  
  [1X51 [33X[0;0YSemigroups and Monoids[133X[101X
  
  [33X[0;0YThis  chapter  describes  functions  for creating semigroups and monoids and
  determining information about them.[133X
  
  
  [1X51.1 [33X[0;0YSemigroups[133X[101X
  
  [1X51.1-1 IsSemigroup[101X
  
  [29X[2XIsSemigroup[102X( [3XD[103X ) [32X Synonym
  
  [33X[0;0Yreturns [9Xtrue[109X if the object [3XD[103X is a semigroup. A [13Xsemigroup[113X is a magma (see [14X35[114X)
  with associative multiplication.[133X
  
  
  [1X51.1-2 [33X[0;0YSemigroup[133X[101X
  
  [29X[2XSemigroup[102X( [3Xgen1[103X, [3Xgen2[103X, [3X...[103X ) [32X function
  [29X[2XSemigroup[102X( [3Xgens[103X ) [32X function
  
  [33X[0;0YIn  the  first  form,  [2XSemigroup[102X  returns  the  semigroup  generated  by the
  arguments  [3Xgen1[103X,  [3Xgen2[103X,  [22X...[122X,  that  is, the closure of these elements under
  multiplication.   In  the  second  form,  [2XSemigroup[102X  returns  the  semigroup
  generated  by  the elements in the homogeneous list [3Xgens[103X; a square matrix as
  only argument is treated as one generator, not as a list of generators.[133X
  
  [33X[0;0YIt  is [13Xnot[113X checked whether the underlying multiplication is associative, use
  [2XMagma[102X  ([14X35.2-1[114X)  and  [2XIsAssociative[102X  ([14X35.4-7[114X) if you want to check whether a
  magma is in fact a semigroup.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xa:= Transformation( [ 2, 3, 4, 1 ] );[127X[104X
    [4X[28XTransformation( [ 2, 3, 4, 1 ] )[128X[104X
    [4X[25Xgap>[125X [27Xb:= Transformation( [ 2, 2, 3, 4 ] );[127X[104X
    [4X[28XTransformation( [ 2, 2 ] )[128X[104X
    [4X[25Xgap>[125X [27Xs:= Semigroup(a, b);[127X[104X
    [4X[28X<transformation semigroup of degree 4 with 2 generators>[128X[104X
  [4X[32X[104X
  
  [1X51.1-3 Subsemigroup[101X
  
  [29X[2XSubsemigroup[102X( [3XS[103X, [3Xgens[103X ) [32X function
  [29X[2XSubsemigroupNC[102X( [3XS[103X, [3Xgens[103X ) [32X function
  
  [33X[0;0Yare   just   synonyms   of   [2XSubmagma[102X   ([14X35.2-7[114X)  and  [2XSubmagmaNC[102X  ([14X35.2-7[114X),
  respectively.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xa:=GeneratorsOfSemigroup(s)[1];[127X[104X
    [4X[28XTransformation( [ 2, 3, 4, 1 ] )[128X[104X
    [4X[25Xgap>[125X [27Xt:=Subsemigroup(s,[a]);[127X[104X
    [4X[28X<commutative transformation semigroup of degree 4 with 1 generator>[128X[104X
  [4X[32X[104X
  
  [1X51.1-4 IsSubsemigroup[101X
  
  [29X[2XIsSubsemigroup[102X( [3XS[103X, [3XT[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Y[9Xtrue[109X or [9Xfalse[109X.[133X
  
  [33X[0;0YThis  operation  returns  [9Xtrue[109X  if  the semigroup [3XT[103X is a subsemigroup of the
  semigroup [3XS[103X and [9Xfalse[109X if it is not.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xf:=Transformation( [ 5, 6, 7, 1, 4, 3, 2, 7 ] );[127X[104X
    [4X[28XTransformation( [ 5, 6, 7, 1, 4, 3, 2, 7 ] )[128X[104X
    [4X[25Xgap>[125X [27XT:=Semigroup(f);;[127X[104X
    [4X[25Xgap>[125X [27XIsSubsemigroup(FullTransformationSemigroup(4), T);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XS:=Semigroup(f);; T:=Semigroup(f^2);;[127X[104X
    [4X[25Xgap>[125X [27XIsSubsemigroup(S, T);                             [127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [1X51.1-5 SemigroupByGenerators[101X
  
  [29X[2XSemigroupByGenerators[102X( [3Xgens[103X ) [32X operation
  
  [33X[0;0Yis the underlying operation of [2XSemigroup[102X ([14X51.1-2[114X).[133X
  
  [1X51.1-6 AsSemigroup[101X
  
  [29X[2XAsSemigroup[102X( [3XC[103X ) [32X attribute
  
  [33X[0;0YIf  [3XC[103X  is  a  collection  whose  elements  form a semigroup (see [2XIsSemigroup[102X
  ([14X51.1-1[114X))  then  [2XAsSemigroup[102X  returns  this  semigroup.  Otherwise  [9Xfail[109X  is
  returned.[133X
  
  [1X51.1-7 AsSubsemigroup[101X
  
  [29X[2XAsSubsemigroup[102X( [3XD[103X, [3XC[103X ) [32X operation
  
  [33X[0;0YLet  [3XD[103X  be  a  domain and [3XC[103X a collection. If [3XC[103X is a subset of [3XD[103X that forms a
  semigroup  then  [2XAsSubsemigroup[102X  returns  this  semigroup,  with  parent  [3XD[103X.
  Otherwise [9Xfail[109X is returned.[133X
  
  [1X51.1-8 GeneratorsOfSemigroup[101X
  
  [29X[2XGeneratorsOfSemigroup[102X( [3XS[103X ) [32X attribute
  
  [33X[0;0YSemigroup  generators  of  a  semigroup  [3XD[103X are the same as magma generators,
  see [2XGeneratorsOfMagma[102X ([14X35.4-1[114X).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfSemigroup(s);[127X[104X
    [4X[28X[ Transformation( [ 2, 3, 4, 1 ] ), Transformation( [ 2, 2 ] ) ][128X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfSemigroup(t);[127X[104X
    [4X[28X[ Transformation( [ 2, 3, 4, 1 ] ) ][128X[104X
  [4X[32X[104X
  
  [1X51.1-9 IsGeneratorsOfSemigroup[101X
  
  [29X[2XIsGeneratorsOfSemigroup[102X( [3XC[103X ) [32X property
  
  [33X[0;0YThis  property  reflects  wheter  the  list  or  collection  [3XC[103X  generates  a
  semigroup.       [2XIsAssociativeElementCollection[102X       ([14X31.15-1[114X)      implies
   [2XIsGeneratorsOfSemigroup[102X,  but  is  not  used  directly  in  semigroup code,
  because of conflicts with matrices.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XIsGeneratorsOfSemigroup([Transformation([2,3,1])]);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  
  [1X51.1-10 [33X[0;0YFreeSemigroup[133X[101X
  
  [29X[2XFreeSemigroup[102X( [[3Xwfilt[103X, ][3Xrank[103X[, [3Xname[103X] ) [32X function
  [29X[2XFreeSemigroup[102X( [[3Xwfilt[103X, ][3Xname1[103X, [3Xname2[103X, [3X...[103X ) [32X function
  [29X[2XFreeSemigroup[102X( [[3Xwfilt[103X, ][3Xnames[103X ) [32X function
  [29X[2XFreeSemigroup[102X( [[3Xwfilt[103X, ][3Xinfinity[103X, [3Xname[103X, [3Xinit[103X ) [32X function
  
  [33X[0;0YCalled  with a positive integer [3Xrank[103X, [2XFreeSemigroup[102X returns a free semigroup
  on  [3Xrank[103X  generators.  If  the  optional  argument  [3Xname[103X  is  given then the
  generators  are  printed  as  [3Xname[103X[10X1[110X,  [3Xname[103X[10X2[110X  etc., that is, each name is the
  concatenation of the string [3Xname[103X and an integer from [10X1[110X to [3Xrange[103X. The default
  for [3Xname[103X is the string [10X"s"[110X.[133X
  
  [33X[0;0YCalled in the second form, [2XFreeSemigroup[102X returns a free semigroup on as many
  generators as arguments, printed as [3Xname1[103X, [3Xname2[103X etc.[133X
  
  [33X[0;0YCalled  in the third form, [2XFreeSemigroup[102X returns a free semigroup on as many
  generators as the length of the list [3Xnames[103X, the [22Xi[122X-th generator being printed
  as [3Xnames[103X[22X[i][122X.[133X
  
  [33X[0;0YCalled  in  the  fourth  form,  [2XFreeSemigroup[102X  returns  a  free semigroup on
  infinitely  many  generators,  where the first generators are printed by the
  names  in  the  list  [3Xinit[103X, and the other generators by [3Xname[103X and an appended
  number.[133X
  
  [33X[0;0YIf    the   extra   argument   [3Xwfilt[103X   is   given,   it   must   be   either
  [2XIsSyllableWordsFamily[102X    ([14X37.6-6[114X)   or   [2XIsLetterWordsFamily[102X   ([14X37.6-2[114X)   or
  [2XIsWLetterWordsFamily[102X  ([14X37.6-4[114X) or [2XIsBLetterWordsFamily[102X ([14X37.6-4[114X). This filter
  then  specifies  the  representation  used  for  the  elements  of  the free
  semigroup (see [14X37.6[114X). If no such filter is given, a letter representation is
  used.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xf1 := FreeSemigroup( 3 );[127X[104X
    [4X[28X<free semigroup on the generators [ s1, s2, s3 ]>[128X[104X
    [4X[25Xgap>[125X [27Xf2 := FreeSemigroup( 3 , "generator" );[127X[104X
    [4X[28X<free semigroup on the generators [128X[104X
    [4X[28X[ generator1, generator2, generator3 ]>[128X[104X
    [4X[25Xgap>[125X [27Xf3 := FreeSemigroup( "gen1" , "gen2" );[127X[104X
    [4X[28X<free semigroup on the generators [ gen1, gen2 ]>[128X[104X
    [4X[25Xgap>[125X [27Xf4 := FreeSemigroup( ["gen1" , "gen2"] );[127X[104X
    [4X[28X<free semigroup on the generators [ gen1, gen2 ]>[128X[104X
  [4X[32X[104X
  
