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#############################################################################
##
#W utils.gd GAP 4 package AtlasRep Thomas Breuer
##
## This file contains the declarations of utility functions for the
## &ATLAS; of Group Representations.
##

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##
## Class Names Used in the AtlasRep Package
##
## <#GAPDoc Label="classnames">
## <Subsection Label="subsect:Definition of ATLAS Class Names">
## <Heading>Definition of &ATLAS; Class Names</Heading>
##
## For the definition of class names of an almost simple group,
## we assume that the ordinary character tables of all nontrivial normal
## subgroups are shown in
## the &ATLAS; of Finite Groups <Cite Key="CCN85"/>.
## 
## Each class name is a string consisting of the element order of the class
## in question followed by a combination of capital letters, digits, and
## the characters <C>'</C> and <C>-</C> (starting with a capital letter).
## For example, <C>1A</C>, <C>12A1</C>, and <C>3B'</C> denote
## the class that contains the identity element,
## a class of element order <M>12</M>,
## and a class of element order <M>3</M>, respectively.
## 
## <Enum>
## <Item>
## For the table of a <E>simple</E> group, the class names are the same
## as returned by the two argument version of the &GAP; function
## <Ref Func="ClassNames" BookName="ref"/>,
## cf. <Cite Key="CCN85" Where="Chapter 7, Section 5"/>:
## The classes are arranged w. r. t. increasing element
## order and for each element order w. r. t. decreasing
## centralizer order, the conjugacy classes that contain elements of
## order <M>n</M> are named <M>n</M><C>A</C>, <M>n</M><C>B</C>,
## <M>n</M><C>C</C>, <M>\ldots</M>;
## the alphabet used here is potentially infinite, and reads
## <C>A</C>, <C>B</C>, <C>C</C>, <M>\ldots</M>, <C>Z</C>, <C>A1</C>,
## <C>B1</C>, <M>\ldots</M>, <C>A2</C>, <C>B2</C>, <M>\ldots</M>.
## 
## For example, the classes of the alternating group <M>A_5</M> have the
## names <C>1A</C>, <C>2A</C>, <C>3A</C>, <C>5A</C>, and <C>5B</C>.
## </Item>
## <Item>
## Next we consider the case of an <E>upward extension</E> <M>G.A</M>
## of a simple group <M>G</M> by a <E>cyclic</E> group of order
## <M>A</M>.
## The &ATLAS; defines class names for each element <M>g</M> of
## <M>G.A</M> only w. r. t. the group <M>G.a</M>, say,
## that is generated by <M>G</M> and <M>g</M>;
## namely, there is a power of <M>g</M> (with the exponent coprime to
## the order of <M>g</M>) for which the class has a name of the same
## form as the class names for simple groups,
## and the name of the class of <M>g</M>
## w. r. t. <M>G.a</M> is then
## obtained from this name by appending a suitable number of
## dashes <C>'</C>.
## So dashed class names refer exactly to those classes that are not
## printed in the &ATLAS;.
## 
## For example, those classes of the symmetric group <M>S_5</M> that do
## not lie in <M>A_5</M> have the names <C>2B</C>, <C>4A</C>,
## and <C>6A</C>.
## The outer classes of the group <M>L_2(8).3</M> have the names
## <C>3B</C>, <C>6A</C>, <C>9D</C>, and <C>3B'</C>, <C>6A'</C>,
## <C>9D'</C>.
## The outer elements of order <M>5</M> in the group <M>Sz(32).5</M>
## lie in the classes with names <C>5B</C>, <C>5B'</C>, <C>5B''</C>,
## and <C>5B'''</C>.
## 
## In the group <M>G.A</M>, the class of <M>g</M> may fuse with other
## classes.
