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12 Attributes and operations for semigroups
 12.1 Random elements of a semigroup
 12.2 Expressing semigroup elements as words in generators
 12.3 Generating sets
 12.4 Minimal ideals and multiplicative zeros
 12.5 Group of units and identity elements
 12.6 Idempotents
 12.7 Maximal subsemigroups
 12.8 The normalizer of a semigroup
 12.9 Attributes of transformation semigroups
 12.10 Attributes of partial perm semigroups

12 Attributes and operations for semigroups

In this chapter we decribe the methods that are available in Semigroups for determining the attributes of a semigroup, and the operations which can be applied to a semigroup.

12.1 Random elements of a semigroup

12.1-1 Random
‣ Random( S )( method )

Returns: A random element.

This function returns a random element of the semigroup S. If the elements of S have been calculated, then one of these is chosen randomly. Otherwise, if the data structure for S is known, then a random element of a randomly chosen \(\mathscr{R}\)-class is returned. If the data structure for S has not been calculated, then a short product (at most 2*Length(GeneratorsOfSemigroup(S))) of generators is returned.

12.2 Expressing semigroup elements as words in generators

It is possible to express an element of a semigroup as a word in the generators of that semigroup. This section describes how to accomplish this in Semigroups.

12.2-1 EvaluateWord
‣ EvaluateWord( gens, w )( operation )

Returns: A semigroup element.

The argument gens should be a collection of generators of a semigroup and the argument w should be a list of positive integers less than or equal to the length of gens. This operation evaluates the word w in the generators gens. More precisely, EvaluateWord returns the equivalent of:

Product(List(w, i-> gens[i]));

see also Factorization (12.2-2).

for elements of a semigroup

When gens is a list of elements of a semigroup and w is a list of positive integers less than or equal to the length of gens, this operation returns the product gens[w[1]]*gens[w[2]]*...*gens[w[n]] when the length of w is n.

for elements of an inverse semigroup

When gens is a list of elements with a semigroup inverse and w is a list of non-zero integers whose absolute value does not exceed the length of gens, this operation returns the product gens[AbsInt(w[1])]^SignInt(w[1])*...*gens[AbsInt(w[n])]^SignInt(w[n]) where n is the length of w.

Note that EvaluateWord(gens, []) returns One(gens) if gens belongs to the category IsMultiplicativeElementWithOne (Reference: IsMultiplicativeElementWithOne).

gap> gens:=[ Transformation( [ 2, 4, 4, 6, 8, 8, 6, 6 ] ), 
> Transformation( [ 2, 7, 4, 1, 4, 6, 5, 2 ] ), 
> Transformation( [ 3, 6, 2, 4, 2, 2, 2, 8 ] ), 
> Transformation( [ 4, 3, 6, 4, 2, 1, 2, 6 ] ), 
> Transformation( [ 4, 5, 1, 3, 8, 5, 8, 2 ] ) ];;
gap> S:=Semigroup(gens);;
gap> f:=Transformation( [ 1, 4, 6, 1, 7, 2, 7, 6 ] );;
gap> Factorization(S, f);
[ 4, 2 ]
gap> EvaluateWord(gens, last);
Transformation( [ 1, 4, 6, 1, 7, 2, 7, 6 ] )
gap> S:=SymmetricInverseMonoid(10);;
gap> f:=PartialPerm( [ 1, 2, 3, 6, 8, 10 ], [ 2, 6, 7, 9, 1, 5 ] );
[3,7][8,1,2,6,9][10,5]
gap> Factorization(S, f);
[ -2, -2, -2, -2, -3, -4, -3, -2, -2, -2, -2, -3, -2, 2, 2, 2, 2, 4, 
  4, 4, 4, 2, 2, 2, 2, 2, 3, 4, -3, -2, -3, -2, -3, -2, 2, 2, 2, 2, 
  2, 3, 4, -3, -2, -3, -2, -3, -2, 2, 2, 2, 2, 2, 3, 4, -3, -2, -3, 
  -2, -3, -2, 2, 2, 2, 2, 2, 3, 4, -3, -2, -3, -2, -3, -2, 2, 2, 2, 
  2, 2, 3, 4, -3, -2, -3, -2, -3, -2, 3, 2, 2, 2, 2, 2, 3, 4, -3, -2, 
  -3, -2, -3, -2, 2, 3, 2, 3, 2, 2, 2, 3, 2, 2, 2, 2, 2, 3, 2, 3, 2 ]
gap> EvaluateWord(GeneratorsOfSemigroup(S), last); 
[3,7][8,1,2,6,9][10,5]

12.2-2 Factorization
‣ Factorization( S, x )( operation )

Returns: A word in the generators.

for semigroups

When S is a semigroup and x belongs to S, Factorization return a word in the generators of S that is equal to x. In this case, a word is a list of positive integers where an entry i corresponds to GeneratorsOfSemigroups(S)[i]. More specifically,

EvaluateWord(GeneratorsOfSemigroup(S), Factorization(S, x))=x;
for inverse semigroups

When S is a inverse semigroup and x belongs to S, Factorization return a word in the generators of S that is equal to x. In this case, a word is a list of non-zero integers where an entry i corresponds to GeneratorsOfSemigroup(S)[i] and -i corresponds to GeneratorsOfSemigroup(S)[i]^-1. As in the previous case,

EvaluateWord(GeneratorsOfSemigroup(S), Factorization(S, x))=x;

Note that Factorization does not always return a word of minimum length; see MinimalFactorization (12.2-3).

See also EvaluateWord (12.2-1) and GeneratorsOfSemigroup (Reference: GeneratorsOfSemigroup).

gap> gens := [ Transformation( [ 2, 2, 9, 7, 4, 9, 5, 5, 4, 8 ] ), 
> Transformation( [ 4, 10, 5, 6, 4, 1, 2, 7, 1, 2 ] ) ];;
gap> S := Semigroup(gens);;
gap> x := Transformation( [ 1, 10, 2, 10, 1, 2, 7, 10, 2, 7 ] );;
gap> word := Factorization(S, x);
[ 2, 2, 1, 2 ]
gap> EvaluateWord(gens, word);
Transformation( [ 1, 10, 2, 10, 1, 2, 7, 10, 2, 7 ] )
gap> S := SymmetricInverseMonoid(8);
<symmetric inverse semigroup on 8 pts>
gap> x := PartialPerm( [ 1, 2, 3, 4, 5, 8 ], [ 7, 1, 4, 3, 2, 6 ] );
[5,2,1,7][8,6](3,4)
gap> word := Factorization(S, f);
[ -2, -2, -2, -2, -2, -2, -2, 2, 2, 4, 4, 2, 3, 2, 3, -2, -2, -2, 2, 
  3, 2, 3, -2, -2, -2, 2, 3, 2, 3, -2, -2, -2, 3, 2, 3, 2, 3, -2, -2, 
  -2, 3, 2, 3, 2, 3, -2, -2, -2, 2, 3, 2, 3, -2, -2, -2, 2, 3, 2, 3, 
  -2, -2, -2, 2, 3, 2, 3, -2, -2, -2, 2, 3, 2, 3, -2, -2, -2, 3, 2, 
  3, 2, 3, -2, -2, -2, 2, 3, 2, 3, -2, -2, -2, 2, 3, 2, 3, -2, -2, 
  -2, 2, 3, 2, 3, -2, -2, -2, 2, 3, 2, 2, 3, 2, 2, 2, 2 ]
gap> EvaluateWord(GeneratorsOfSemigroup(S), word);
[5,2,1,7][8,6](3,4)
gap> S := DualSymmetricInverseMonoid(6);;
gap> x := S.1 * S.2 * S.3 * S.2 * S.1;
<block bijection: [ 1, 6, -4 ], [ 2, -2, -3 ], [ 3, -5 ], [ 4, -6 ], 
 [ 5, -1 ]>
gap> word := Factorization(S, x);
[ -2, -2, -2, -2, -2, 4, 2 ]
gap> EvaluateWord(GeneratorsOfSemigroup(S), word);
<block bijection: [ 1, 6, -4 ], [ 2, -2, -3 ], [ 3, -5 ], [ 4, -6 ], 
 [ 5, -1 ]>