  [33X[0;0YAlso see Chapter [14X51[114X.[133X
  
  [33X[0;0YEach  free  object defines a unique alphabet (and a unique family of words).
  Its generators are the letters of this alphabet, thus words of length one.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XFreeGroup( 5 );[127X[104X
    [4X[28X<free group on the generators [ f1, f2, f3, f4, f5 ]>[128X[104X
    [4X[25Xgap>[125X [27XFreeGroup( "a", "b" );[127X[104X
    [4X[28X<free group on the generators [ a, b ]>[128X[104X
    [4X[25Xgap>[125X [27XFreeGroup( infinity );[127X[104X
    [4X[28X<free group with infinity generators>[128X[104X
    [4X[25Xgap>[125X [27XFreeSemigroup( "x", "y" );[127X[104X
    [4X[28X<free semigroup on the generators [ x, y ]>[128X[104X
    [4X[25Xgap>[125X [27XFreeMonoid( 7 );[127X[104X
    [4X[28X<free monoid on the generators [ m1, m2, m3, m4, m5, m6, m7 ]>[128X[104X
  [4X[32X[104X
  
  [33X[0;0YRemember  that  names  are  just  a help for printing and do not necessarily
  distinguish  letters.  It is possible to create arbitrarily weird situations
  by choosing strange names for the letters.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xf:= FreeGroup( "x", "x" );  gens:= GeneratorsOfGroup( f );;[127X[104X
    [4X[28X<free group on the generators [ x, x ]>[128X[104X
    [4X[25Xgap>[125X [27Xgens[1] = gens[2];[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xf:= FreeGroup( "f1*f2", "f2^-1", "Group( [ f1, f2 ] )" );[127X[104X
    [4X[28X<free group on the generators [ f1*f2, f2^-1, Group( [ f1, f2 ] ) ]>[128X[104X
    [4X[25Xgap>[125X [27Xgens:= GeneratorsOfGroup( f );;[127X[104X
    [4X[25Xgap>[125X [27Xgens[1]*gens[2];[127X[104X
    [4X[28Xf1*f2*f2^-1[128X[104X
    [4X[25Xgap>[125X [27Xgens[1]/gens[3];[127X[104X
    [4X[28Xf1*f2*Group( [ f1, f2 ] )^-1[128X[104X
    [4X[25Xgap>[125X [27Xgens[3]/gens[1]/gens[2];[127X[104X
    [4X[28XGroup( [ f1, f2 ] )*f1*f2^-1*f2^-1^-1[128X[104X
  [4X[32X[104X
  
  [1X51.1-11 SemigroupByMultiplicationTable[101X
  
  [29X[2XSemigroupByMultiplicationTable[102X( [3XA[103X ) [32X function
  
  [33X[0;0Yreturns the semigroup whose multiplication is defined by the square matrix [3XA[103X
  (see [2XMagmaByMultiplicationTable[102X   ([14X35.3-1[114X))  if  such  a  semigroup  exists.
  Otherwise [9Xfail[109X is returned.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XSemigroupByMultiplicationTable([[1,2,3],[2,3,1],[3,1,2]]);[127X[104X
    [4X[28X<semigroup of size 3, with 3 generators>[128X[104X
    [4X[25Xgap>[125X [27XSemigroupByMultiplicationTable([[1,2,3],[2,3,1],[3,2,1]]);[127X[104X
    [4X[28Xfail[128X[104X
  [4X[32X[104X
  
  
  [1X51.2 [33X[0;0YMonoids[133X[101X
  
  [1X51.2-1 IsMonoid[101X
  
  [29X[2XIsMonoid[102X( [3XD[103X ) [32X Synonym
  
  [33X[0;0YA [13Xmonoid[113X is a magma-with-one (see [14X35[114X) with associative multiplication.[133X
  
  
  [1X51.2-2 [33X[0;0YMonoid[133X[101X
  
  [29X[2XMonoid[102X( [3Xgen1[103X, [3Xgen2[103X, [3X...[103X ) [32X function
  [29X[2XMonoid[102X( [3Xgens[103X[, [3Xid[103X] ) [32X function
  
  [33X[0;0YIn  the  first  form,  [2XMonoid[102X  returns the monoid generated by the arguments
  [3Xgen1[103X, [3Xgen2[103X, [22X...[122X, that is, the closure of these elements under multiplication
  and  taking  the  0-th  power. In the second form, [2XMonoid[102X returns the monoid
  generated  by  the elements in the homogeneous list [3Xgens[103X; a square matrix as
  only  argument  is treated as one generator, not as a list of generators. In
  the  second  form,  the  identity  element  [3Xid[103X  may  be  given as the second
  argument.[133X
  
  [33X[0;0YIt  is [13Xnot[113X checked whether the underlying multiplication is associative, use
  [2XMagmaWithOne[102X  ([14X35.2-2[114X)  and  [2XIsAssociative[102X  ([14X35.4-7[114X)  if  you  want to check
  whether a magma-with-one is in fact a monoid.[133X
  
  [1X51.2-3 Submonoid[101X
  
  [29X[2XSubmonoid[102X( [3XM[103X, [3Xgens[103X ) [32X function
  [29X[2XSubmonoidNC[102X( [3XM[103X, [3Xgens[103X ) [32X function
  
  [33X[0;0Yare   just   synonyms  of  [2XSubmagmaWithOne[102X  ([14X35.2-8[114X)  and  [2XSubmagmaWithOneNC[102X
  ([14X35.2-8[114X), respectively.[133X
  
  [1X51.2-4 MonoidByGenerators[101X
  
  [29X[2XMonoidByGenerators[102X( [3Xgens[103X[, [3Xone[103X] ) [32X operation
  
  [33X[0;0Yis the underlying operation of [2XMonoid[102X ([14X51.2-2[114X).[133X
  
  [1X51.2-5 AsMonoid[101X
  
  [29X[2XAsMonoid[102X( [3XC[103X ) [32X attribute
  
  [33X[0;0YIf  [3XC[103X  is  a collection whose elements form a monoid (see [2XIsMonoid[102X ([14X51.2-1[114X))
  then [2XAsMonoid[102X returns this monoid. Otherwise [9Xfail[109X is returned.[133X
  
  [1X51.2-6 AsSubmonoid[101X
  
  [29X[2XAsSubmonoid[102X( [3XD[103X, [3XC[103X ) [32X operation
  
  [33X[0;0YLet  [3XD[103X  be  a  domain and [3XC[103X a collection. If [3XC[103X is a subset of [3XD[103X that forms a
  monoid  then  [2XAsSubmonoid[102X returns this monoid, with parent [3XD[103X. Otherwise [9Xfail[109X
  is returned.[133X
  
  [1X51.2-7 GeneratorsOfMonoid[101X
  
  [29X[2XGeneratorsOfMonoid[102X( [3XM[103X ) [32X attribute
  
  [33X[0;0YMonoid  generators  of  a monoid [3XM[103X are the same as magma-with-one generators
  (see [2XGeneratorsOfMagmaWithOne[102X ([14X35.4-2[114X)).[133X
  
  [1X51.2-8 TrivialSubmonoid[101X
  
  [29X[2XTrivialSubmonoid[102X( [3XM[103X ) [32X attribute
  
  [33X[0;0Yis just a synonym for [2XTrivialSubmagmaWithOne[102X ([14X35.4-13[114X).[133X
  
  
  [1X51.2-9 [33X[0;0YFreeMonoid[133X[101X
  
  [29X[2XFreeMonoid[102X( [[3Xwfilt[103X, ][3Xrank[103X[, [3Xname[103X] ) [32X function
  [29X[2XFreeMonoid[102X( [[3Xwfilt[103X, ][3Xname1[103X, [3Xname2[103X, [3X...[103X ) [32X function
  [29X[2XFreeMonoid[102X( [[3Xwfilt[103X, ][3Xnames[103X ) [32X function
  [29X[2XFreeMonoid[102X( [[3Xwfilt[103X, ][3Xinfinity[103X, [3Xname[103X, [3Xinit[103X ) [32X function
  
  [33X[0;0YCalled  with  a  positive  integer [3Xrank[103X, [2XFreeMonoid[102X returns a free monoid on
  [3Xrank[103X  generators. If the optional argument [3Xname[103X is given then the generators
  are printed as [3Xname[103X[10X1[110X, [3Xname[103X[10X2[110X etc., that is, each name is the concatenation of
  the  string [3Xname[103X and an integer from [10X1[110X to [3Xrange[103X. The default for [3Xname[103X is the
  string [10X"m"[110X.[133X
  
  [33X[0;0YCalled  in  the  second  form,  [2XFreeMonoid[102X  returns a free monoid on as many
  generators as arguments, printed as [3Xname1[103X, [3Xname2[103X etc.[133X
  
  [33X[0;0YCalled  in  the  third  form,  [2XFreeMonoid[102X  returns  a free monoid on as many
  generators as the length of the list [3Xnames[103X, the [22Xi[122X-th generator being printed
  as [3Xnames[103X[10X[[110X[22Xi[122X[10X][110X.[133X
  
  [33X[0;0YCalled  in  the  fourth form, [2XFreeMonoid[102X returns a free monoid on infinitely
  many  generators, where the first generators are printed by the names in the
  list [3Xinit[103X, and the other generators by [3Xname[103X and an appended number.[133X
  
  [33X[0;0YIf    the   extra   argument   [3Xwfilt[103X   is   given,   it   must   be   either
  [2XIsSyllableWordsFamily[102X    ([14X37.6-6[114X)   or   [2XIsLetterWordsFamily[102X   ([14X37.6-2[114X)   or
  [2XIsWLetterWordsFamily[102X  ([14X37.6-4[114X) or [2XIsBLetterWordsFamily[102X ([14X37.6-4[114X). This filter
  then  specifies  the representation used for the elements of the free monoid
  (see [14X37.6[114X). If no such filter is given, a letter representation is used.[133X
  
  [33X[0;0YAlso see Chapter [14X51[114X.[133X
  
  [1X51.2-10 MonoidByMultiplicationTable[101X
  
  [29X[2XMonoidByMultiplicationTable[102X( [3XA[103X ) [32X function
  
  [33X[0;0Yreturns  the  monoid  whose multiplication is defined by the square matrix [3XA[103X
  (see [2XMagmaByMultiplicationTable[102X ([14X35.3-1[114X)) if such a monoid exists. Otherwise
  [9Xfail[109X is returned.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XMonoidByMultiplicationTable([[1,2,3],[2,3,1],[3,1,2]]);[127X[104X
    [4X[28X<monoid of size 3, with 3 generators>[128X[104X
    [4X[25Xgap>[125X [27XMonoidByMultiplicationTable([[1,2,3],[2,3,1],[1,3,2]]);[127X[104X
    [4X[28Xfail[128X[104X
  [4X[32X[104X
  