## The name of the class of <M>g</M> in <M>G.A</M> is obtained from the
## names of the involved classes of <M>G.a</M> by concatenating their
## names after removing the element order part from all of them except
## the first one.
## 
## For example, the elements of order <M>9</M> in the group
## <M>L_2(27).6</M> are contained in the subgroup <M>L_2(27).3</M>
## but not in <M>L_2(27)</M>.
## In <M>L_2(27).3</M>, they lie in the classes <C>9A</C>, <C>9A'</C>,
## <C>9B</C>, and <C>9B'</C>;
## in <M>L_2(27).6</M>, these classes fuse to <C>9AB</C> and
## <C>9A'B'</C>.
## </Item>
## <Item>
## Now we define class names for <E>general upward extensions</E>
## <M>G.A</M> of a simple group <M>G</M>.
## Each element <M>g</M> of such a group lies in an upward extension
## <M>G.a</M> by a cyclic group, and the class names
## w. r. t. <M>G.a</M> are already defined.
## The name of the class of <M>g</M> in <M>G.A</M> is obtained by
## concatenating the names of the classes in the orbit of <M>G.A</M> on
## the classes of cyclic upward extensions of <M>G</M>,
## after ordering the names lexicographically and removing the element
## order part from all of them except the first one.
## An <E>exception</E> is the situation where dashed and non-dashed
## class names appear in an orbit;
## in this case, the dashed names are omitted.
## 
## For example, the classes <C>21A</C> and <C>21B</C> of the group
## <M>U_3(5).3</M> fuse in <M>U_3(5).S_3</M> to the class <C>21AB</C>,
## and the class <C>2B</C> of <M>U_3(5).2</M> fuses with the involution
## classes <C>2B'</C>, <C>2B''</C> in the groups
## <M>U_3(5).2^{\prime}</M> and <M>U_3(5).2^{{\prime\prime}}</M>
## to the class <C>2B</C> of <M>U_3(5).S_3</M>.
## 
## It may happen that some names in the <C>outputs</C> component of a
## record returned by <Ref Func="AtlasProgram"/>
## do not uniquely determine the classes of the corresponding elements.
## For example, the (algebraically conjugate) classes <C>39A</C> and
## <C>39B</C> of the group <M>Co_1</M> have not been distinguished yet.
## In such cases, the names used contain a minus sign <C>-</C>,
## and mean <Q>one of the classes in the range described by the name
## before and the name after the minus sign</Q>;
## the element order part of the name does not appear after the minus
## sign.
## So the name <C>39A-B</C> for the group <M>Co_1</M> means
## <C>39A</C> or <C>39B</C>,
## and the name <C>20A-B'''</C> for the group <M>Sz(32).5</M> means
## one of the classes of element order <M>20</M> in this group
## (these classes lie outside the simple group <M>Sz</M>).
## </Item>
## <Item>
## For a <E>downward extension</E> <M>m.G.A</M> of an almost simple
## group <M>G.A</M> by a cyclic group of order <M>m</M>,
## let <M>\pi</M> denote the natural epimorphism from <M>m.G.A</M>
## onto <M>G.A</M>.
## Each class name of <M>m.G.A</M> has the form <C>nX_0</C>,
## <C>nX_1</C> etc.,
## where <C>nX</C> is the class name of the image under <M>\pi</M>,
## and the indices <C>0</C>, <C>1</C> etc. are chosen according to the
## position of the class in the lifting order rows for <M>G</M>,
## see <Cite Key="CCN85" Where="Chapter 7, Section 7,
## and the example in Section 8"/>).
## 
## For example, if <M>m = 6</M> then <C>1A_1</C> and <C>1A_5</C> denote
## the classes containing the generators of the kernel of <M>\pi</M>,
## that is, central elements of order <M>6</M>.
## </Item>
## </Enum>
##
## </Subsection>
## <#/GAPDoc>
##
#T missing:
#T general central downward extensions (<M>2^2</M>, <M>2 \times 4</M>, ...)
##