12.2-3 MinimalFactorization
‣ MinimalFactorization( S, x )( operation )

Returns: A minimal word in the generators.

This operation returns a minimal length word in the generators of the semigroup S that equals the element x. In this case, a word is a list of positive integers where an entry i corresponds to GeneratorsOfSemigroups(S)[i]. More specifically,

EvaluateWord(GeneratorsOfSemigroup(S), MinimalFactorization(S, x)) = x;

MinimalFactorization involves exhaustively enumerating S until the element x is found, and so MinimalFactorization may be less efficient than Factorization (12.2-2) for some semigroups.

Unlike Factorization (12.2-2) this operation does not distinguish between semigroups and inverse semigroups. See also EvaluateWord (12.2-1) and GeneratorsOfSemigroup (Reference: GeneratorsOfSemigroup).

gap> S := Semigroup(Transformation([2, 2, 9, 7, 4, 9, 5, 5, 4, 8]), 
>                   Transformation([4, 10, 5, 6, 4, 1, 2, 7, 1, 2]));
<transformation semigroup of degree 10 with 2 generators>
gap> Transformation( [ 8, 8, 2, 2, 9, 2, 8, 8, 9, 9 ] );
Transformation( [ 8, 8, 2, 2, 9, 2, 8, 8, 9, 9 ] )
gap> Factorization(S, x);
[ 1, 2, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 2, 1 ]
gap> MinimalFactorization(S, x);
[ 1, 2, 1, 1, 1, 1, 2, 2, 1 ]

12.3 Generating sets

12.3-1 Generators
‣ Generators( S )( attribute )

Returns: A list of generators.

Generators returns a generating set that can be used to define the semigroup S. The generators of a monoid or inverse semigroup S, say, can be defined in several ways, for example, including or excluding the identity element, including or not the inverses of the generators. Generators uses the definition that returns the least number of generators. If no generating set for S is known, then GeneratorsOfSemigroup is used by default.

for a group

Generators(S) is a synonym for GeneratorsOfGroup (Reference: GeneratorsOfGroup).

for an ideal of semigroup

Generators(S) is a synonym for GeneratorsOfSemigroupIdeal (7.2-1).

for a semigroup

Generators(S) is a synonym for GeneratorsOfSemigroup (Reference: GeneratorsOfSemigroup).

for a monoid

Generators(S) is a synonym for GeneratorsOfMonoid (Reference: GeneratorsOfMonoid).

for an inverse semigroup

Generators(S) is a synonym for GeneratorsOfInverseSemigroup (Reference: GeneratorsOfInverseSemigroup).

for an inverse monoid

Generators(S) is a synonym for GeneratorsOfInverseMonoid (Reference: GeneratorsOfInverseMonoid).

gap> M:=Monoid(Transformation( [ 1, 4, 6, 2, 5, 3, 7, 8, 9, 9 ] ),
> Transformation( [ 6, 3, 2, 7, 5, 1, 8, 8, 9, 9 ] ) );;
gap> GeneratorsOfSemigroup(M);
[ IdentityTransformation, 
  Transformation( [ 1, 4, 6, 2, 5, 3, 7, 8, 9, 9 ] ), 
  Transformation( [ 6, 3, 2, 7, 5, 1, 8, 8, 9, 9 ] ) ]
gap> GeneratorsOfMonoid(M);
[ Transformation( [ 1, 4, 6, 2, 5, 3, 7, 8, 9, 9 ] ), 
  Transformation( [ 6, 3, 2, 7, 5, 1, 8, 8, 9, 9 ] ) ]
gap> Generators(M);
[ Transformation( [ 1, 4, 6, 2, 5, 3, 7, 8, 9, 9 ] ), 
  Transformation( [ 6, 3, 2, 7, 5, 1, 8, 8, 9, 9 ] ) ]
gap> S:=Semigroup(Generators(M));;
gap> Generators(S);
[ Transformation( [ 1, 4, 6, 2, 5, 3, 7, 8, 9, 9 ] ), 
  Transformation( [ 6, 3, 2, 7, 5, 1, 8, 8, 9, 9 ] ) ]
gap> GeneratorsOfSemigroup(S);
[ Transformation( [ 1, 4, 6, 2, 5, 3, 7, 8, 9, 9 ] ), 
  Transformation( [ 6, 3, 2, 7, 5, 1, 8, 8, 9, 9 ] ) ]

12.3-2 SmallGeneratingSet
‣ SmallGeneratingSet( coll )( attribute )
‣ SmallSemigroupGeneratingSet( coll )( attribute )
‣ SmallMonoidGeneratingSet( coll )( attribute )
‣ SmallInverseSemigroupGeneratingSet( coll )( attribute )
‣ SmallInverseMonoidGeneratingSet( coll )( attribute )

Returns: A small generating set for a semigroup.

The attributes SmallXGeneratingSet return a relatively small generating subset of the collection of elements coll, which can also be a semigroup. The returned value of SmallXGeneratingSet, where applicable, has the property that

      X(SmallXGeneratingSet(coll))=X(coll);
    

where X is any of Semigroup (Reference: Semigroup), Monoid (Reference: Monoid), InverseSemigroup (Reference: InverseSemigroup), or InverseMonoid (Reference: InverseMonoid).

If the number of generators for S is already relatively small, then these functions will often return the original generating set. These functions may return different results in different GAP sessions.

SmallGeneratingSet returns the smallest of the returned values of SmallXGeneratingSet which is applicable to coll; see Generators (12.3-1).