  
  [1X51.3 [33X[0;0YInverse semigroups and monoids[133X[101X
  
  [1X51.3-1 InverseSemigroup[101X
  
  [29X[2XInverseSemigroup[102X( [3Xobj1[103X, [3Xobj2[103X, [3X...[103X ) [32X function
  [6XReturns:[106X  [33X[0;10YAn inverse semigroup.[133X
  
  [33X[0;0YIf [3Xobj1[103X, [3Xobj2[103X, ... are (any combination) of associative elements with unique
  semigroup  inverses,  semigroups  of  such  elements, or collections of such
  elements,  then  [10XInverseSemigroup[110X returns the inverse semigroup generated by
  the  union  of  [3Xobj1[103X,  [3Xobj2[103X, .... This equals the semigroup generated by the
  union of [3Xobj1[103X, [3Xobj2[103X, ... and their inverses.[133X
  
  [33X[0;0YFor  example  if [10XS[110X and [10XT[110X are inverse semigroups, then [10XInverseSemigroup(S, f,
  Idempotents(T));[110X     is     the     inverse     semigroup    generated    by
  [10XUnion(GeneratorsOfInverseSemigroup(S), [f], Idempotents(T)));[110X.[133X
  
  [33X[0;0YAs  present,  the  only associative elements with unique semigroup inverses,
  which  do not always generate a group, are partial permutations; see Chapter
  [14X54[114X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS := InverseSemigroup([127X[104X
    [4X[25X>[125X [27XPartialPerm( [ 1, 2, 3, 6, 8, 10 ], [ 2, 6, 7, 9, 1, 5 ] ) );;[127X[104X
    [4X[25Xgap>[125X [27Xf := PartialPerm( [ 1, 2, 3, 4, 5, 8, 10 ], [127X[104X
    [4X[25X>[125X [27X[ 7, 1, 4, 3, 2, 6, 5 ] );;[127X[104X
    [4X[25Xgap>[125X [27XS := InverseSemigroup(S, f, Idempotents(SymmetricInverseSemigroup(5)));[127X[104X
    [4X[28X<inverse partial perm semigroup of rank 10 with 34 generators>[128X[104X
    [4X[25Xgap>[125X [27XSize(S);[127X[104X
    [4X[28X1233[128X[104X
  [4X[32X[104X
  
  [1X51.3-2 InverseMonoid[101X
  
  [29X[2XInverseMonoid[102X( [3Xobj1[103X, [3Xobj2[103X, [3X...[103X ) [32X function
  [6XReturns:[106X  [33X[0;10YAn inverse monoid.[133X
  
  [33X[0;0YIf [3Xobj1[103X, [3Xobj2[103X, ... are (any combination) of associative elements with unique
  semigroup  inverses,  semigroups  of  such  elements, or collections of such
  elements,  then  [10XInverseMonoid[110X  returns  the inverse monoid generated by the
  union  of  [3Xobj1[103X, [3Xobj2[103X, .... This equals the monoid generated by the union of
  [3Xobj1[103X, [3Xobj2[103X, ... and their inverses.[133X
  
  [33X[0;0YAs present, the only associative elements with unique semigroup inverses are
  partial permutations; see Chapter [14X54[114X.[133X
  
  [33X[0;0YFor  example  if  [10XS[110X  and  [10XT[110X  are  inverse  monoids, then [10XInverseMonoid(S, f,
  Idempotents(T));[110X      is     the     inverse     monoid     generated     by
  [10XUnion(GeneratorsOfInverseMonoid(S), [f], Idempotents(T)));[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS := InverseMonoid([127X[104X
    [4X[25X>[125X [27XPartialPerm( [ 1, 2, 3, 6, 8, 10 ], [ 2, 6, 7, 9, 1, 5 ] ) );;[127X[104X
    [4X[25Xgap>[125X [27Xf := PartialPerm( [ 1, 2, 3, 4, 5, 8, 10 ], [127X[104X
    [4X[25X>[125X [27X[ 7, 1, 4, 3, 2, 6, 5 ] );;[127X[104X
    [4X[25Xgap>[125X [27XS := InverseMonoid(S, f, Idempotents(SymmetricInverseSemigroup(5)));[127X[104X
    [4X[28X<inverse partial perm monoid of rank 10 with 35 generators>[128X[104X
    [4X[25Xgap>[125X [27XSize(S);[127X[104X
    [4X[28X1243[128X[104X
  [4X[32X[104X
  
  [1X51.3-3 GeneratorsOfInverseSemigroup[101X
  
  [29X[2XGeneratorsOfInverseSemigroup[102X( [3XS[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10YThe generators of an inverse semigroup.[133X
  
  [33X[0;0YIf  [3XS[103X is an inverse semigroup, then [10XGeneratorsOfInverseSemigroup[110X returns the
  generators used to define [3XS[103X, i.e. an inverse semigroup generating set for [3XS[103X.[133X
  
  [33X[0;0YThe  value  of  [10XGeneratorsOfSemigroup([3XS[103X[10X)[110X, for an inverse semigroup [3XS[103X, is the
  union  of  inverse  semigroup  generator  and  their  inverses. So, [3XS[103X is the
  semigroup,  as  opposed  to  inverse semigroup, generated by the elements of
  [10XGeneratorsOfInverseSemigroup([3XS[103X[10X)[110X and their inverses.[133X
  
  [33X[0;0YIf  [3XS[103X  is  an  inverse monoid, then [10XGeneratorsOfInverseSemigroup[110X returns the
  generators used to define [3XS[103X, as described above, and the identity of [3XS[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS:=InverseMonoid([127X[104X
    [4X[25X>[125X [27X PartialPerm( [ 1, 2 ], [ 1, 4 ] ),[127X[104X
    [4X[25X>[125X [27X PartialPerm( [ 1, 2, 4 ], [ 3, 4, 1 ] ) );;[127X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfSemigroup(S);[127X[104X
    [4X[28X[ <identity partial perm on [ 1, 2, 3, 4 ]>, [2,4](1), [2,4,1,3], [128X[104X
    [4X[28X  [4,2](1), [3,1,4,2] ][128X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfInverseSemigroup(S);[127X[104X
    [4X[28X[ [2,4](1), [2,4,1,3], <identity partial perm on [ 1, 2, 3, 4 ]> ][128X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfMonoid(S);[127X[104X
    [4X[28X[ [2,4](1), [2,4,1,3], [4,2](1), [3,1,4,2] ][128X[104X
  [4X[32X[104X
  
  [1X51.3-4 GeneratorsOfInverseMonoid[101X
  
  [29X[2XGeneratorsOfInverseMonoid[102X( [3XS[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10YThe generators of an inverse monoid.[133X
  
  [33X[0;0YIf  [3XS[103X  is  an  inverse  monoid,  then  [10XGeneratorsOfInverseMonoid[110X returns the
  generators used to define [3XS[103X, i.e. an inverse monoid generating set for [3XS[103X.[133X
  
  [33X[0;0YThere  are  four  different possible generating sets which define an inverse
  monoid.  More  precisely,  an  inverse monoid can be generated as an inverse
  monoid,  inverse  semigroup,  monoid, or semigroup. The different generating
  sets   in   each  case  can  be  obtained  using  [2XGeneratorsOfInverseMonoid[102X,
  [2XGeneratorsOfInverseSemigroup[102X   ([14X51.3-3[114X),  [2XGeneratorsOfMonoid[102X  ([14X51.2-7[114X),  and
  [2XGeneratorsOfSemigroup[102X ([14X51.1-8[114X), respectively.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS:=InverseMonoid([127X[104X
    [4X[25X>[125X [27X PartialPerm( [ 1, 2 ], [ 1, 4 ] ),[127X[104X
    [4X[25X>[125X [27X PartialPerm( [ 1, 2, 4 ], [ 3, 4, 1 ] ) );;[127X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfInverseMonoid(S);[127X[104X
    [4X[28X[ [2,4](1), [2,4,1,3] ][128X[104X
  [4X[32X[104X
  
  [1X51.3-5 IsInverseSubsemigroup[101X
  
  [29X[2XIsInverseSubsemigroup[102X( [3XS[103X, [3XT[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Y[9Xtrue[109X or [9Xfalse[109X.[133X
  
  [33X[0;0YIf  the semigroup [3XT[103X is an inverse subsemigroup of the semigroup [3XS[103X, then this
  operation returns [9Xtrue[109X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XT:=InverseSemigroup(RandomPartialPerm(4));;[127X[104X
    [4X[25Xgap>[125X [27XIsInverseSubsemigroup(SymmetricInverseSemigroup(4), T); [127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XT:=Semigroup(Transformation( [ 1, 2, 4, 5, 6, 3, 7, 8 ] ),[127X[104X
    [4X[25X>[125X [27XTransformation( [ 3, 3, 4, 5, 6, 2, 7, 8 ] ),[127X[104X
    [4X[25X>[125X [27XTransformation([ 1, 2, 5, 3, 6, 8, 4, 4 ] ));;[127X[104X
    [4X[25Xgap>[125X [27XIsInverseSubsemigroup(FullTransformationSemigroup(8), T);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  
  [1X51.4 [33X[0;0YProperties of Semigroups[133X[101X
  
  [33X[0;0YThe following functions determine information about semigroups.[133X
  
  [1X51.4-1 IsRegularSemigroup[101X
  
  [29X[2XIsRegularSemigroup[102X( [3XS[103X ) [32X property
  
  [33X[0;0Yreturns [9Xtrue[109X if [3XS[103X is regular, i.e., if every [13XD[113X-class of [3XS[103X is regular.[133X
  
  [1X51.4-2 IsRegularSemigroupElement[101X
  
  [29X[2XIsRegularSemigroupElement[102X( [3XS[103X, [3Xx[103X ) [32X operation
  
  [33X[0;0Yreturns  [9Xtrue[109X if [3Xx[103X has a general inverse in [3XS[103X, i.e., there is an element [22Xy ∈
  [3XS[103X[122X such that [22X[3Xx[103X y [3Xx[103X = [3Xx[103X[122X and [22Xy [3Xx[103X y = y[122X.[133X
  
  [1X51.4-3 InversesOfSemigroupElement[101X
  
  [29X[2XInversesOfSemigroupElement[102X( [3XS[103X, [3Xx[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10YThe inverses of an element of a semigroup.[133X
  
  [33X[0;0Y[10XInversesOfSemigroupElement[110X  returns  a list of the inverses of the element [3Xx[103X
  in the semigroup [3XS[103X.[133X
  