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##
#F AtlasClassNames( <tbl> )
##
## <#GAPDoc Label="AtlasClassNames">
## <ManSection>
## <Func Name="AtlasClassNames" Arg='tbl'/>
##
## <Returns>
## a list of class names.
## </Returns>
## <Description>
## Let <A>tbl</A> be the ordinary or modular character table of a group
## <M>G</M>, say, that is almost simple or a downward extension of an
## almost simple group and such that <A>tbl</A> is an &ATLAS; table
## from the &GAP; Character Table Library,
## according to its <Ref Func="InfoText" BookName="ref"/> value.
## Then <Ref Func="AtlasClassNames"/> returns the list of class names for
## <M>G</M>, as defined
## in Section <Ref Subsect="subsect:Definition of ATLAS Class Names"/>.
## The ordering of class names is the same as the ordering of the columns
## of <A>tbl</A>.
## 
## (The function may work also for character tables that are not
## &ATLAS; tables,
## but then clearly the class names returned are somewhat arbitrary.)
## 
## <Example><![CDATA[
## gap> AtlasClassNames( CharacterTable( "L3(4).3" ) );
## [ "1A", "2A", "3A", "4ABC", "5A", "5B", "7A", "7B", "3B", "3B'",
## "3C", "3C'", "6B", "6B'", "15A", "15A'", "15B", "15B'", "21A",
## "21A'", "21B", "21B'" ]
## gap> AtlasClassNames( CharacterTable( "U3(5).2" ) );
## [ "1A", "2A", "3A", "4A", "5A", "5B", "5CD", "6A", "7AB", "8AB",
## "10A", "2B", "4B", "6D", "8C", "10B", "12B", "20A", "20B" ]
## gap> AtlasClassNames( CharacterTable( "L2(27).6" ) );
## [ "1A", "2A", "3AB", "7ABC", "13ABC", "13DEF", "14ABC", "2B", "4A",
## "26ABC", "26DEF", "28ABC", "28DEF", "3C", "3C'", "6A", "6A'",
## "9AB", "9A'B'", "6B", "6B'", "12A", "12A'" ]
## gap> AtlasClassNames( CharacterTable( "L3(4).3.2_2" ) );
## [ "1A", "2A", "3A", "4ABC", "5AB", "7A", "7B", "3B", "3C", "6B",
## "15A", "15B", "21A", "21B", "2C", "4E", "6E", "8D", "14A", "14B" ]
## gap> AtlasClassNames( CharacterTable( "3.A6" ) );
## [ "1A_0", "1A_1", "1A_2", "2A_0", "2A_1", "2A_2", "3A_0", "3B_0",
## "4A_0", "4A_1", "4A_2", "5A_0", "5A_1", "5A_2", "5B_0", "5B_1",
## "5B_2" ]
## gap> AtlasClassNames( CharacterTable( "2.A5.2" ) );
## [ "1A_0", "1A_1", "2A_0", "3A_0", "3A_1", "5AB_0", "5AB_1", "2B_0",
## "4A_0", "4A_1", "6A_0", "6A_1" ]
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "AtlasClassNames" );

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##
#V AtlasClassNamesOffsetInfo
##
## <ManSection>
## <Var Name="AtlasClassNamesOffsetInfo"/>
##
## <Description>
## This global variable describes the cyclic upwards extensions of those
## simple groups whose ordinary character tables are contained in the
## &ATLAS; of Finite Groups and for which the outer automorphism groups
## are noncyclic.
## </Description>
## </ManSection>
##
DeclareGlobalVariable( "AtlasClassNamesOffsetInfo" );