As neither irredundancy, nor minimal length are proven, these functions usually return an answer much more quickly than IrredundantGeneratingSubset (12.3-3). These functions can be used whenever a small generating set is desired which does not necessarily needs to be minimal.

gap> S := Semigroup( Transformation( [ 1, 2, 3, 2, 4 ] ), 
> Transformation( [ 1, 5, 4, 3, 2 ] ),
> Transformation( [ 2, 1, 4, 2, 2 ] ), 
> Transformation( [ 2, 4, 4, 2, 1 ] ),
> Transformation( [ 3, 1, 4, 3, 2 ] ), 
> Transformation( [ 3, 2, 3, 4, 1 ] ),
> Transformation( [ 4, 4, 3, 3, 5 ] ), 
> Transformation( [ 5, 1, 5, 5, 3 ] ),
> Transformation( [ 5, 4, 3, 5, 2 ] ), 
> Transformation( [ 5, 5, 4, 5, 5 ] ) );;
gap> SmallGeneratingSet(S);                  
[ Transformation( [ 1, 5, 4, 3, 2 ] ), Transformation( [ 3, 2, 3, 4, 1 ] ), 
  Transformation( [ 5, 4, 3, 5, 2 ] ), Transformation( [ 1, 2, 3, 2, 4 ] ), 
  Transformation( [ 4, 4, 3, 3, 5 ] ) ]
gap> S := RandomInverseMonoid(10000,10);;
gap> SmallGeneratingSet(S);
[ [ 1 .. 10 ] -> [ 3, 2, 4, 5, 6, 1, 7, 10, 9, 8 ], 
  [ 1 .. 10 ] -> [ 5, 10, 8, 9, 3, 2, 4, 7, 6, 1 ], 
  [ 1, 3, 4, 5, 6, 7, 8, 9, 10 ] -> [ 1, 6, 4, 8, 2, 10, 7, 3, 9 ] ]
gap> M := MathieuGroup(24);;
gap> mat := List([1..1000], x-> Random(G));;
gap> Append(mat, [1..1000]*0);
gap> mat := List([1..138], x-> List([1..57], x-> Random(mat)));;
gap> R := ReesZeroMatrixSemigroup(G, mat);;
gap> U := Semigroup(List([1..200], x-> Random(R)));
<subsemigroup of 57x138 Rees 0-matrix semigroup with 100 generators>
gap> Length(SmallGeneratingSet(U));
84
gap> S := RandomBipartitionSemigroup(100,4);
<bipartition semigroup on 4 pts with 96 generators>
gap> Length(SmallGeneratingSet(S));       
13

12.3-3 IrredundantGeneratingSubset
‣ IrredundantGeneratingSubset( coll )( operation )

Returns: A list of irredundant generators.

If coll is a collection of elements of a semigroup, then this function returns a subset U of coll such that no element of U is generated by the other elements of U.

gap> S := Semigroup( Transformation( [ 5, 1, 4, 6, 2, 3 ] ),
> Transformation( [ 1, 2, 3, 4, 5, 6 ] ),
> Transformation( [ 4, 6, 3, 4, 2, 5 ] ),
> Transformation( [ 5, 4, 6, 3, 1, 3 ] ),
> Transformation( [ 2, 2, 6, 5, 4, 3 ] ),
> Transformation( [ 3, 5, 5, 1, 2, 4 ] ),
> Transformation( [ 6, 5, 1, 3, 3, 4 ] ),
> Transformation( [ 1, 3, 4, 3, 2, 1 ] ) );;
gap> IrredundantGeneratingSubset(S);
[ Transformation( [ 1, 3, 4, 3, 2, 1 ] ), 
  Transformation( [ 2, 2, 6, 5, 4, 3 ] ), 
  Transformation( [ 3, 5, 5, 1, 2, 4 ] ), 
  Transformation( [ 5, 1, 4, 6, 2, 3 ] ), 
  Transformation( [ 5, 4, 6, 3, 1, 3 ] ), 
  Transformation( [ 6, 5, 1, 3, 3, 4 ] ) ]
gap> S := RandomInverseMonoid(1000,10);
<inverse partial perm monoid on 10 pts with 1000 generators>
gap> SmallGeneratingSet(S);
[ [ 1 .. 10 ] -> [ 6, 5, 1, 9, 8, 3, 10, 4, 7, 2 ], 
  [ 1 .. 10 ] -> [ 1, 4, 6, 2, 8, 5, 7, 10, 3, 9 ], 
  [ 1, 2, 3, 4, 6, 7, 8, 9 ] -> [ 7, 5, 10, 1, 8, 4, 9, 6 ]
  [ 1 .. 9 ] -> [ 4, 3, 5, 7, 10, 9, 1, 6, 8 ] ]
gap> IrredundantGeneratingSubset(last);
[ [ 1 .. 9 ] -> [ 4, 3, 5, 7, 10, 9, 1, 6, 8 ], 
  [ 1 .. 10 ] -> [ 1, 4, 6, 2, 8, 5, 7, 10, 3, 9 ], 
  [ 1 .. 10 ] -> [ 6, 5, 1, 9, 8, 3, 10, 4, 7, 2 ] ]
gap> S := RandomBipartitionSemigroup(1000,4);
<bipartition semigroup on 4 pts with 749 generators>
gap> SmallGeneratingSet(S);
[ <bipartition: [ 1, -3 ], [ 2, -2 ], [ 3, -1 ], [ 4, -4 ]>, 
  <bipartition: [ 1, 3, -2 ], [ 2, -1, -3 ], [ 4, -4 ]>, 
  <bipartition: [ 1, -4 ], [ 2, 4, -1, -3 ], [ 3, -2 ]>, 
  <bipartition: [ 1, -1, -3 ], [ 2, -4 ], [ 3, 4, -2 ]>, 
  <bipartition: [ 1, -2, -4 ], [ 2 ], [ 3, -3 ], [ 4, -1 ]>, 
  <bipartition: [ 1, -2 ], [ 2, -1, -3 ], [ 3, 4, -4 ]>, 
  <bipartition: [ 1, 3, -1 ], [ 2, -3 ], [ 4, -2, -4 ]>, 
  <bipartition: [ 1, -1 ], [ 2, 4, -4 ], [ 3, -2, -3 ]>, 
  <bipartition: [ 1, 3, -1 ], [ 2, -2 ], [ 4, -3, -4 ]>, 
  <bipartition: [ 1, 2, -2 ], [ 3, -1, -4 ], [ 4, -3 ]>, 
  <bipartition: [ 1, -2, -3 ], [ 2, -4 ], [ 3 ], [ 4, -1 ]>, 
  <bipartition: [ 1, -1 ], [ 2, 4, -3 ], [ 3, -2 ], [ -4 ]>, 
  <bipartition: [ 1, -3 ], [ 2, -1 ], [ 3, 4, -4 ], [ -2 ]>, 
  <bipartition: [ 1, 2, -4 ], [ 3, -1 ], [ 4, -2 ], [ -3 ]>, 
  <bipartition: [ 1, -3 ], [ 2, -4 ], [ 3, -1, -2 ], [ 4 ]> ]
gap> IrredundantGeneratingSubset(last);
[ <bipartition: [ 1, 2, -4 ], [ 3, -1 ], [ 4, -2 ], [ -3 ]>, 
  <bipartition: [ 1, 3, -1 ], [ 2, -2 ], [ 4, -3, -4 ]>, 
  <bipartition: [ 1, 3, -2 ], [ 2, -1, -3 ], [ 4, -4 ]>, 
  <bipartition: [ 1, -1 ], [ 2, 4, -3 ], [ 3, -2 ], [ -4 ]>, 
  <bipartition: [ 1, -3 ], [ 2, -1 ], [ 3, 4, -4 ], [ -2 ]>, 
  <bipartition: [ 1, -3 ], [ 2, -2 ], [ 3, -1 ], [ 4, -4 ]>, 
  <bipartition: [ 1, -3 ], [ 2, -4 ], [ 3, -1, -2 ], [ 4 ]>, 
  <bipartition: [ 1, -2, -3 ], [ 2, -4 ], [ 3 ], [ 4, -1 ]>, 
  <bipartition: [ 1, -2, -4 ], [ 2 ], [ 3, -3 ], [ 4, -1 ]> ]

