

#############################################################################
##
#F BestSplittingMatrix(<D>)
##
## <#GAPDoc Label="BestSplittingMatrix">
## <ManSection>
## <Func Name="BestSplittingMatrix" Arg='D'/>
##
## <Description>
## returns the number of the class sum matrix that is assumed to yield the
## best (cost/earning ration) split. This matrix then will be the next one
## computed and used.
## 
## The global option <C>maxclasslen</C>
## (defaulting to <Ref Var="infinity"/>) is recognized
## by <Ref Func="BestSplittingMatrix"/>:
## Only classes whose length is limited by the value of this option will be
## considered for splitting. If no usable class remains,
## <K>fail</K> is returned.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction("BestSplittingMatrix");

#############################################################################
##
#F DixonInit( <G> ) . . . . . . . . . . initialize Dixon-Schneider algorithm
##
## <#GAPDoc Label="DixonInit">
## <ManSection>
## <Func Name="DixonInit" Arg='G'/>
##
## <Description>
## This function does all the initializations for the Dixon-Schneider
## algorithm. This includes calculation of conjugacy classes, power maps,
## linear characters and character morphisms.
## It returns a record (see <Ref Attr="DixonRecord"/> and
## Section <Ref Sect="Components of a Dixon Record"/>)
## that can be used when calculating the irreducible characters of <A>G</A>
## interactively.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "DixonInit" );

#############################################################################
##
#F DixonSplit( <D> ) . calculate matrix, split spaces and obtain characters
##
## <#GAPDoc Label="DixonSplit">
## <ManSection>
## <Func Name="DixonSplit" Arg='D'/>
##
## <Description>
## This function performs one splitting step in the Dixon-Schneider
## algorithm. It selects a class, computes the (partial) class sum matrix,
## uses it to split character spaces and stores all the irreducible
## characters obtained that way.
## 
## The class to use for splitting is chosen via
## <Ref Func="BestSplittingMatrix"/> and the options described for this
## function apply here.
## 
## <Ref Func="DixonSplit"/> returns the number of the class that was
## used for splitting if a split was performed, and <K>fail</K> otherwise.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "DixonSplit" );
DeclareGlobalFunction( "SplitStep" );

#############################################################################
##
#F DixontinI( <D> ) . . . . . . . . . . . . . . . . reverse initialisation
##
## <#GAPDoc Label="DixontinI">
## <ManSection>
## <Func Name="DixontinI" Arg='D'/>
##
## <Description>
## This function ends a Dixon-Schneider calculation.
## It sorts the characters according to the degree and
## unbinds components in the Dixon record that are not of use any longer.
## It returns a list of irreducible characters.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "DixontinI" );

#############################################################################
##
#A IrrDixonSchneider( <G> ) . . . . irreducible characters of finite group G
##
## <#GAPDoc Label="IrrDixonSchneider">
## <ManSection>
## <Attr Name="IrrDixonSchneider" Arg='G'/>
##
## <Description>
## computes the irreducible characters of the finite group <A>G</A>,
## using the Dixon-Schneider method
## (see <Ref Sect="The Dixon-Schneider Algorithm"/>).
## It calls <Ref Func="DixonInit"/> and <Ref Func="DixonSplit"/>,
## 
## and finally returns the list returned by <Ref Func="DixontinI"/>.
## See also the sections
## <Ref Sect="Components of a Dixon Record"/> and
## <Ref Sect="An Example of Advanced Dixon-Schneider Calculations"/>.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute( "IrrDixonSchneider", IsGroup );
DeclareOperation( "IrrDixonSchneider", [ IsGroup, IsRecord ] );

#############################################################################
##
#F IrreducibleRepresentationsDixon( <G>[,<chi>] )
##
## <#GAPDoc Label="IrreducibleRepresentationsDixon">
## <ManSection>
## <Func Name="IrreducibleRepresentationsDixon" Arg='G[, chi]'/>
##
## <Description>
## Called with one argument, a group <A>G</A>,
## <Ref Func="IrreducibleRepresentationsDixon"/>
## computes (representatives of) all irreducible complex representations for
## the finite group <A>G</A>, using the method of <Cite Key="Dix93"/>,
## which computes the character table and computes the representation
## as constituent of an induced monomial representation of a subgroup.
## 
## This method can be quite expensive for larger groups, for example it
## might involve calculation of the subgroup lattice of <A>G</A>.
## 
## A character <A>chi</A> of <A>G</A> can be given as the second argument,
## in this case only a representation affording <A>chi</A> is returned.
## 
## The second argument can also be a list of characters of <A>G</A>,
## in this case only representations for characters in this list are
## computed.
## 
## Note that this method might fail if for an irreducible representation
## there is no subgroup in which its reduction has a linear constituent
## with multiplicity one.
## 
## If the option <A>unitary</A> is given, &GAP; tries, at extra cost, to find a
## unitary representation (and will issue an error if it cannot do so).
## <Example><![CDATA[
## gap> a5:= AlternatingGroup( 5 );
## Alt( [ 1 .. 5 ] )
## gap> char:= First( Irr( a5 ), x -> x[1] = 4 );
## Character( CharacterTable( Alt( [ 1 .. 5 ] ) ), [ 4, 0, 1, -1, -1 ] )
## gap> hom:=IrreducibleRepresentationsDixon( a5, char: unitary );;
## gap> Order( a5.1*a5.2 ) = Order( Image( hom, a5.1 )*Image( hom, a5.2 ) );
## true
## gap> reps:= List( ConjugacyClasses( a5 ), Representative );;
## gap> List( reps, g -> TraceMat( Image( hom, g ) ) );
## [ 4, 0, 1, -1, -1 ]
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction("IrreducibleRepresentationsDixon");

#############################################################################
##
#F RepresentationsPermutationIrreducibleCharacters(<G>,<chars>,<reps>)
##
## <ManSection>
## <Func Name="RepresentationsPermutationIrreducibleCharacters"
## Arg='G,chars,reps'/>
##
## <Description>
## Given a group <A>G</A> and a list of characters and representations of
## <A>G</A>, this function returns a permutation of the representations
## (via <Ref Func="Permuted"/>),
## that will ensure characters and representations are ordered compatibly.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("RepresentationsPermutationIrreducibleCharacters");

# the following function is in this file only for dependency reasons.
#############################################################################
##
#F NthRootsInGroup( <G>, <e>, <n> )
##
## <#GAPDoc Label="NthRootsInGroup">
## <ManSection>
## <Func Name="NthRootsInGroup" Arg='G, e, n'/>
##
## <Description>
## Let <A>e</A> be an element in the group <A>G</A>.
## This function returns a list of all those elements in <A>G</A>
## whose <A>n</A>-th power is <A>e</A>.
## 
## <Example><![CDATA[
## gap> NthRootsInGroup(g,(1,2)(3,4),2);
## [ (1,3,2,4), (1,4,2,3) ]
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction("NthRootsInGroup");

DeclareGlobalName( "ComputeAllPowerMaps" );

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