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##
#F AtlasCharacterNames( <tbl> )
##
## <#GAPDoc Label="AtlasCharacterNames">
## <ManSection>
## <Func Name="AtlasCharacterNames" Arg='tbl'/>
##
## <Returns>
## a list of character names.
## </Returns>
## <Description>
## Let <A>tbl</A> be the ordinary or modular character table of a simple
## group.
## <Ref Func="AtlasCharacterNames"/> returns a list of strings,
## the <M>i</M>-th entry being the name of the <M>i</M>-th irreducible
## character of <A>tbl</A>;
## this name consists of the degree of this character followed by
## distinguishing lowercase letters.
## 
## <Example><![CDATA[
## gap> AtlasCharacterNames( CharacterTable( "A5" ) );
## [ "1a", "3a", "3b", "4a", "5a" ]
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "AtlasCharacterNames" );

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##
#F StringOfAtlasProgramCycToCcls( <prgstring>, <tbl>, <mode> )
##
## <ManSection>
## <Func Name="StringOfAtlasProgramCycToCcls" Arg='prgstring, tbl, mode'/>
##
## <Description>
## The purpose of this utility program is to construct a straight line
## program for computing conjugacy class representatives of a group <M>G</M>
## from a straight line program for computing representatives of classes
## of maximally cyclic subgroups of <M>G</M>;
## the latter program is assumed to be given by the string <A>prgstring</A>.
## The second argument <A>tbl</A> must be the ordinary character table of
## <M>G</M>.
## The third argument <A>mode</A> must be one of the strings <C>"names"</C>
## or <C>"numbers"</C>; in the former case, the labels used are class names,
## in the latter case they are numbers.
## (Note that the labels used for the inputs are the outputs of the program
## given by <A>prgstring</A>, which may be names even if <C>"numbers"</C> is
## chosen for <A>mode</A>.)
## 
## <M>G</M> must be an &ATLAS; group, and the classes of <A>tbl</A> must be
## sorted compatibly with the
## &ATLAS; of Finite Groups <Cite Key="CCN85"/>,
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "StringOfAtlasProgramCycToCcls" );

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##
#F CurrentDateTimeString( [<options>] )
##
## <ManSection>
## <Func Name="CurrentDateTimeString" Arg='[options]'/>
##
## <Description>
## If the system function <C>date</C> is available then the return value is
## a string that describes the current date and time,
## otherwise the string <C>"unknown"</C> is returned.
## 
## If the argument <A>options</A> is given it must be a list of options for
## <C>date</C>;
## for example an empty list means the local time,
## and the value <C>[ "-u" ]</C> means Greenwich mean time (UTC).
## 
## If no argument is given then the format of the result refers to Greenwich
## mean time and is compatible with <Ref Func="StringDate" BookName="ref"/>
## and <Ref Func="StringTime" BookName="ref"/>;
## in this case the option <C>+%s</C> is used.
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "CurrentDateTimeString" );

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##
#F SendMail( <sendto>, <copyto>, <subject>, <text> )
##
## <ManSection>
## <Func Name="SendMail" Arg='sendto, copyto, subject, text'/>
##
## <Description>
## Let <A>sendto</A> and <A>copyto</A> be lists of email addresses,
## and <A>subject</A> and <A>text</A> be strings.
## <Ref Func="SendMail"/> sends email messages with subject <A>subject</A>
## and body <A>text</A> to the addresses in <A>sendto</A> and <A>copyto</A>.
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "SendMail" );

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##
#F ParseBackwards( <string>, <format> )
#F ParseBackwardsWithPrefix( <string>, <format> )
#F ParseForwards( <string>, <format> )
#F ParseForwardsWithSuffix( <string>, <format> )
##
## <ManSection>
## <Func Name="ParseBackwards" Arg='string, format'/>
## <Func Name="ParseBackwardsWithPrefix" Arg='string, format'/>
## <Func Name="ParseForwards" Arg='string, format'/>
## <Func Name="ParseForwardsWithSuffix" Arg='string, format'/>
##
## <Description>
## 
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "ParseBackwards" );
DeclareGlobalFunction( "ParseBackwardsWithPrefix" );
DeclareGlobalFunction( "ParseForwards" );
DeclareGlobalFunction( "ParseForwardsWithSuffix" );