12.4 Minimal ideals and multiplicative zeros

In this section we describe the attributes of a semigroup that can be found using the Semigroups package.

12.4-1 MinimalIdeal
‣ MinimalIdeal( S )( attribute )

Returns: The minimal ideal of a semigroup.

The minimal ideal of a semigroup is the least ideal with respect to containment.

It is significantly easier to find the minimal \(\mathscr{D}\)-class of a semigroup, than to find its \(\mathscr{D}\)-classes.

See also RepresentativeOfMinimalIdeal (12.4-2), PartialOrderOfDClasses (11.1-10), IsGreensLessThanOrEqual (Reference: IsGreensLessThanOrEqual), and MinimalDClass (11.1-6).

gap> S := Semigroup( Transformation( [ 3, 4, 1, 3, 6, 3, 4, 6, 10, 1 ] ), 
> Transformation( [ 8, 2, 3, 8, 4, 1, 3, 4, 9, 7 ] ));;
gap> MinimalIdeal(S);
<simple transformation semigroup ideal on 10 pts with 1 generator>
gap> Elements(MinimalIdeal(S));
[ Transformation( [ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 ] ), 
  Transformation( [ 3, 3, 3, 3, 3, 3, 3, 3, 3, 3 ] ), 
  Transformation( [ 4, 4, 4, 4, 4, 4, 4, 4, 4, 4 ] ), 
  Transformation( [ 6, 6, 6, 6, 6, 6, 6, 6, 6, 6 ] ), 
  Transformation( [ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8 ] ) ]
gap> x := Transformation( [ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8 ] );;
gap> D := DClass(S, x);
<Green's D-class: Transformation( [ 3, 3, 3, 3, 3, 3, 3, 3, 3, 3 ] )>
gap> ForAll(GreensDClasses(S), x-> IsGreensLessThanOrEqual(D, x));
true
gap> MinimalIdeal(POI(10));
<partial perm group on 10 pts with 1 generator>
gap> MinimalIdeal(BrauerMonoid(6));
<simple bipartition semigroup ideal on 6 pts with 1 generator>

12.4-2 RepresentativeOfMinimalIdeal
‣ RepresentativeOfMinimalIdeal( S )( attribute )
‣ RepresentativeOfMinimalDClass( S )( attribute )

Returns: An element of the minimal ideal of a semigroup.

The minimal ideal of a semigroup is the least ideal with respect to containment.

This method returns a representative element of the minimal ideal of S without having to create the minimal ideal itself. In general, beyond being a member of the minimal ideal, the returned element is not guaranteed to have any special properties. However, the element will coincide with the zero element of S if one exists.

This method works particularly well if S is a semigroup of transformations or partial permutations.

See also MinimalIdeal (12.4-1) and MinimalDClass (11.1-6).

gap> S := SymmetricInverseSemigroup(10);;
gap> RepresentativeOfMinimalIdeal(S);
<empty partial perm>
gap> B := Semigroup([
> Bipartition( [ [ 1, 2 ], [ 3, 6, -2 ], [ 4, 5, -3, -4 ],
>  [ -1, -6 ], [ -5 ] ] ),
> Bipartition( [ [ 1, -1 ], [ 2 ], [ 3 ], [ 4, -3 ], 
>  [ 5, 6, -5, -6 ], [ -2, -4 ] ] ) ]);;
gap> RepresentativeOfMinimalIdeal(B);
<bipartition: [ 1, 2 ], [ 3, 6 ], [ 4, 5 ], [ -1, -5, -6 ], 
 [ -2, -4 ], [ -3 ]>
gap> S := Semigroup([ Transformation( [ 5, 1, 6, 2, 2, 4 ] ),
> Transformation( [ 3, 5, 5, 1, 6, 2 ] ) ]);;
gap> RepresentativeOfMinimalDClass(S);
Transformation( [ 1, 2, 2, 5, 5, 1 ] )
gap> MinimalDClass(S);
<Green's D-class: Transformation( [ 1, 2, 2, 5, 5, 1 ] )>

12.4-3 MultiplicativeZero
‣ MultiplicativeZero( S )( attribute )

Returns: The zero element of a semigroup.

MultiplicativeZero returns the zero element of the semigroup S if it exists and fail if it does not. See also MultiplicativeZero (Reference: MultiplicativeZero).

gap> S := Semigroup( Transformation( [ 1, 4, 2, 6, 6, 5, 2 ] ), 
> Transformation( [ 1, 6, 3, 6, 2, 1, 6 ] ));;
gap> MultiplicativeZero(S);
Transformation( [ 1, 1, 1, 1, 1, 1, 1 ] )
gap> S := Semigroup(Transformation( [ 2, 8, 3, 7, 1, 5, 2, 6 ] ), 
> Transformation( [ 3, 5, 7, 2, 5, 6, 3, 8 ] ), 
> Transformation( [ 6, 7, 4, 1, 4, 1, 6, 2 ] ), 
> Transformation( [ 8, 8, 5, 1, 7, 5, 2, 8 ] ));;
gap> MultiplicativeZero(S);
fail
gap> S := InverseSemigroup( PartialPerm( [ 1, 3, 4 ], [ 5, 3, 1 ] ),
> PartialPerm( [ 1, 2, 3, 4 ], [ 4, 3, 1, 2 ] ),
> PartialPerm( [ 1, 3, 4, 5 ], [ 2, 4, 5, 3 ] ) );;
gap> MultiplicativeZero(S);
<empty partial perm>
gap> S := PartitionMonoid(6); 
<regular bipartition monoid on 6 pts with 4 generators>
gap> MultiplicativeZero(S);
fail
gap> S := DualSymmetricInverseMonoid(6);
<inverse bipartition monoid on 6 pts with 3 generators>
gap> MultiplicativeZero(S);
<block bijection: [ 1, 2, 3, 4, 5, 6, -1, -2, -3, -4, -5, -6 ]>

12.5 Group of units and identity elements

12.5-1 GroupOfUnits
‣ GroupOfUnits( S )( attribute )

Returns: The group of units of a semigroup or fail.