  [33X[0;0YAn  element  [3Xy[103X in [3XS[103X is an [13Xinverse[113X of [3Xx[103X if [10X[3Xx[103X[10X*y*[3Xx[103X[10X=[3Xx[103X[10X[110X and [10Xy*[3Xx[103X[10X*y=y[110X. The element [3Xx[103X
  has an inverse if and only if [3Xx[103X is a regular element of [3XS[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS:=Semigroup([ Transformation( [ 3, 1, 4, 2, 5, 2, 1, 6, 1 ] ), [127X[104X
    [4X[25X>[125X [27X Transformation( [ 5, 7, 8, 8, 7, 5, 9, 1, 9 ] ), [127X[104X
    [4X[25X>[125X [27X Transformation( [ 7, 6, 2, 8, 4, 7, 5, 8, 3 ] ) ]);;[127X[104X
    [4X[25Xgap>[125X [27Xx:=Transformation( [ 3, 1, 4, 2, 5, 2, 1, 6, 1 ] );;[127X[104X
    [4X[25Xgap>[125X [27XInversesOfSemigroupElement(S, x);[127X[104X
    [4X[28X[  ][128X[104X
    [4X[25Xgap>[125X [27XIsRegularSemigroupElement(S, x);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xx:=Transformation( [ 1, 9, 7, 5, 5, 1, 9, 5, 1 ] );;[127X[104X
    [4X[25Xgap>[125X [27XSet(InversesOfSemigroupElement(S, x));[127X[104X
    [4X[28X[ Transformation( [ 1, 2, 3, 5, 5, 1, 3, 5, 2 ] ), [128X[104X
    [4X[28X  Transformation( [ 1, 5, 1, 1, 5, 1, 3, 1, 2 ] ), [128X[104X
    [4X[28X  Transformation( [ 1, 5, 1, 2, 5, 1, 3, 2, 2 ] ) ][128X[104X
    [4X[25Xgap>[125X [27XIsRegularSemigroupElement(S, x);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XS:=ReesZeroMatrixSemigroup(Group((1,2,3)), [127X[104X
    [4X[25X>[125X [27X[ [ (), () ], [ (), 0 ], [ (), (1,2,3) ] ]);;[127X[104X
    [4X[25Xgap>[125X [27Xx:=ReesZeroMatrixSemigroupElement(S, 2, (1,2,3), 3);;[127X[104X
    [4X[25Xgap>[125X [27XInversesOfSemigroupElement(S, x);[127X[104X
    [4X[28X[ (1,(1,2,3),3), (1,(1,3,2),1), (2,(),3), (2,(1,2,3),1) ][128X[104X
  [4X[32X[104X
  
  [1X51.4-4 IsSimpleSemigroup[101X
  
  [29X[2XIsSimpleSemigroup[102X( [3XS[103X ) [32X property
  
  [33X[0;0Yis [9Xtrue[109X if and only if the semigroup [3XS[103X has no proper ideals.[133X
  
  [1X51.4-5 IsZeroSimpleSemigroup[101X
  
  [29X[2XIsZeroSimpleSemigroup[102X( [3XS[103X ) [32X property
  
  [33X[0;0Yis  [9Xtrue[109X  if  and  only  if the semigroup has no proper ideals except for 0,
  where  [3XS[103X  is a semigroup with zero. If the semigroup does not find its zero,
  then a break-loop is entered.[133X
  
  [1X51.4-6 IsZeroGroup[101X
  
  [29X[2XIsZeroGroup[102X( [3XS[103X ) [32X property
  
  [33X[0;0Yis [9Xtrue[109X if and only if the semigroup [3XS[103X is a group with zero adjoined.[133X
  
  [1X51.4-7 IsReesCongruenceSemigroup[101X
  
  [29X[2XIsReesCongruenceSemigroup[102X( [3XS[103X ) [32X property
  
  [33X[0;0Yreturns  [9Xtrue[109X  if  [3XS[103X  is  a  Rees  Congruence  semigroup,  that  is,  if all
  congruences of [3XS[103X are Rees Congruences.[133X
  
  [1X51.4-8 IsInverseSemigroup[101X
  
  [29X[2XIsInverseSemigroup[102X( [3XS[103X ) [32X property
  [29X[2XIsInverseMonoid[102X( [3XS[103X ) [32X Category
  [6XReturns:[106X  [33X[0;10Y[9Xtrue[109X or [9Xfalse[109X.[133X
  
  [33X[0;0YA  semigroup  is  an  [13Xinverse  semigroup[113X  if  every  element  [10Xx[110X has a unique
  semigroup  inverse,  that  is,  a  unique  element  [10Xy[110X  such that [10Xx*y*x=x[110X and
  [10Xy*x*y=y[110X.[133X
  
  [33X[0;0YA  monoid  that  happens  to  be  an  inverse semigroup is called an [13Xinverse
  monoid[113X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS:=Semigroup( Transformation( [ 1, 2, 4, 5, 6, 3, 7, 8 ] ),[127X[104X
    [4X[25X>[125X [27XTransformation( [ 3, 3, 4, 5, 6, 2, 7, 8 ] ),[127X[104X
    [4X[25X>[125X [27XTransformation( [ 1, 2, 5, 3, 6, 8, 4, 4 ] ) );;[127X[104X
    [4X[25Xgap>[125X [27XIsInverseSemigroup(S);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  
  [1X51.5 [33X[0;0YIdeals of semigroups[133X[101X
  
  [33X[0;0YIdeals of semigroups are the same as ideals of the semigroup when considered
  as a magma. For documentation on ideals for magmas, see [2XMagma[102X ([14X35.2-1[114X).[133X
  
  [1X51.5-1 SemigroupIdealByGenerators[101X
  
  [29X[2XSemigroupIdealByGenerators[102X( [3XS[103X, [3Xgens[103X ) [32X operation
  
  [33X[0;0Y[3XS[103X  is  a  semigroup,  [3Xgens[103X is a list of elements of [3XS[103X. Returns the two-sided
  ideal of [3XS[103X generated by [3Xgens[103X.[133X
  
  [1X51.5-2 ReesCongruenceOfSemigroupIdeal[101X
  
  [29X[2XReesCongruenceOfSemigroupIdeal[102X( [3XI[103X ) [32X attribute
  
  [33X[0;0YA  two sided ideal [3XI[103X of a semigroup [3XS[103X defines a congruence on [3XS[103X given by [22X∆ ∪
  I × I[122X.[133X
  
  [1X51.5-3 IsLeftSemigroupIdeal[101X
  
  [29X[2XIsLeftSemigroupIdeal[102X( [3XI[103X ) [32X property
  [29X[2XIsRightSemigroupIdeal[102X( [3XI[103X ) [32X property
  [29X[2XIsSemigroupIdeal[102X( [3XI[103X ) [32X property
  
  [33X[0;0YCategories of semigroup ideals.[133X
  
  
  [1X51.6 [33X[0;0YCongruences for semigroups[133X[101X
  
  [33X[0;0YAn  equivalence  or  a  congruence  on  a  semigroup  is  the equivalence or
  congruence  on  the  semigroup  considered  as  a  magma.  So,  to deal with
  equivalences  and  congruences  on semigroups, magma functions are used. For
  documentation   on  equivalences  and  congruences  for  magmas,  see  [2XMagma[102X
  ([14X35.2-1[114X).[133X
  
  [1X51.6-1 IsSemigroupCongruence[101X
  
  [29X[2XIsSemigroupCongruence[102X( [3Xc[103X ) [32X property
  
  [33X[0;0Ya magma congruence [3Xc[103X on a semigroup.[133X
  
  [1X51.6-2 IsReesCongruence[101X
  
  [29X[2XIsReesCongruence[102X( [3Xc[103X ) [32X property
  
  [33X[0;0Yreturns  [9Xtrue[109X  if  and only if the congruence [3Xc[103X has at most one nonsingleton
  congruence class.[133X
  
  
  [1X51.7 [33X[0;0YQuotients[133X[101X
  
  [33X[0;0YGiven a semigroup and a congruence on the semigroup, one can construct a new
  semigroup:  the  quotient  semigroup.  The  following  functions  deal  with
  quotient  semigroups  in  [5XGAP[105X.  For  a  semigroup  [22XS[122X, elements of a quotient
  semigroup     are     equivalence     classes    of    elements    of    the
  [2XQuotientSemigroupPreimage[102X  ([14X51.7-3[114X)  value under the congruence given by the
  value of [2XQuotientSemigroupCongruence[102X ([14X51.7-3[114X).[133X
  
  [33X[0;0YIt  is  probably most useful for calculating the elements of the equivalence
  classes  by using [2XElements[102X ([14X30.3-11[114X) or by looking at the images of elements
  of   [2XQuotientSemigroupPreimage[102X   ([14X51.7-3[114X)   under   the   map   returned  by
  [2XQuotientSemigroupHomomorphism[102X       ([14X51.7-3[114X),       which      maps      the
  [2XQuotientSemigroupPreimage[102X ([14X51.7-3[114X) value to [3XS[103X.[133X
  
  [33X[0;0YFor   intensive  computations  in  a  quotient  semigroup,  it  is  probably
  worthwhile finding another representation as the equality test could involve
  enumeration of the elements of the congruence classes being compared.[133X
  
  [1X51.7-1 IsQuotientSemigroup[101X
  
  [29X[2XIsQuotientSemigroup[102X( [3XS[103X ) [32X Category
  
  [33X[0;0Yis  the  category  of  semigroups  constructed  from another semigroup and a
  congruence on it.[133X
  
  [1X51.7-2 HomomorphismQuotientSemigroup[101X
  
  [29X[2XHomomorphismQuotientSemigroup[102X( [3Xcong[103X ) [32X function
  
  [33X[0;0Yfor  a congruence [3Xcong[103X and a semigroup [3XS[103X. Returns the homomorphism from [3XS[103X to
  the quotient of [3XS[103X by [3Xcong[103X.[133X
  
  [1X51.7-3 QuotientSemigroupPreimage[101X
  
  [29X[2XQuotientSemigroupPreimage[102X( [3XS[103X ) [32X attribute
  [29X[2XQuotientSemigroupCongruence[102X( [3XS[103X ) [32X attribute
  [29X[2XQuotientSemigroupHomomorphism[102X( [3XS[103X ) [32X attribute
  
  [33X[0;0Yfor a quotient semigroup [3XS[103X.[133X
  
  
  [1X51.8 [33X[0;0YGreen's Relations[133X[101X
  
  [33X[0;0YGreen's  equivalence  relations  play  a  very  important  role in semigroup
  theory. In this section we describe how they can be used in [5XGAP[105X.[133X
  