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##
#F AtlasRepIdentifier( <oldid> )
#F AtlasRepIdentifier( <id>, "old" )
##
## <#GAPDoc Label="AtlasRepIdentifier">
## <ManSection>
## <Heading>AtlasRepIdentifier</Heading>
## <Func Name="AtlasRepIdentifier" Arg='oldid'
## Label="convert an old type identifier to a new type one"/>
## <Func Name="AtlasRepIdentifier" Arg='id, "old"'
## Label="convert a new type identifier to an old type one"/>
##
## <Description>
## This function converts between the <Q>old format</Q> (the one used up to
## version 1.5.1 of the package) and the <Q>new format</Q> (the one used
## since version 2.0) of the <C>identifier</C> component of the records
## returned by &AtlasRep; functions.
## Note that the two formats differ only for <C>identifier</C> components
## that describe data from non-core parts of the database.
## 
## If the only argument is a list <A>oldid</A> that is an <C>identifier</C>
## in old format then the function returns the corresponding
## <C>identifier</C> in new format.
## If there are two arguments, a list <A>id</A> that is an <C>identifier</C>
## in new format and the string <A>"old"</A>,
## then the function returns the corresponding <C>identifier</C> in old
## format if this is possible, and <K>fail</K> otherwise.
## 
## <Example><![CDATA[
## gap> id:= [ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5 ];;
## gap> AtlasRepIdentifier( id ) = id;
## true
## gap> id:= [ "L2(8)", "L28G1-check1", 1, 1 ];;
## gap> AtlasRepIdentifier( id ) = id;
## true
## gap> oldid:= [ [ "priv", "C4" ], [ "C4G1-p4B0.m1" ], 1, 4 ];;
## gap> newid:= AtlasRepIdentifier( oldid );
## [ "C4", [ [ "priv", "C4G1-p4B0.m1" ] ], 1, 4 ]
## gap> oldid = AtlasRepIdentifier( newid, "old" );
## true
## gap> oldid:= [ [ "priv", "C4" ], "C4G1-max1W1", 1 ];;
## gap> newid:= AtlasRepIdentifier( oldid );
## [ "C4", [ [ "priv", "C4G1-max1W1" ] ], 1 ]
## gap> oldid = AtlasRepIdentifier( newid, "old" );
## true
## gap> oldid:= [ [ "priv", "C4" ], "C4G1-Ar1aB0.g", 1, 1 ];;
## gap> newid:= AtlasRepIdentifier( oldid );
## [ "C4", [ [ "priv", "C4G1-Ar1aB0.g" ] ], 1, 1 ]
## gap> oldid = AtlasRepIdentifier( newid, "old" );
## true
## gap> oldid:= [ [ "priv", "C4" ], "C4G1-XtestW1", 1 ];;
## gap> newid:= AtlasRepIdentifier( oldid );
## [ "C4", [ [ "priv", "C4G1-XtestW1" ] ], 1 ]
## gap> oldid = AtlasRepIdentifier( newid, "old" );
## true
## gap> oldid:= [ [ "mfer", "2.M12" ],
## > [ "2M12G1-p264aB0.m1", "2M12G1-p264aB0.m2" ], 1, 264 ];;
## gap> newid:= AtlasRepIdentifier( oldid );
## [ "2.M12",
## [ [ "mfer", "2M12G1-p264aB0.m1" ], [ "mfer", "2M12G1-p264aB0.m2" ] ]
## , 1, 264 ]
## gap> oldid = AtlasRepIdentifier( newid, "old" );
## true
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "AtlasRepIdentifier" );

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##
#F CompositionOfSLDAndSLP( <sld>, <slp> )
##
## <ManSection>
## <Func Name="CompositionOfSLDAndSLP" Arg='sld, slp'/>
##
## <Description>
## Return a straight line decision that first applies the straight line
## program <A>slp</A> to its inputs and then returns the result of the
## straight line decision <A>sld</A> on the outputs.
## 
## A typical situation is that <A>slp</A> is a restandardization script
## and <A>sld</A> is a presentation.
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "CompositionOfSLDAndSLP" );