GroupOfUnits returns the group of units of the semigroup S as a subsemigroup of S if it exists and returns fail if it does not. Use IsomorphismPermGroup (6.5-2) if you require a permutation representation of the group of units.

If a semigroup S has an identity e, then the group of units of S is the set of those s in S such that there exists t in S where s*t=t*s=e. Equivalently, the group of units is the \(\mathscr{H}\)-class of the identity of S.

See also GreensHClassOfElement (Reference: GreensHClassOfElement), IsMonoidAsSemigroup (14.1-11), and MultiplicativeNeutralElement (Reference: MultiplicativeNeutralElement).

gap> S := Semigroup(Transformation( [ 1, 2, 5, 4, 3, 8, 7, 6 ] ),
>   Transformation( [ 1, 6, 3, 4, 7, 2, 5, 8 ] ),
>   Transformation( [ 2, 1, 6, 7, 8, 3, 4, 5 ] ),
>   Transformation( [ 3, 2, 3, 6, 1, 6, 1, 2 ] ),
>   Transformation( [ 5, 2, 3, 6, 3, 4, 7, 4 ] ) );;
gap> Size(S);
5304
gap> StructureDescription(GroupOfUnits(S));
"C2 x S4"
gap> S := InverseSemigroup( PartialPerm( [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ], 
> [ 2, 4, 5, 3, 6, 7, 10, 9, 8, 1 ] ),
> PartialPerm( [ 1, 2, 3, 4, 5, 6, 7, 8, 10 ], 
> [ 8, 2, 3, 1, 4, 5, 10, 6, 9 ] ) );;
gap> StructureDescription(GroupOfUnits(S));
"C8"
gap> S := InverseSemigroup( PartialPerm( [ 1, 3, 4 ], [ 4, 3, 5 ] ),
> PartialPerm( [ 1, 2, 3, 5 ], [ 3, 1, 5, 2 ] ) );;
gap> GroupOfUnits(S);
fail
gap> S := Semigroup( Bipartition( [ [ 1, 2, 3, -1, -3 ], [ -2 ] ] ), 
> Bipartition( [ [ 1, -1 ], [ 2, 3, -2, -3 ] ] ), 
> Bipartition( [ [ 1, -2 ], [ 2, -3 ], [ 3, -1 ] ] ), 
> Bipartition( [ [ 1 ], [ 2, 3, -2 ], [ -1, -3 ] ] ) );;
gap> StructureDescription(GroupOfUnits(S));
"C3"

12.6 Idempotents

12.6-1 Idempotents
‣ Idempotents( obj[, n] )( attribute )

Returns: A list of idempotents.

The argument obj should be a semigroup, \(\mathscr{D}\)-class, \(\mathscr{H}\)-class, \(\mathscr{L}\)-class, or \(\mathscr{R}\)-class.

If the optional second argument n is present and obj is a semigroup, then a list of the idempotents in obj of rank n is returned. If you are only interested in the idempotents of a given rank, then the second version of the function will probably be faster. However, if the optional second argument is present, then nothing is stored in obj and so every time the function is called the computation must be repeated.

This functions produce essentially the same output as the GAP library function with the same name; see Idempotents (Reference: Idempotents). The main difference is that this function can be applied to a wider class of objects as described above.

See also IsRegularDClass (Reference: IsRegularDClass), IsRegularClass (11.3-2) IsGroupHClass (Reference: IsGroupHClass), NrIdempotents (12.6-2), and GroupHClass (11.4-1).

gap> S := Semigroup([ Transformation( [ 2, 3, 4, 1 ] ), 
> Transformation( [ 3, 3, 1, 1 ] ) ]);;
gap> Idempotents(S, 1);
[  ]
gap> Idempotents(S, 2);
[ Transformation( [ 1, 1, 3, 3 ] ), Transformation( [ 1, 3, 3, 1 ] ), 
  Transformation( [ 2, 2, 4, 4 ] ), Transformation( [ 4, 2, 2, 4 ] ) ]
gap> Idempotents(S);
[ IdentityTransformation, Transformation( [ 1, 1, 3, 3 ] ), 
  Transformation( [ 1, 3, 3, 1 ] ), Transformation( [ 2, 2, 4, 4 ] ), 
  Transformation( [ 4, 2, 2, 4 ] ) ]
gap> x := Transformation( [ 2, 2, 4, 4 ] );;
gap> R := GreensRClassOfElement(S, x);
<Green's R-class: Transformation( [ 3, 3, 1, 1 ] )>
gap> Idempotents(R);
[ Transformation( [ 1, 1, 3, 3 ] ), Transformation( [ 2, 2, 4, 4 ] ) ]
gap> x := Transformation( [ 4, 2, 2, 4 ] );;
gap> L := GreensLClassOfElement(S, x);;
gap> Idempotents(L);
[ Transformation( [ 2, 2, 4, 4 ] ), Transformation( [ 4, 2, 2, 4 ] ) ]
gap> D := DClassOfLClass(L);
<Green's D-class: Transformation( [ 1, 1, 3, 3 ] )>
gap> Idempotents(D);
[ Transformation( [ 1, 1, 3, 3 ] ), Transformation( [ 2, 2, 4, 4 ] ), 
  Transformation( [ 1, 3, 3, 1 ] ), Transformation( [ 4, 2, 2, 4 ] ) ]
gap> L := GreensLClassOfElement(S, Transformation( [ 3, 1, 1, 3 ] ));;
gap> Idempotents(L);
[ Transformation( [ 1, 1, 3, 3 ] ), Transformation( [ 1, 3, 3, 1 ] ) ]
gap> H := GroupHClass(D);
<Green's H-class: Transformation( [ 1, 1, 3, 3 ] )>
gap> Idempotents(H);
[ Transformation( [ 1, 1, 3, 3 ] ) ]
gap> S := InverseSemigroup(
> [ PartialPerm( [ 1, 2, 3, 4, 5, 7 ], [ 10, 6, 3, 4, 9, 1 ] ),
> PartialPerm( [ 1, 2, 3, 4, 5, 6, 7, 8 ], 
> [ 6, 10, 7, 4, 8, 2, 9, 1 ] ) ]);;
gap> Idempotents(S, 1);
[ <identity partial perm on [ 4 ]> ]
gap> Idempotents(S, 0);
[  ]

12.6-2 NrIdempotents
‣ NrIdempotents( obj )( attribute )

Returns: A positive integer.

This function returns the number of idempotents in obj where obj can be a semigroup, \(\mathscr{D}\)-, \(\mathscr{L}\)-, \(\mathscr{H}\)-, or \(\mathscr{R}\)-class. If the actual idempotents are not required, then it is more efficient to use NrIdempotents(obj) than Length(Idempotents(obj)) since the idempotents themselves are not created when NrIdempotents is called.