  [33X[0;0YThe  five  Green's  relations  are  [22XR[122X,  [22XL[122X, [22XJ[122X, [22XH[122X, [22XD[122X: two elements [22Xx[122X, [22Xy[122X from a
  semigroup  [22XS[122X are [22XR[122X-related if and only if [22XxS^1 = yS^1[122X, [22XL[122X-related if and only
  if  [22XS^1 x = S^1 y[122X and [22XJ[122X-related if and only if [22XS^1 xS^1 = S^1 yS^1[122X; finally,
  [22XH = R ∧ L[122X, and [22XD = R ∘ L[122X.[133X
  
  [33X[0;0YRecall  that  relations [22XR[122X, [22XL[122X and [22XJ[122X induce a partial order among the elements
  of the semigroup [22XS[122X: for two elements [22Xx[122X, [22Xy[122X from [22XS[122X, we say that [22Xx[122X is less than
  or equal to [22Xy[122X in the order on [22XR[122X if [22XxS^1 ⊆ yS^1[122X; similarly, [22Xx[122X is less than or
  equal  to  [22Xy[122X  under  [22XL[122X  if [22XS^1x ⊆ S^1y[122X; finally [22Xx[122X is less than or equal to [22Xy[122X
  under  [22XJ[122X  if [22XS^1 xS^1 ⊆ S^1 tS^1[122X. We extend this preorder to a partial order
  on equivalence classes in the natural way.[133X
  
  [1X51.8-1 GreensRRelation[101X
  
  [29X[2XGreensRRelation[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensLRelation[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensJRelation[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensDRelation[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensHRelation[102X( [3Xsemigroup[103X ) [32X attribute
  
  [33X[0;0YThe  Green's  relations  (which are equivalence relations) are attributes of
  the semigroup [3Xsemigroup[103X.[133X
  
  [1X51.8-2 IsGreensRelation[101X
  
  [29X[2XIsGreensRelation[102X( [3Xbin-relation[103X ) [32X filter
  [29X[2XIsGreensRRelation[102X( [3Xequiv-relation[103X ) [32X filter
  [29X[2XIsGreensLRelation[102X( [3Xequiv-relation[103X ) [32X filter
  [29X[2XIsGreensJRelation[102X( [3Xequiv-relation[103X ) [32X filter
  [29X[2XIsGreensHRelation[102X( [3Xequiv-relation[103X ) [32X filter
  [29X[2XIsGreensDRelation[102X( [3Xequiv-relation[103X ) [32X filter
  
  [33X[0;0YCategories for the Green's relations.[133X
  
  [1X51.8-3 IsGreensClass[101X
  
  [29X[2XIsGreensClass[102X( [3Xequiv-class[103X ) [32X filter
  [29X[2XIsGreensRClass[102X( [3Xequiv-class[103X ) [32X filter
  [29X[2XIsGreensLClass[102X( [3Xequiv-class[103X ) [32X filter
  [29X[2XIsGreensJClass[102X( [3Xequiv-class[103X ) [32X filter
  [29X[2XIsGreensHClass[102X( [3Xequiv-class[103X ) [32X filter
  [29X[2XIsGreensDClass[102X( [3Xequiv-class[103X ) [32X filter
  
  [33X[0;0Yreturn  [9Xtrue[109X  if the equivalence class [3Xequiv-class[103X is a Green's class of any
  type, or of [22XR[122X, [22XL[122X, [22XJ[122X, [22XH[122X, [22XD[122X type, respectively, or [9Xfalse[109X otherwise.[133X
  
  [1X51.8-4 IsGreensLessThanOrEqual[101X
  
  [29X[2XIsGreensLessThanOrEqual[102X( [3XC1[103X, [3XC2[103X ) [32X operation
  
  [33X[0;0Yreturns  [9Xtrue[109X  if the Green's class [3XC1[103X is less than or equal to [3XC2[103X under the
  respective ordering (as defined above), and [9Xfalse[109X otherwise.[133X
  
  [33X[0;0YOnly defined for [22XR[122X, [22XL[122X and [22XJ[122X classes.[133X
  
  [1X51.8-5 RClassOfHClass[101X
  
  [29X[2XRClassOfHClass[102X( [3XH[103X ) [32X attribute
  [29X[2XLClassOfHClass[102X( [3XH[103X ) [32X attribute
  
  [33X[0;0Yare  attributes  reflecting  the  natural  ordering over the various Green's
  classes.  [2XRClassOfHClass[102X  and  [2XLClassOfHClass[102X  return  the  [22XR[122X and [22XL[122X classes,
  respectively, in which an [22XH[122X class is contained.[133X
  
  [1X51.8-6 EggBoxOfDClass[101X
  
  [29X[2XEggBoxOfDClass[102X( [3XDclass[103X ) [32X attribute
  
  [33X[0;0Yreturns for a Green's [22XD[122X class [3XDclass[103X a matrix whose rows represent [22XR[122X classes
  and columns represent [22XL[122X classes. The entries are the [22XH[122X classes.[133X
  
  [1X51.8-7 DisplayEggBoxOfDClass[101X
  
  [29X[2XDisplayEggBoxOfDClass[102X( [3XDclass[103X ) [32X function
  
  [33X[0;0Ydisplays  a  [21Xpicture[121X  of  the  [22XD[122X class [3XDclass[103X, as an array of 1s and 0s. A 1
  represents a group [22XH[122X class.[133X
  
  [1X51.8-8 GreensRClassOfElement[101X
  
  [29X[2XGreensRClassOfElement[102X( [3XS[103X, [3Xa[103X ) [32X operation
  [29X[2XGreensLClassOfElement[102X( [3XS[103X, [3Xa[103X ) [32X operation
  [29X[2XGreensDClassOfElement[102X( [3XS[103X, [3Xa[103X ) [32X operation
  [29X[2XGreensJClassOfElement[102X( [3XS[103X, [3Xa[103X ) [32X operation
  [29X[2XGreensHClassOfElement[102X( [3XS[103X, [3Xa[103X ) [32X operation
  
  [33X[0;0YCreates the [22XX[122X class of the element [3Xa[103X in the semigroup [3XS[103X where [22XX[122X is one of [22XL[122X,
  [22XR[122X, [22XD[122X, [22XJ[122X, or [22XH[122X.[133X
  
  [1X51.8-9 GreensRClasses[101X
  
  [29X[2XGreensRClasses[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensLClasses[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensJClasses[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensDClasses[102X( [3Xsemigroup[103X ) [32X attribute
  [29X[2XGreensHClasses[102X( [3Xsemigroup[103X ) [32X attribute
  
  [33X[0;0Yreturn  the  [22XR[122X,  [22XL[122X,  [22XJ[122X,  [22XH[122X, or [22XD[122X Green's classes, respectively for semigroup
  [3Xsemigroup[103X. [2XEquivalenceClasses[102X ([14X33.7-3[114X) for a Green's relation lead to one of
  these functions.[133X
  
  [1X51.8-10 GroupHClassOfGreensDClass[101X
  
  [29X[2XGroupHClassOfGreensDClass[102X( [3XDclass[103X ) [32X attribute
  
  [33X[0;0Yfor a [22XD[122X class [3XDclass[103X of a semigroup, returns a group [22XH[122X class of the [22XD[122X class,
  or [9Xfail[109X if there is no group [22XH[122X class.[133X
  
  [1X51.8-11 IsGroupHClass[101X
  
  [29X[2XIsGroupHClass[102X( [3XHclass[103X ) [32X property
  
  [33X[0;0Yreturns [9Xtrue[109X if the Green's [22XH[122X class [3XHclass[103X is a group, which in turn is true
  if and only if [3XHclass[103X[22X^2[122X intersects [3XHclass[103X.[133X
  
  [1X51.8-12 IsRegularDClass[101X
  
  [29X[2XIsRegularDClass[102X( [3XDclass[103X ) [32X property
  
  [33X[0;0Yreturns  [9Xtrue[109X  if the Greens [22XD[122X class [3XDclass[103X is regular. A [22XD[122X class is regular
  if and only if each of its elements is regular, which in turn is true if and
  only  if any one element of [3XDclass[103X is regular. Idempotents are regular since
  [22Xeee  =  e[122X  so  it follows that a Green's [22XD[122X class containing an idempotent is
  regular. Conversely, it is true that a regular [22XD[122X class must contain at least
  one idempotent. (See [How76, Prop. 3.2].)[133X
  
  
  [1X51.9 [33X[0;0YRees Matrix Semigroups[133X[101X
  
  [33X[0;0YIn  this  section,  we  describe  the  functions  in [5XGAP[105X for Rees matrix and
  0-matrix  semigroups  and  their  subsemigroups.  The  importance  of  these
  semigroups  lies  in  the  fact  that Rees matrix semigroups over groups are
  exactly  the completely simple semigroups, and Rees 0-matrix semigroups over
  groups are the completely 0-simple semigroups.[133X
  
  [33X[0;0YLet  [22XI[122X and [22XJ[122X be sets, let [22XS[122X be a semigroup, and let [22XP=(p_ji)_j∈ J, i∈ I[122X be a
  [22X|J|×  |I|[122X  matrix  with  entries  in  [22XS[122X. Then the [13XRees matrix semigroup[113X with
  underlying semigroup [22XS[122X and matrix [22XP[122X is just the direct product [22XI× S × J[122X with
  multiplication defined by[133X
  
  
        [33X[1;6Y[24X[33X[0;0Y(i, s, j)(k, t, l)=(i,s\cdot p_{j,k}\cdot t, l).[133X [124X[133X
  
  
  [33X[0;0YRees  0-matrix  semigroups  are defined as follows. If [22XI[122X, [22XJ[122X, [22XS[122X, and [22XP[122X are as
  above  and  [22X0[122X  denotes  a new element, then the [13XRees 0-matrix semigroup[113X with
  underlying  semigroup  [22XS[122X  and matrix [22XP[122X is [22X(I× S× J)∪ {0}[122X with multiplication
  defined by[133X
  
  
        [33X[1;6Y[24X[33X[0;0Y(i, s, j)(k, t, l)=(i, s\cdot p_{j,k}\cdot t, l)[133X [124X[133X
  
  
  [33X[0;0Ywhen [22Xp_j,k[122X is not [22X0[122X and [22X0[122X if [22Xp_j,k[122X is 0.[133X
  
  [33X[0;0YIf [22XR[122X is a Rees matrix or 0-matrix semigroup, then the [13Xrows[113X of [22XR[122X is the index
  set  [22XI[122X,  the  [13Xcolumns[113X  of  [22XR[122X  is  the  index  set  [22XJ[122X, the semigroup [22XS[122X is the
  [13Xunderlying semigroup[113X of [22XR[122X, and the [13Xmatrix[113X [22XP[122X is the matrix of [22XS[122X.[133X
  