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##
#F AtlasRepComputedKernelGenerators( <gapname>, <std>,
#F <factgapname>, <factstd>,
#F <bound> )
##
## <ManSection>
## <Func Name="AtlasRepComputedKernelGenerators"
## Arg='gapname, std, factgapname, factstd, bound'/>
##
## <Description>
## We assume that <A>gapname</A> and <A>factgapname</A> are valid arguments
## of <Ref Func="AtlasGroup"/>,
## and that the <A>std</A>-th and <A>factstd</A>-th standard generators of
## the two groups <M>G</M> and <M>F</M>, say, are compatible
## in the sense that mapping the generators of <M>G</M> to those of <M>F</M>
## defines an epimorphism.
## 
## If representations for the two groups in the given standardizations
## are locally available then the following happens.
## 
## The function runs over the elements of a free monoid and collects those
## elements that evaluate to elements of different orders in the two groups
## and thus lie in the kernel of the epimorphism from <M>G</M> to <M>F</M>.
## Only those words in the free generators are considered for which the
## exponents of all syllables are smaller than the orders of the
## corresponding generators of <M>G</M>.
## 
## If <A>gapname</A> and <A>factgapname</A> are two identifiers of
## character tables from the &GAP; Character Table Library such that
## a factor fusion from the table of <A>gapname</A> to that of
## <A>factgapname</A> is stored then the character tables are used
## to determine those orders of elements in <A>F</A> for which a preimage
## in <M>G</M> has larger order.
## In this case, only those elements of <M>G</M> are computed for which
## the order of the corresponding element of <M>F</M> admits a preimage of
## larger order in <M>G</M>.
## 
## At most the first <A>bound</A> words in the free generators are checked
## for which an element of <M>G</M> is actually computed according to these
## rules.
## 
## The function returns <K>fail</K> if it finds out that the generators
## are not compatible;
## in this case, a message about the details is printed
## if the info level of <Ref Var="InfoAtlasRep"/> is at least <M>3</M>.
## Otherwise, the function returns a list <M>[ l, flag ]</M>,
## where <M>l</M> is a list of pairs <M>[ w, o ]</M> such that <M>w^o</M>
## describes an element in the kernel,
## and <M>flag</M> is <K>true</K> if these words are known to generate
## the kernel, and <K>false</K> otherwise.
## 
## Yes, the strategy used is quite simpleminded:
## First, although the words in the free monoid are checked in an ordering
## that respects the length of the words, it may happen that some longer
## word can be evaluated with a straight line program that needs less
## multiplications.
## Second, the checks of many words are unnecessary because these words
## evaluate to the same elements as words that have been checked already.
## 
## Moreover, the strategy is suitable only for computing <E>small</E>
## kernels, since membership tests for the kernel are needed if it is not
## cyclic. Large kernels occur for example in maximal subgroups of the
## Monster group; if such a kernel is an irreducible module then it is
## a better approach to find one nontrivial element in the kernel and
## suitable conjugating elements of the maximal subgroup.
## 
## <Example><![CDATA[
## gap> AtlasRepComputedKernelGenerators( "2.A5", 1, "A5", 1, 10^6 );
## [ [ [ m1, 2 ] ], true ]
## gap> g:= AtlasGroup( "A5" );;
## gap> 2g:= AtlasGroup( "2.A5" );;
## gap> List( GeneratorsOfGroup( g ), Order );
## [ 2, 3 ]
## gap> List( GeneratorsOfGroup( 2g ), Order );
## [ 4, 3 ]
## ]]></Example>
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "AtlasRepComputedKernelGenerators" );

#############################################################################
##
#E

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