See also Idempotents (Reference: Idempotents) and Idempotents (12.6-1), IsRegularDClass (Reference: IsRegularDClass), IsRegularClass (11.3-2) IsGroupHClass (Reference: IsGroupHClass), and GroupHClass (11.4-1).

gap> S := Semigroup([ Transformation( [ 2, 3, 4, 1 ] ), 
> Transformation( [ 3, 3, 1, 1 ] ) ]);;
gap> NrIdempotents(S);   
5
gap> f := Transformation( [ 2, 2, 4, 4 ] );;
gap> R := GreensRClassOfElement(S, f);;
gap> NrIdempotents(R);
2
gap> f := Transformation( [ 4, 2, 2, 4 ] );;
gap> L := GreensLClassOfElement(S, f);;
gap> NrIdempotents(L);
2
gap> D := DClassOfLClass(L);;
gap> NrIdempotents(D);
4
gap> L := GreensLClassOfElement(S, Transformation( [ 3, 1, 1, 3 ] ));;
gap> NrIdempotents(L);
2
gap> H := GroupHClass(D);;
gap> NrIdempotents(H);
1
gap> S := InverseSemigroup(
> [ PartialPerm( [ 1, 2, 3, 5, 7, 9, 10 ], [ 6, 7, 2, 9, 1, 5, 3 ] ),
> PartialPerm( [ 1, 2, 3, 5, 6, 7, 9, 10 ], 
> [ 8, 1, 9, 4, 10, 5, 6, 7 ] ) ]);;
gap> NrIdempotents(S);
236
gap> f := PartialPerm([ 2, 3, 7, 9, 10 ], [ 7, 2, 1, 5, 3 ]);;
gap> d := DClassNC(S, f);;
gap> NrIdempotents(d);
13

12.6-3 IdempotentGeneratedSubsemigroup
‣ IdempotentGeneratedSubsemigroup( S )( attribute )

Returns: A semigroup.

IdempotentGeneratedSubsemigroup returns the subsemigroup of the semigroup S generated by the idempotents of S.

See also Idempotents (12.6-1) and SmallGeneratingSet (12.3-2).

gap> S := Semigroup( [ Transformation( [ 1, 1 ] ), 
>  Transformation( [ 2, 1 ] ), 
> Transformation( [ 1, 2, 2 ] ), 
>  Transformation( [ 1, 2, 3, 4, 5, 1 ] ), 
>  Transformation( [ 1, 2, 3, 4, 5, 5 ] ), 
>  Transformation( [ 1, 2, 3, 4, 6, 5 ] ), 
>  Transformation( [ 1, 2, 3, 5, 4 ] ),
>  Transformation( [ 1, 2, 3, 7, 4, 5, 7 ] ), 
>  Transformation( [ 1, 2, 4, 8, 8, 3, 8, 7 ] ), 
>  Transformation( [ 1, 2, 8, 4, 5, 6, 7, 8 ] ), 
>  Transformation( [ 7, 7, 7, 4, 5, 6, 1 ] ) ] );;
gap> IdempotentGeneratedSubsemigroup(S);
<transformation monoid on 8 pts with 18 generators>
gap> S := SymmetricInverseSemigroup(5);
<symmetric inverse semigroup on 5 pts>
gap> IdempotentGeneratedSubsemigroup(S);
<inverse partial perm monoid on 5 pts with 5 generators>
gap> S := DualSymmetricInverseSemigroup(5); 
<inverse bipartition monoid on 5 pts with 3 generators>
gap> IdempotentGeneratedSubsemigroup(S);
<inverse bipartition monoid on 5 pts with 10 generators>
gap> IsSemilatticeAsSemigroup(last);
true

12.7 Maximal subsemigroups

12.7-1 MaximalSubsemigroups
‣ MaximalSubsemigroups( S )( attribute )

Returns: The maximal subsemigroups of S.

If S is a semigroup, then MaximalSubsemigroups returns a list of the maximal subsemigroups of S.

A maximal subsemigroup of S is a proper subsemigroup of S which is contained in no other proper subsemigroups of S.

The method for this function are based on [GGR68].

Please note: the Grape package version 4.5 or higher must be available and compiled for this function to work.

gap> S := FullTransformationSemigroup(4);
<full transformation semigroup on 4 pts>
gap> MaximalSubsemigroups(S);
[ <transformation semigroup on 4 pts with 3 generators>, 
  <transformation semigroup on 4 pts with 5 generators>, 
  <transformation semigroup on 4 pts with 4 generators>, 
  <transformation semigroup on 4 pts with 4 generators>, 
  <transformation semigroup on 4 pts with 5 generators>, 
  <transformation semigroup on 4 pts with 4 generators>, 
  <transformation semigroup on 4 pts with 5 generators>, 
  <transformation semigroup on 4 pts with 5 generators>, 
  <transformation semigroup on 4 pts with 4 generators> ]
gap> D := DClass(S, Transformation([ 2, 2 ]));
<Green's D-class: Transformation( [ 2, 3, 1, 2 ] )>
gap> R := PrincipalFactor(D);
<Rees 0-matrix semigroup 6x4 over Group([ (1,2,3), (1,2) ])>
gap> MaximalSubsemigroups(R);                                       
[ <Rees 0-matrix semigroup 6x3 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 6x3 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 6x3 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 6x3 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 5x4 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 5x4 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 5x4 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 5x4 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 5x4 over Group([ (1,2,3), (1,2) ])>, 
  <Rees 0-matrix semigroup 5x4 over Group([ (1,2,3), (1,2) ])>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 23 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 23 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 21 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 23 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 21 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 21 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 23 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 21 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 21 generators>, 
  <subsemigroup of 6x4 Rees 0-matrix semigroup with 21 generators> ]

12.7-2 MaximalSubsemigroups
‣ MaximalSubsemigroups( R, H )( operation )

Returns: The maximal subsemigroups of a Rees (0)-matrix semigroup corresponding to a maximal subgroup of the underlying group.

Suppose that R is a regular Rees (0-)matrix semigroup of the form mathcalM[G; I, J; P] where G is a group and P is a |J| by |I| matrix with entries in G∪{0} . If H is a maximal subgroup of G, then this function returns the maximal subsemigroups of R which are isomorphic to mathcalM[H; I, J; P].

The method used in this function is based on Remark 1 of [GGR68].

Please note: the Grape package version 4.5 or higher must be available and compiled for this function to work, when the argument R is a Rees 0-matrix semigroup.

gap> R := ReesZeroMatrixSemigroup(Group([ (1,2), (3,4) ]), 
> [ [ (), (1,2) ], [ (), (1,2) ] ]);
<Rees 0-matrix semigroup 2x2 over Group([ (1,2), (3,4) ])>
gap> G := UnderlyingSemigroup(R);
Group([ (1,2), (3,4) ])
gap> H := Group((1,2));  
Group([ (1,2) ])
gap> max := MaximalSubsemigroups(R, H);
[ <subsemigroup of 2x2 Rees 0-matrix semigroup with 6 generators> ]
gap> IsMaximalSubsemigroup(R, max[1]);
true

12.7-3 IsMaximalSubsemigroup
‣ IsMaximalSubsemigroup( S, T )( operation )

Returns: true or false

If S and T are semigroups, then IsMaximalSubsemigroup returns true if and only if T is a maximal subsemigroup of S.