  [33X[0;0YThoroughout  this  section, wherever the distinction is unimportant, we will
  refer  to  Rees  matrix  or  0-matrix semigroups collectively as Rees matrix
  semigroups.[133X
  
  [33X[0;0YMultiplication  of  elements of a Rees matrix semigroup obviously depends on
  the  matrix  used  to  create the semigroup. Hence elements of a Rees matrix
  semigroup  can only be created with reference to the semigroup to which they
  belong.  More  specifically,  every  collection  or semigroup of Rees matrix
  semigroup  elements  is created from a specific Rees matrix semigroup, which
  contains  the  whole  family  of  its  elements.  So,  it is not possible to
  multiply  or  compare elements belonging to distinct Rees matrix semigroups,
  since  they belong to different families. This situation is similar to, say,
  free  groups,  and different to, say, permutations, which belong to a single
  family,  and  where  arbitrary  permutations  can be compared and multiplied
  without reference to any group containing them.[133X
  
  [33X[0;0YA  subsemigroup  of a Rees matrix semigroup is not necessarily a Rees matrix
  semigroup. Every semigroup consisting of elements of a Rees matrix semigroup
  satisfies the property [2XIsReesMatrixSubsemigroup[102X ([14X51.9-6[114X) and every semigroup
  of  Rees  0-matrix semigroup elements satisfies [2XIsReesZeroMatrixSubsemigroup[102X
  ([14X51.9-6[114X).[133X
  
  [33X[0;0YRees  matrix  and  0-matrix  semigroups  can be created using the operations
  [2XReesMatrixSemigroup[102X    ([14X51.9-1[114X)    and   [2XReesZeroMatrixSemigroup[102X   ([14X51.9-1[114X),
  respectively,  from  an  underlying  semigroup  and  a  matrix.  Rees matrix
  semigroups  created  in this way contain the whole family of their elements.
  Every  element  of  a  Rees  matrix  semigroup belongs to a unique semigroup
  created  in  this  way;  every  subsemigroup of a Rees matrix semigroup is a
  subsemigroup of a unique semigroup created in this way.[133X
  
  [33X[0;0YSubsemigroups  of  Rees  matrix semigroups can also be created by specifying
  generators.  A  subsemigroup  of  a  Rees matrix semigroup [22XI× U× J[122X satisfies
  [2XIsReesMatrixSemigroup[102X  ([14X51.9-7[114X)  if  and  only  if it is equal to [22XI'× U'× J'[122X
  where  [22XI'⊆ I[122X, [22XJ'⊆ J[122X, and [22XU'[122X is a subsemigroup of [22XU[122X. The analogous statements
  holds for Rees 0-matrix semigroups.[133X
  
  [33X[0;0YIt  is  not  necessarily the case that a simple subsemigroups of Rees matrix
  semigroups satisfies [2XIsReesMatrixSemigroup[102X ([14X51.9-7[114X). A Rees matrix semigroup
  is  simple  if  and  only  if  its  underlying semigroup is simple. A finite
  semigroup  is  simple  if  and  only  if  it  is isomorphic to a Rees matrix
  semigroup  over  a  group; this isomorphism can be obtained explicitly using
  [2XIsomorphismReesMatrixSemigroup[102X ([14X51.9-3[114X).[133X
  
  [33X[0;0YSimilarly, 0-simple subsemigroups of Rees 0-matrix semigroups do not have to
  satisfy  [2XIsReesZeroMatrixSemigroup[102X  ([14X51.9-7[114X). A Rees 0-matrix semigroup with
  more  than  2 elements is 0-simple if and only if every row and every column
  of  its  matrix  contains  a non-zero entry, and its underlying semigroup is
  simple.  A finite semigroup is 0-simple if and only if it is isomorphic to a
  Rees 0-matrix semigroup over a group; again this isomorphism can be found by
  using [2XIsomorphismReesZeroMatrixSemigroup[102X ([14X51.9-3[114X).[133X
  
  [33X[0;0YElements  of  a  Rees  matrix or 0-matrix semigroup belong to the categories
  [2XIsReesMatrixSemigroupElement[102X  ([14X51.9-4[114X)  and [2XIsReesZeroMatrixSemigroupElement[102X
  ([14X51.9-4[114X),  respectively.  Such  elements  can  be created directly using the
  functions          [2XReesMatrixSemigroupElement[102X          ([14X51.9-5[114X)          and
  [2XReesZeroMatrixSemigroupElement[102X ([14X51.9-5[114X).[133X
  
  [33X[0;0YA  semigroup  in  [5XGAP[105X can either satisfies [2XIsReesMatrixSemigroup[102X ([14X51.9-7[114X) or
  [2XIsReesZeroMatrixSemigroup[102X ([14X51.9-7[114X) but not both.[133X
  
  [1X51.9-1 ReesMatrixSemigroup[101X
  
  [29X[2XReesMatrixSemigroup[102X( [3XS[103X, [3Xmat[103X ) [32X operation
  [29X[2XReesZeroMatrixSemigroup[102X( [3XS[103X, [3Xmat[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10YA Rees matrix or 0-matrix semigroup.[133X
  
  [33X[0;0YWhen  [3XS[103X  is  a  semigroup and [3Xmat[103X is an [10Xm[110X by [10Xn[110X matrix with entries in [3XS[103X, the
  function [10XReesMatrixSemigroup[110X returns the [10Xn[110X by [10Xm[110X Rees matrix semigroup over [3XS[103X
  with multiplication defined by [3Xmat[103X.[133X
  
  [33X[0;0YThe arguments of [10XReesZeroMatrixSemigroup[110X should be a semigroup [3XS[103X and an [10Xm[110X by
  [10Xn[110X   matrix   [3Xmat[103X   with   entries   in   [3XS[103X   or  equal  to  the  integer  [10X0[110X.
  [10XReesZeroMatrixSemigroup[110X  returns  the  [10Xn[110X by [10Xm[110X Rees 0-matrix semigroup over [3XS[103X
  with  multiplication defined by [3Xmat[103X. In [5XGAP[105X a Rees 0-matrix semigroup always
  contains  a multiplicative zero element, regardless of whether there are any
  entries in [3Xmat[103X which are equal to [10X0[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=Random(AllGroups(Size, 32));;[127X[104X
    [4X[25Xgap>[125X [27Xmat:=List([1..5], x-> List([1..3], y-> Random(G)));;[127X[104X
    [4X[25Xgap>[125X [27XS:=ReesMatrixSemigroup(G, mat);[127X[104X
    [4X[28X<Rees matrix semigroup 3x5 over <pc group of size 32 with [128X[104X
    [4X[28X 5 generators>>[128X[104X
    [4X[25Xgap>[125X [27Xmat:=[[(), 0, (), ()], [0, 0, 0, 0]];;[127X[104X
    [4X[25Xgap>[125X [27XS:=ReesZeroMatrixSemigroup(DihedralGroup(IsPermGroup, 8), mat);[127X[104X
    [4X[28X<Rees 0-matrix semigroup 4x2 over Group([ (1,2,3,4), (2,4) ])>[128X[104X
  [4X[32X[104X
  
  [1X51.9-2 ReesMatrixSubsemigroup[101X
  
  [29X[2XReesMatrixSubsemigroup[102X( [3XR[103X, [3XI[103X, [3XU[103X, [3XJ[103X ) [32X operation
  [29X[2XReesZeroMatrixSubsemigroup[102X( [3XR[103X, [3XI[103X, [3XU[103X, [3XJ[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10YA Rees matrix or 0-matrix subsemigroup.[133X
  
  [33X[0;0YThe arguments of [10XReesMatrixSubsemigroup[110X should be a Rees matrix semigroup [3XR[103X,
  subsets  [3XI[103X  and  [3XJ[103X  of  the  rows  and  columns  of  [3XR[103X,  respectively, and a
  subsemigroup  [3XS[103X  of  the  underlying  semigroup of [3XR[103X. [10XReesMatrixSubsemigroup[110X
  returns  the  subsemigroup of [3XR[103X generated by the direct product of [3XI[103X, [3XU[103X, and
  [3XJ[103X.[133X
  
  [33X[0;0YThe usage and returned value of [10XReesZeroMatrixSubsemigroup[110X is analogous when
  [3XR[103X is a Rees 0-matrix semigroup.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=CyclicGroup(IsPermGroup, 1007);;[127X[104X
    [4X[25Xgap>[125X [27Xmat:=[[(), 0, 0], [0, (), 0], [0, 0, ()], [127X[104X
    [4X[25X>[125X [27X[(), (), ()], [0, 0, ()]];;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesZeroMatrixSemigroup(G, mat);[127X[104X
    [4X[28X<Rees 0-matrix semigroup 3x5 over [128X[104X
    [4X[28X  <permutation group of size 1007 with 1 generators>>[128X[104X
    [4X[25Xgap>[125X [27XReesZeroMatrixSubsemigroup(R, [1,3], G, [1..5]);[127X[104X
    [4X[28X<Rees 0-matrix semigroup 2x5 over [128X[104X
    [4X[28X  <permutation group of size 1007 with 1 generators>>[128X[104X
  [4X[32X[104X
  
  [1X51.9-3 IsomorphismReesMatrixSemigroup[101X
  
  [29X[2XIsomorphismReesMatrixSemigroup[102X( [3XS[103X ) [32X attribute
  [29X[2XIsomorphismReesZeroMatrixSemigroup[102X( [3XS[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10YAn isomorphism.[133X
  
  [33X[0;0YEvery  finite simple semigroup is isomorphic to a Rees matrix semigroup over
  a  group,  and  every  finite  0-simple  semigroup  is  isomorphic to a Rees
  0-matrix semigroup over a group.[133X
  
  [33X[0;0YIf the argument [3XS[103X is a simple semigroup, then [10XIsomorphismReesMatrixSemigroup[110X
  returns  an isomorphism to a Rees matrix semigroup over a group. If [3XS[103X is not
  simple, then [10XIsomorphismReesMatrixSemigroup[110X returns an error.[133X
  
  [33X[0;0YIf     the     argument     [3XS[103X     is     a    0-simple    semigroup,    then
  [10XIsomorphismReesZeroMatrixSemigroup[110X returns an isomorphism to a Rees 0-matrix
  semigroup    over    a    group.    If    [3XS[103X    is    not    0-simple,   then
  [10XIsomorphismReesMatrixSemigroup[110X returns an error.[133X
  