A proper subsemigroup T of a semigroup S is a maximal if T is not contained in any other proper subsemigroups of S.

gap> S := FullTransformationSemigroup(4);                              
<full transformation semigroup on 4 pts>
gap> T := Semigroup([ Transformation( [ 3, 4, 1, 2 ] ),
>  Transformation( [ 1, 4, 2, 3 ] ),
>  Transformation( [ 2, 1, 1, 3 ] ) ]);
<transformation semigroup on 4 pts with 3 generators>
gap> IsMaximalSubsemigroup(S, T);
true
gap> R := Semigroup([ Transformation( [ 3, 4, 1, 2 ] ),
>  Transformation( [ 1, 4, 2, 2 ] ),
>  Transformation( [ 2, 1, 1, 3 ] ) ]);
<transformation semigroup on 4 pts with 3 generators>
gap> IsMaximalSubsemigroup(S, R); 
false

12.8 The normalizer of a semigroup

12.8-1 Normalizer
‣ Normalizer( G, S[, opts] )( operation )
‣ Normalizer( S[, opts] )( operation )

Returns: A permutation group.

In its first form, this function returns the normalizer of the transformation, partial perm, or bipartition semigroup S in the permutation group G. In its second form, the normalizer of S in the symmetric group on [1..n] where n is the degree of S is returned.

The normalizer of a transformation semigroup S in a permutation group G in the subgroup H of G consisting of those elements in g in G conjugating S to S, i.e. S^g=S.

Analogous definitions can be given for a partial perm and bipartition semigroups.

The method used by this operation is based on Section 3 in [ABMN10].

The optional final argument opts allows you to specify various options, which determine how the normalizer is calculated. The values of these options can dramatically change the time it takes for this operation to complete. In different situations, different options give the best performance.

The argument opts should be a record, and the available options are:

random

If this option has the value true and the genss package is loaded, then the non-deterministic algorithms in genss are used in Normalizer. So, there is some chance that Normalizer will return an incorrect result in this case, but these methods can also be much faster than the deterministic algorithms which are used if this option is false.

If genss is not loaded, then this option is ignored.

The default value for this option is false.

lambdastab

If this option has the value true, then Normalizer initially finds the setwise stabilizer of the images or right blocks of the semigroup S. Sometimes this improves the performance of Normalizer and sometimes it does not. If this option in false, then this setwise stabilizer is not found.

The default value for this option is true.

rhostab

If this option has the value true, then Normalizer initially finds the setwise stabilizer of the kernels, domains, or left blocks of the semigroup S. Sometimes this improves the performance of Normalizer and sometimes it does not. If this option is false, the this setwise stabilizer is not found.

If S is an inverse semigroup, then this option is ignored.

The default value for this option is true.

gap> S:=BrauerMonoid(8);
<regular bipartition monoid on 8 pts with 3 generators>
gap> StructureDescription(Normalizer(S));
"S8"
gap> S:=InverseSemigroup( 
> PartialPerm( [ 1, 2, 3, 4, 5 ], [ 2, 5, 6, 3, 8 ] ), 
> PartialPerm( [ 1, 2, 4, 7, 8 ], [ 3, 6, 2, 5, 7 ] ) );;
gap> Normalizer(S, rec(random:=true, lambdastab:=false));
#I  Have 33389 points.
#I  Have 40136 points in new orbit.
Group(())

12.9 Attributes of transformation semigroups

12.9-1 ComponentRepsOfTransformationSemigroup
‣ ComponentRepsOfTransformationSemigroup( S )( attribute )

Returns: The representatives of components of a transformation semigroup.

This function returns the representatives of the components of the action of the transformation semigroup S on the set of positive integers not greater than the degree of S.

The representatives are the least set of points such that every point can be reached from some representative under the action of S.

gap> S:=Semigroup( 
> Transformation( [ 11, 11, 9, 6, 4, 1, 4, 1, 6, 7, 12, 5 ] ), 
> Transformation( [ 12, 10, 7, 10, 4, 1, 12, 9, 11, 9, 1, 12 ] ) );;
gap> ComponentRepsOfTransformationSemigroup(S);
[ 2, 3, 8 ]

12.9-2 ComponentsOfTransformationSemigroup
‣ ComponentsOfTransformationSemigroup( S )( attribute )

Returns: The components of a transformation semigroup.

This function returns the components of the action of the transformation semigroup S on the set of positive integers not greater than the degree of S; the components of S partition this set.

gap> S:=Semigroup( 
> Transformation( [ 11, 11, 9, 6, 4, 1, 4, 1, 6, 7, 12, 5 ] ), 
> Transformation( [ 12, 10, 7, 10, 4, 1, 12, 9, 11, 9, 1, 12 ] ) );;
gap> ComponentsOfTransformationSemigroup(S);
[ [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 ] ]

12.9-3 CyclesOfTransformationSemigroup
‣ CyclesOfTransformationSemigroup( S )( attribute )

Returns: The cycles of a transformation semigroup.

This function returns the cycles, or strongly connected components, of the action of the transformation semigroup S on the set of positive integers not greater than the degree of S.

gap> S:=Semigroup( 
> Transformation( [ 11, 11, 9, 6, 4, 1, 4, 1, 6, 7, 12, 5 ] ), 
> Transformation( [ 12, 10, 7, 10, 4, 1, 12, 9, 11, 9, 1, 12 ] ) );;
gap> CyclesOfTransformationSemigroup(S);
[ [ 1, 11, 12, 5, 4, 6, 10, 7, 9 ] ]

12.9-4 IsTransitive
‣ IsTransitive( S[, X] )( operation )
‣ IsTransitive( S[, n] )( operation )

Returns: true or false.

A transformation semigroup S is transitive or strongly connected on the set X if for every i,j in X there is an element s in S such that i^s=j.

If the optional second argument is a positive integer n, then IsTransitive returns true if S is transitive on [1..n], and false if it is not.

If the optional second argument is not provided, then the degree of S is used by default; see DegreeOfTransformationSemigroup (Reference: DegreeOfTransformationSemigroup).

gap> S:=Semigroup( [ Bipartition( [ [ 1, 2 ], [ 3, 6, -2 ], 
> [ 4, 5, -3, -4 ], [ -1, -6 ], [ -5 ] ] ), 
> Bipartition( [ [ 1, -4 ], [ 2, 3, 4, 5 ], [ 6 ], [ -1, -6 ], 
> [ -2, -3 ], [ -5 ] ] ) ] );
<bipartition semigroup on 6 pts with 2 generators>
gap> AsTransformationSemigroup(S);
<transformation semigroup on 12 pts with 2 generators>
gap> IsTransitive(last);
false
gap> IsTransitive(AsSemigroup(Group((1,2,3))));
true

12.9-5 SmallestElementSemigroup
‣ SmallestElementSemigroup( S )( attribute )
‣ LargestElementSemigroup( S )( attribute )

Returns: A transformation.