  [33X[0;0YSee [2XIsSimpleSemigroup[102X ([14X51.4-4[114X) and [2XIsZeroSimpleSemigroup[102X ([14X51.4-5[114X).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS := Semigroup(Transformation([2, 1, 1, 2, 1]), [127X[104X
    [4X[25X>[125X [27X                  Transformation([3, 4, 3, 4, 4]), [127X[104X
    [4X[25X>[125X [27X                  Transformation([3, 4, 3, 4, 3]),  [127X[104X
    [4X[25X>[125X [27X                  Transformation([4, 3, 3, 4, 4]));;[127X[104X
    [4X[25Xgap>[125X [27XIsSimpleSemigroup(S);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XRange(IsomorphismReesMatrixSemigroup(S));[127X[104X
    [4X[28X<Rees matrix semigroup 4x2 over Group([ (1,2) ])>[128X[104X
    [4X[25Xgap>[125X [27Xmat := [[(), 0, 0], [127X[104X
    [4X[25X>[125X [27X           [0, (), 0], [127X[104X
    [4X[25X>[125X [27X           [0, 0, ()]];;[127X[104X
    [4X[25Xgap>[125X [27XR := ReesZeroMatrixSemigroup(Group((1,2,4,5,6)), mat);[127X[104X
    [4X[28X<Rees 0-matrix semigroup 3x3 over Group([ (1,2,4,5,6) ])>[128X[104X
    [4X[25Xgap>[125X [27XU := ReesZeroMatrixSubsemigroup(R, [1, 2], Group(()), [2, 3]);[127X[104X
    [4X[28X<subsemigroup of 3x3 Rees 0-matrix semigroup with 4 generators>[128X[104X
    [4X[25Xgap>[125X [27XIsZeroSimpleSemigroup(U);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XU := ReesZeroMatrixSubsemigroup(R, [2, 3], Group(()), [2, 3]);[127X[104X
    [4X[28X<subsemigroup of 3x3 Rees 0-matrix semigroup with 3 generators>[128X[104X
    [4X[25Xgap>[125X [27XIsZeroSimpleSemigroup(U);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XRows(U); Columns(U);[127X[104X
    [4X[28X[ 2, 3 ][128X[104X
    [4X[28X[ 2, 3 ][128X[104X
    [4X[25Xgap>[125X [27XV := Range(IsomorphismReesZeroMatrixSemigroup(U));[127X[104X
    [4X[28X<Rees 0-matrix semigroup 2x2 over Group(())>[128X[104X
    [4X[25Xgap>[125X [27XRows(V); Columns(V); [127X[104X
    [4X[28X[ 1, 2 ][128X[104X
    [4X[28X[ 1, 2 ][128X[104X
  [4X[32X[104X
  
  [1X51.9-4 IsReesMatrixSemigroupElement[101X
  
  [29X[2XIsReesMatrixSemigroupElement[102X( [3Xelt[103X ) [32X Category
  [29X[2XIsReesZeroMatrixSemigroupElement[102X( [3Xelt[103X ) [32X Category
  [6XReturns:[106X  [33X[0;10Y[9Xtrue[109X or [9Xfalse[109X.[133X
  
  [33X[0;0YEvery   element   of  a  Rees  matrix  semigroup  belongs  to  the  category
  [10XIsReesMatrixSemigroupElement[110X, and every element of a Rees 0-matrix semigroup
  belongs to the category [10XIsReesZeroMatrixSemigroupElement[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=Group((1,2,3));;[127X[104X
    [4X[25Xgap>[125X [27Xmat:=[ [ (), (1,3,2) ], [ (1,3,2), () ] ];;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesMatrixSemigroup(G, mat);[127X[104X
    [4X[28X<Rees matrix semigroup 2x2 over Group([ (1,2,3) ])>[128X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfSemigroup(R);[127X[104X
    [4X[28X[ (1,(1,2,3),1), (2,(),2) ][128X[104X
    [4X[25Xgap>[125X [27XIsReesMatrixSemigroupElement(last[1]);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsReesZeroMatrixSemigroupElement(last2[1]);[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [1X51.9-5 ReesMatrixSemigroupElement[101X
  
  [29X[2XReesMatrixSemigroupElement[102X( [3XR[103X, [3Xi[103X, [3Xx[103X, [3Xj[103X ) [32X function
  [29X[2XReesZeroMatrixSemigroupElement[102X( [3XR[103X, [3Xi[103X, [3Xx[103X, [3Xj[103X ) [32X function
  [6XReturns:[106X  [33X[0;10YAn element of a Rees matrix or [10X0[110X-matrix semigroup.[133X
  
  [33X[0;0YThe   arguments  of  [3XReesMatrixSemigroupElement[103X  should  be  a  Rees  matrix
  subsemigroup  [3XR[103X,  elements  [3Xi[103X  and  [3Xj[103X  of  the  the  rows  and columns of [3XR[103X,
  respectively,   and   an  element  [3Xx[103X  of  the  underlying  semigroup  of  [3XR[103X.
  [10XReesMatrixSemigroupElement[110X  returns  the  element  of  [3XR[103X  with  row index [3Xi[103X,
  underlying  element  [3Xx[103X in the underlying semigroup of [3XR[103X, and column index [3Xj[103X,
  if such an element exist, if such an element exists.[133X
  
  [33X[0;0YThe   usage  of  [10XReesZeroMatrixSemigroupElement[110X  is  analogous  to  that  of
  [10XReesMatrixSemigroupElement[110X, when [3XR[103X is a Rees 0-matrix semigroup.[133X
  
  [33X[0;0YThe  row  [3Xi[103X,  underlying  element  [3Xx[103X, and column [3Xj[103X of an element [10Xy[110X of a Rees
  matrix (or 0-matrix) semigroup can be recovered from [10Xy[110X using [10Xy[1][110X, [10Xy[2][110X, and
  [10Xy[3][110X, respectively.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=Group((1,2,3));;[127X[104X
    [4X[25Xgap>[125X [27Xmat:=[ [ 0, () ], [ (1,3,2), (1,3,2) ] ];;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesZeroMatrixSemigroup(G, mat);[127X[104X
    [4X[28X<Rees 0-matrix semigroup 2x2 over Group([ (1,2,3) ])>[128X[104X
    [4X[25Xgap>[125X [27XReesZeroMatrixSemigroupElement(R, 1, (1,2,3), 2);[127X[104X
    [4X[28X(1,(1,2,3),2)[128X[104X
    [4X[25Xgap>[125X [27XMultiplicativeZero(R);[127X[104X
    [4X[28X0[128X[104X
  [4X[32X[104X
  
  [1X51.9-6 IsReesMatrixSubsemigroup[101X
  
  [29X[2XIsReesMatrixSubsemigroup[102X( [3XR[103X ) [32X Synonym
  [29X[2XIsReesZeroMatrixSubsemigroup[102X( [3XR[103X ) [32X Synonym
  [6XReturns:[106X  [33X[0;10Y[9Xtrue[109X or [9Xfalse[109X.[133X
  
  [33X[0;0YEvery  semigroup consisting of elements of a Rees matrix semigroup satisfies
  the  property  [2XIsReesMatrixSubsemigroup[102X and every semigroup of Rees 0-matrix
  semigroup elements satisfies [2XIsReesZeroMatrixSubsemigroup[102X.[133X
  
  [33X[0;0YNote  that  a  subsemigroup  of a Rees matrix semigroup is not necessarily a
  Rees matrix semigroup.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=DihedralGroup(32);;[127X[104X
    [4X[25Xgap>[125X [27Xmat:=List([1..2], x-> List([1..10], x-> Random(G)));;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesMatrixSemigroup(G, mat);[127X[104X
    [4X[28X<Rees matrix semigroup 10x2 over <pc group of size 32 with [128X[104X
    [4X[28X 5 generators>>[128X[104X
    [4X[25Xgap>[125X [27XS:=Semigroup(GeneratorsOfSemigroup(R));      [127X[104X
    [4X[28X<subsemigroup of 10x2 Rees matrix semigroup with 14 generators>[128X[104X
    [4X[25Xgap>[125X [27XIsReesMatrixSubsemigroup(S); [127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XS:=Semigroup(GeneratorsOfSemigroup(R)[1]);[127X[104X
    [4X[28X<subsemigroup of 10x2 Rees matrix semigroup with 1 generator>[128X[104X
    [4X[25Xgap>[125X [27XIsReesMatrixSubsemigroup(S);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [1X51.9-7 IsReesMatrixSemigroup[101X
  
  [29X[2XIsReesMatrixSemigroup[102X( [3XR[103X ) [32X property
  [29X[2XIsReesZeroMatrixSemigroup[102X( [3XR[103X ) [32X property
  [6XReturns:[106X  [33X[0;10Y[9Xtrue[109X or [9Xfalse[109X.[133X
  
  [33X[0;0YA   subsemigroup   of   a   Rees   matrix   semigroup   [22XI×  U×  J[122X  satisfies
  [2XIsReesMatrixSemigroup[102X  if and only if it is equal to [22XI'× U'× J'[122X where [22XI'⊆ I[122X,
  [22XJ'⊆  J[122X,  and  [22XU'[122X  is  a  subsemigroup of [22XU[122X. It can be costly to check that a
  subsemigroup  defined  by  generators  satisfies  [10XIsReesMatrixSemigroup[110X. The
  analogous statements holds for Rees 0-matrix semigroups.[133X
  
  [33X[0;0YIt  is  not  necessarily the case that a simple subsemigroups of Rees matrix
  semigroups  satisfies  [2XIsReesMatrixSemigroup[102X.  A  Rees  matrix  semigroup is
  simple if and only if its underlying semigroup is simple. A finite semigroup
  is  simple if and only if it is isomorphic to a Rees matrix semigroup over a
  group;    this    isomorphism    can    be    obtained    explicitly   using
  [2XIsomorphismReesMatrixSemigroup[102X ([14X51.9-3[114X).[133X
  
  [33X[0;0YSimilarly, 0-simple subsemigroups of Rees 0-matrix semigroups do not have to
  satisfy  [2XIsReesZeroMatrixSemigroup[102X. A Rees 0-matrix semigroup with more than
  2  elements  is  0-simple  if  and only if every row and every column of its
  matrix  contains a non-zero entry, and its underlying semigroup is simple. A
  finite  semigroup  is  0-simple  if  and  only if it is isomorphic to a Rees
  0-matrix  semigroup  over  a  group;  again this isomorphism can be found by
  using [2XIsomorphismReesMatrixSemigroup[102X ([14X51.9-3[114X).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=PSL(2,5);;[127X[104X
    [4X[25Xgap>[125X [27Xmat:=[ [ 0, (), 0, (2,6,3,5,4) ], [127X[104X
    [4X[25X>[125X [27X[ (), 0, (), 0 ], [ 0, 0, 0, () ] ];;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesZeroMatrixSemigroup(G, mat);[127X[104X
    [4X[28X<Rees 0-matrix semigroup 4x3 over Group([ (3,5)(4,6), (1,2,5)[128X[104X
    [4X[28X(3,4,6) ])>[128X[104X
    [4X[25Xgap>[125X [27XIsReesZeroMatrixSemigroup(R);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XU:=ReesZeroMatrixSubsemigroup(R, [1..3], Group(()), [1..2]);[127X[104X
    [4X[28X<subsemigroup of 4x3 Rees 0-matrix semigroup with 4 generators>[128X[104X
    [4X[25Xgap>[125X [27XIsReesZeroMatrixSemigroup(U);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XV:=Semigroup(GeneratorsOfSemigroup(U));[127X[104X
    [4X[28X<subsemigroup of 4x3 Rees 0-matrix semigroup with 4 generators>[128X[104X
    [4X[25Xgap>[125X [27XIsReesZeroMatrixSemigroup(V);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XS:=Semigroup(Transformation([1,1]), Transformation([1,2]));[127X[104X
    [4X[28X<commutative transformation monoid of degree 2 with 1 generator>[128X[104X
    [4X[25Xgap>[125X [27XIsSimpleSemigroup(S);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xmat:=[[0, One(S), 0, One(S)], [One(S), 0, One(S), 0], [127X[104X
    [4X[25X>[125X [27X[0, 0, 0, One(S)]];;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesZeroMatrixSemigroup(S, mat);;[127X[104X
    [4X[25Xgap>[125X [27XU:=ReesZeroMatrixSubsemigroup(R, [1..3], [127X[104X
    [4X[25X>[125X [27XSemigroup(Transformation([1,1])), [1..2]);[127X[104X
    [4X[28X<subsemigroup of 4x3 Rees 0-matrix semigroup with 6 generators>[128X[104X
    [4X[25Xgap>[125X [27XV:=Semigroup(GeneratorsOfSemigroup(U));[127X[104X
    [4X[28X<subsemigroup of 4x3 Rees 0-matrix semigroup with 6 generators>[128X[104X
    [4X[25Xgap>[125X [27XIsReesZeroMatrixSemigroup(V);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XT:=Semigroup([127X[104X
    [4X[25X>[125X [27XReesZeroMatrixSemigroupElement(R, 3, Transformation( [ 1, 1 ] ), 3), [127X[104X
    [4X[25X>[125X [27XReesZeroMatrixSemigroupElement(R, 2, Transformation( [ 1, 1 ] ), 2));[127X[104X
    [4X[28X<subsemigroup of 4x3 Rees 0-matrix semigroup with 2 generators>[128X[104X
    [4X[25Xgap>[125X [27XIsReesZeroMatrixSemigroup(T);[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [1X51.9-8 Matrix[101X
  
  [29X[2XMatrix[102X( [3XR[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10YA matrix.[133X
  
  [33X[0;0YIf  [3XR[103X is a Rees matrix or 0-matrix semigroup, then [10XMatrix[110X returns the matrix
  used to define multiplication in [3XR[103X.[133X
  
  [33X[0;0YMore  specifically,  if [3XR[103X is a Rees matrix or 0-matrix semigroup, which is a
  proper  subsemigroup  of  another  such  semigroup,  then [10XMatrix[110X returns the
  matrix  used to define the Rees matrix (or 0-matrix) semigroup consisting of
  the  whole  family to which the elements of [3XR[103X belong. Thus, for example, a [10X1[110X
  by [10X1[110X Rees matrix semigroup can have a [10X65[110X by [10X15[110X matrix.[133X
  
  [33X[0;0YArbitrary  subsemigroups of Rees matrix or 0-matrix semigroups do not have a
  matrix.  Such  a  subsemigroup  [3XR[103X  has  a matrix if and only if it satisfies
  [2XIsReesMatrixSemigroup[102X ([14X51.9-7[114X) or [2XIsReesZeroMatrixSemigroup[102X ([14X51.9-7[114X).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=AlternatingGroup(5);;[127X[104X
    [4X[25Xgap>[125X [27Xmat:=[[(), (), ()], [(), (), ()]];;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesMatrixSemigroup(G, mat);[127X[104X
    [4X[28X<Rees matrix semigroup 3x2 over Alt( [ 1 .. 5 ] )>[128X[104X
    [4X[25Xgap>[125X [27XMatrix(R); [127X[104X
    [4X[28X[ [ (), (), () ], [ (), (), () ] ][128X[104X
    [4X[25Xgap>[125X [27XR:=ReesMatrixSubsemigroup(R, [1,2], Group(()), [2]);[127X[104X
    [4X[28X<subsemigroup of 3x2 Rees matrix semigroup with 2 generators>[128X[104X
    [4X[25Xgap>[125X [27XMatrix(R);[127X[104X
    [4X[28X[ [ (), (), () ], [ (), (), () ] ][128X[104X
  [4X[32X[104X
  
  
  [1X51.9-9 [33X[0;0YRows and columns[133X[101X
  
  [29X[2XRows[102X( [3XR[103X ) [32X attribute
  [29X[2XColumns[102X( [3XR[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10YThe rows or columns of [3XR[103X.[133X
  
  [33X[0;0Y[10XRows[110X  returns the rows of the Rees matrix or 0-matrix semigroup [3XR[103X. Note that
  the  rows  of  the semigroup correspond to the columns of the matrix used to
  define multiplication in [3XR[103X.[133X
  
  [33X[0;0Y[10XColumns[110X returns the columns of the Rees matrix or 0-matrix semigroup [3XR[103X. Note
  that  the columns of the semigroup correspond to the rows of the matrix used
  to define multiplication in [3XR[103X.[133X
  
  [33X[0;0YArbitrary  subsemigroups  of  Rees matrix or 0-matrix semigroups do not have
  rows  or  columns. Such a subsemigroup [3XR[103X has rows and columns if and only if
  it  satisfies  [2XIsReesMatrixSemigroup[102X  ([14X51.9-7[114X)  or [2XIsReesZeroMatrixSemigroup[102X
  ([14X51.9-7[114X).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XG:=Group((1,2,3));;                      [127X[104X
    [4X[25Xgap>[125X [27Xmat:=List([1..100], x-> List([1..200], x->Random(G)));;[127X[104X
    [4X[25Xgap>[125X [27XR:=ReesZeroMatrixSemigroup(G, mat); [127X[104X
    [4X[28X<Rees 0-matrix semigroup 200x100 over Group([ (1,2,3) ])>[128X[104X
    [4X[25Xgap>[125X [27XRows(R);[127X[104X
    [4X[28X[ 1 .. 200 ][128X[104X
    [4X[25Xgap>[125X [27XColumns(R);[127X[104X
    [4X[28X[ 1 .. 100 ][128X[104X
  [4X[32X[104X
  
  [1X51.9-10 UnderlyingSemigroup[101X
  
  [29X[2XUnderlyingSemigroup[102X( [3XR[103X ) [32X attribute
  [29X[2XUnderlyingSemigroup[102X( [3XR[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10YA semigroup.[133X
  
  [33X[0;0Y[10XUnderlyingSemigroup[110X  returns  the underlying semigroup of the Rees matrix or
  0-matrix semigroup [3XR[103X.[133X
  
  [33X[0;0YArbitrary subsemigroups of Rees matrix or 0-matrix semigroups do not have an
  underlying  semigroup.  Such a subsemigroup [3XR[103X has an underlying semigroup if
  and    only    if    it    satisfies   [2XIsReesMatrixSemigroup[102X   ([14X51.9-7[114X)   or
  [2XIsReesZeroMatrixSemigroup[102X ([14X51.9-7[114X).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS:=Semigroup(Transformation( [ 2, 1, 1, 2, 1 ] ), [127X[104X
    [4X[25X>[125X [27XTransformation( [ 3, 4, 3, 4, 4 ] ), Transformation([ 3, 4, 3, 4, 3 ] ),[127X[104X
    [4X[25X>[125X [27XTransformation([ 4, 3, 3, 4, 4 ] ) );;[127X[104X
    [4X[25Xgap>[125X [27XR:=Range(IsomorphismReesMatrixSemigroup(S));    [127X[104X
    [4X[28X<Rees matrix semigroup 4x2 over Group([ (1,2) ])>[128X[104X
    [4X[25Xgap>[125X [27XUnderlyingSemigroup(R);[127X[104X
    [4X[28XGroup([ (1,2) ])[128X[104X
  [4X[32X[104X
  
  [1X51.9-11 AssociatedReesMatrixSemigroupOfDClass[101X
  
  [29X[2XAssociatedReesMatrixSemigroupOfDClass[102X( [3XD[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10YA Rees matrix or 0-matrix semigroup.[133X
  
  [33X[0;0YIf  [3XD[103X is a regular [13XD[113X-class of a finite semigroup [10XS[110X, then there is a standard
  way  of  associating a Rees matrix semigroup to [3XD[103X. If [3XD[103X is a subsemigroup of
  [10XS[110X,  then  [3XD[103X is simple and hence is isomorphic to a Rees matrix semigroup. In
  this case, the associated Rees matrix semigroup of [3XD[103X is just the Rees matrix
  semigroup isomorphic to [3XD[103X.[133X
  
  [33X[0;0YIf  [3XD[103X is not a subsemigroup of [10XS[110X, then we define a semigroup with elements [3XD[103X
  and a new element [10X0[110X with multiplication of [22Xx,y∈ D[122X defined by:[133X
  
  [33X[0;0Y[10Xxy[110X equals the product of [10Xx[110X and [10Xy[110X if it belongs to [3XD[103X and [10X0[110X if it does not.[133X
  
  [33X[0;0YThe  semigroup  thus  defined  is 0-simple and hence is isomorphic to a Rees
  0-matrix  semigroup.  This  semigroup  can  also  be  described  as the Rees
  quotient  of the ideal generated by [3XD[103X by it maximal subideal. The associated
  Rees matrix semigroup of [3XD[103X is just the Rees 0-matrix semigroup isomorphic to
  the semigroup defined above.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XS:=FullTransformationSemigroup(5);;[127X[104X
    [4X[25Xgap>[125X [27XD:=GreensDClasses(S)[3];[127X[104X
    [4X[28X{Transformation( [ 1, 1, 1, 2, 3 ] )}[128X[104X
    [4X[25Xgap>[125X [27XAssociatedReesMatrixSemigroupOfDClass(D);[127X[104X
    [4X[28X<Rees 0-matrix semigroup 25x10 over Group([ (1,2)(3,5)(4,6), (1,3)[128X[104X
    [4X[28X(2,4)(5,6) ])>[128X[104X
  [4X[32X[104X
  