These attributes return the smallest and largest element of the transformation semigroup S, respectively. Smallest means the first element in the sorted set of elements of S and largest means the last element in the set of elements.

It is not necessary to find the elements of the semigroup to determine the smallest or largest element, and this function has considerable better performance than the equivalent Elements(S)[1] and Elements(S)[Size(S)].

gap> S := Monoid( 
> [ Transformation( [ 1, 4, 11, 11, 7, 2, 6, 2, 5, 5, 10 ] ), 
>   Transformation( [ 2, 4, 4, 2, 10, 5, 11, 11, 11, 6, 7 ] ) ] );
<transformation monoid on 11 pts with 2 generators>
gap> SmallestElementSemigroup(S);
IdentityTransformation
gap> LargestElementSemigroup(S);
Transformation( [ 11, 11, 10, 10, 7, 6, 5, 6, 2, 2, 4 ] )

12.9-6 GeneratorsSmallest
‣ GeneratorsSmallest( S )( attribute )

Returns: A generating set of transformations.

GeneratorsSmallest returns the lexicographically least collection X of transformations such that S is generated by X and each X[i] is not generated by X[1], X[2], ..., X[i-1].

Note that it can be difficult to find this set of generators, and that it might contain a substantial proportion of the elements of the semigroup.

The comparison of two transformation semigroups via the lexicographic comparison of their sets of elements is the same relation as the lexicographic comparison of their GeneratorsSmallest. However, due to the complexity of determining the GeneratorsSmallest, this is not the method used by the Semigroups package when comparing transformation semigroups.

gap> S := Monoid( 
> Transformation( [ 1, 3, 4, 1 ] ), Transformation( [ 2, 4, 1, 2 ] ), 
> Transformation( [ 3, 1, 1, 3 ] ), Transformation( [ 3, 3, 4, 1 ] ) );
<transformation monoid on 4 pts with 4 generators>
gap> GeneratorsSmallest(S);
[ Transformation( [ 1, 1, 1, 1 ] ), Transformation( [ 1, 1, 1, 2 ] ), 
  Transformation( [ 1, 1, 1, 3 ] ), Transformation( [ 1, 1, 1 ] ), 
  Transformation( [ 1, 1, 2, 1 ] ), Transformation( [ 1, 1, 2, 2 ] ), 
  Transformation( [ 1, 1, 3, 1 ] ), Transformation( [ 1, 1, 3, 3 ] ), 
  Transformation( [ 1, 1 ] ), Transformation( [ 1, 1, 4, 1 ] ), 
  Transformation( [ 1, 2, 1, 1 ] ), Transformation( [ 1, 2, 2, 1 ] ), 
  IdentityTransformation, Transformation( [ 1, 3, 1, 1 ] ), 
  Transformation( [ 1, 3, 4, 1 ] ), Transformation( [ 2, 1, 1, 2 ] ), 
  Transformation( [ 2, 2, 2 ] ), Transformation( [ 2, 4, 1, 2 ] ), 
  Transformation( [ 3, 3, 3 ] ), Transformation( [ 3, 3, 4, 1 ] ) ]

12.10 Attributes of partial perm semigroups

12.10-1 ComponentRepsOfPartialPermSemigroup
‣ ComponentRepsOfPartialPermSemigroup( S )( attribute )

Returns: The representatives of components of a partial perm semigroup.

This function returns the representatives of the components of the action of the partial perm semigroup S on the set of positive integers where it is defined.

The representatives are the least set of points such that every point can be reached from some representative under the action of S.

gap> S:=Semigroup( 
> PartialPerm( [ 1, 2, 3, 5, 6, 7, 8, 11, 12, 16, 19 ], 
>     [ 9, 18, 20, 11, 5, 16, 8, 19, 14, 13, 1 ] ), 
>  PartialPerm( [ 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14, 16, 18, 19, 20 ], 
>     [ 13, 1, 8, 5, 4, 14, 11, 12, 9, 20, 2, 18, 7, 3, 19 ] ) );;
gap> ComponentRepsOfPartialPermSemigroup(S);
[ 1, 4, 6, 10, 15, 17 ]

12.10-2 ComponentsOfPartialPermSemigroup
‣ ComponentsOfPartialPermSemigroup( S )( attribute )

Returns: The components of a partial perm semigroup.

This function returns the components of the action of the partial perm semigroup S on the set of positive integers where it is defined; the components of S partition this set.

gap> S:=Semigroup( 
> PartialPerm( [ 1, 2, 3, 5, 6, 7, 8, 11, 12, 16, 19 ], 
>     [ 9, 18, 20, 11, 5, 16, 8, 19, 14, 13, 1 ] ), 
>  PartialPerm( [ 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14, 16, 18, 19, 20 ], 
>     [ 13, 1, 8, 5, 4, 14, 11, 12, 9, 20, 2, 18, 7, 3, 19 ] ) );;
gap> ComponentsOfPartialPermSemigroup(S);
[ [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 18, 19, 20 ], 
  [ 15 ], [ 17 ] ]

12.10-3 CyclesOfPartialPerm
‣ CyclesOfPartialPerm( x )( attribute )

Returns: The cycles of a partial perm.

This function returns the cycles, or strongly connected components, of the action of the partial perm x on the set of positive integers where it is defined.

gap> x := PartialPerm( [ 1, 2, 3, 4, 5, 8, 10 ], [ 3, 1, 4, 2, 5, 6, 7 ] );
[8,6][10,7](1,3,4,2)(5)
gap> CyclesOfPartialPerm(x);
[ [ 3, 4, 2, 1 ], [ 5 ] ]

12.10-4 CyclesOfPartialPermSemigroup
‣ CyclesOfPartialPermSemigroup( S )( attribute )

Returns: The cycles of a partial perm semigroup.

This function returns the cycles, or strongly connected components, of the action of the partial perm semigroup S on the set of positive integers where it is defined.

gap> S:=Semigroup( 
> PartialPerm( [ 1, 2, 3, 5, 6, 7, 8, 11, 12, 16, 19 ], 
>     [ 9, 18, 20, 11, 5, 16, 8, 19, 14, 13, 1 ] ), 
>  PartialPerm( [ 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14, 16, 18, 19, 20 ], 
>     [ 13, 1, 8, 5, 4, 14, 11, 12, 9, 20, 2, 18, 7, 3, 19 ] ) );;
gap> CyclesOfPartialPermSemigroup(S);
[ [ 1, 9, 12, 14, 20, 2, 19, 3, 8, 11 ] ]
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