  
  [1X7 [33X[0;0YDomains[133X[101X
  
  [33X[0;0Y[13XDomain[113X is [5XGAP[105X's name for structured sets. We already saw examples of domains
  in  Chapters [14X5[114X  and [14X6[114X:  the  groups  [10Xs8[110X  and  [10Xa8[110X in Section [14X5.1[114X are domains,
  likewise the field [10Xf[110X and the vector space [10Xv[110X in Section [14X6.1[114X are domains. They
  were constructed by functions such as [2XGroup[102X ([14XReference: Group Libraries[114X) and
  [2XGF[102X  ([14XReference: GF for field size[114X), and they could be passed as arguments to
  other  functions  such  as  [2XDerivedSubgroup[102X ([14XReference: DerivedSubgroup[114X) and
  [2XDimension[102X ([14XReference: Dimension[114X).[133X
  
  
  [1X7.1 [33X[0;0YDomains as Sets[133X[101X
  
  [33X[0;0YFirst  of  all,  a  domain  [22XD[122X  is a set. If [22XD[122X is finite then a list with the
  elements  of  this set can be computed with the functions [2XAsList[102X ([14XReference:
  AsList[114X)   and   [2XAsSortedList[102X  ([14XReference:  AsSortedList[114X).  For  infinite  [22XD[122X,
  [2XEnumerator[102X   ([14XReference:   Enumerator[114X)   and   [2XEnumeratorSorted[102X  ([14XReference:
  EnumeratorSorted[114X)  may  work, but it is also possible that one gets an error
  message.[133X
  
  [33X[0;0YDomains  can  be  used  as  arguments  of set functions such as [2XIntersection[102X
  ([14XReference:  Intersection[114X) and [2XUnion[102X ([14XReference: Union[114X). [5XGAP[105X tries to return
  a  domain  in these cases, moreover it tries to return a domain with as much
  structure  as  possible.  For  example,  the  intersection  of two groups is
  (either  empty  or) again a group, and [5XGAP[105X will try to return it as a group.
  For  [2XUnion[102X  ([14XReference: Union[114X), the situation is different because the union
  of two groups is in general not a group.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xg:= Group( (1,2), (3,4) );;[127X[104X
    [4X[25Xgap>[125X [27Xh:= Group( (3,4), (5,6) );;[127X[104X
    [4X[25Xgap>[125X [27XIntersection( g, h );[127X[104X
    [4X[28XGroup([ (3,4) ])[128X[104X
  [4X[32X[104X
  
  [33X[0;0YTwo  domains are regarded as equal w.r.t. the operator [21X[10X=[110X[121X if and only if they
  are equal [13Xas sets[113X, regardless of the additional structure of the domains.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xmats:= [ [ [ 0*Z(2), Z(2)^0 ], [ Z(2)^0, 0*Z(2) ] ],[127X[104X
    [4X[25X>[125X [27X            [ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2), Z(2)^0 ] ] ];;[127X[104X
    [4X[25Xgap>[125X [27XRing( mats ) = VectorSpace( GF(2), mats );[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YAdditionally,  a  domain  is  regarded  as  equal  to the sorted list of its
  elements.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xg:= Group( (1,2) );;[127X[104X
    [4X[25Xgap>[125X [27Xl:= AsSortedList( g );[127X[104X
    [4X[28X[ (), (1,2) ][128X[104X
    [4X[25Xgap>[125X [27Xg = l;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsGroup( l ); IsList( g );[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  
  [1X7.2 [33X[0;0YAlgebraic Structure[133X[101X
  
  [33X[0;0YThe additional structure of [22XD[122X is constituted by the facts that [22XD[122X is known to
  be  closed  under certain operations such as addition or multiplication, and
  that  these  operations  have  additional properties. For example, if [22XD[122X is a
  group  then  it  is  closed under multiplication ([22XD × D → D[122X, [22X(g,h) ↦ g * h[122X),
  under taking inverses ([22XD → D[122X, [22Xg ↦ g^-1[122X) and under taking the identity [22Xg[122X[10X^0[110X of
  each element [22Xg[122X in [22XD[122X; additionally, the multiplication in [22XD[122X is associative.[133X
  
  [33X[0;0YThe  same  set  of  elements  can  carry different algebraic structures. For
  example,  a  semigroup  is  defined  as  being  closed  under an associative
  multiplication,  so  each  group  is also a semigroup. Likewise, a monoid is
  defined  as  a  semigroup  [22XD[122X  in which the identity [22Xg[122X[10X^0[110X is defined for every
  element [22Xg[122X, so each group is a monoid, and each monoid is a semigroup.[133X
  
  [33X[0;0YOther  examples  of domains are vector spaces, which are defined as additive
  groups  that  are  closed  under  (left)  multiplication  with elements in a
  certain  domain of scalars. Also conjugacy classes in a group [22XD[122X are domains,
  they are closed under the conjugation action of [22XD[122X.[133X
  
  
  [1X7.3 [33X[0;0YNotions of Generation[133X[101X
  
  [33X[0;0YWe  have  seen  that  a domain is closed under certain operations. Usually a
  domain   is  constructed  as  the  closure  of  some  elements  under  these
  operations. In this situation, we say that the elements [13Xgenerate[113X the domain.[133X
  
  [33X[0;0YFor example, a list of matrices of the same shape over a common field can be
  used  to generate an additive group or a vector space over a suitable field;
  if  the  matrices are square then we can also use the matrices as generators
  of  a  semigroup,  a  ring,  or  an  algebra.  We  illustrate  some of these
  possibilities:[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xmats:= [ [ [ 0*Z(2), Z(2)^0 ],[127X[104X
    [4X[25X>[125X [27X              [ Z(2)^0, 0*Z(2) ] ], [127X[104X
    [4X[25X>[125X [27X            [ [ Z(2)^0, 0*Z(2) ],[127X[104X
    [4X[25X>[125X [27X              [ 0*Z(2), Z(2)^0 ] ] ];;[127X[104X
    [4X[25Xgap>[125X [27XSize( AdditiveMagma( mats ) );[127X[104X
    [4X[28X4[128X[104X
    [4X[25Xgap>[125X [27XSize( VectorSpace( GF(8), mats ) );[127X[104X
    [4X[28X64[128X[104X
    [4X[25Xgap>[125X [27XSize( Algebra( GF(2), mats ) );    [127X[104X
    [4X[28X4[128X[104X
    [4X[25Xgap>[125X [27XSize( Group( mats ) );         [127X[104X
    [4X[28X2[128X[104X
  [4X[32X[104X
  
  [33X[0;0YEach  combination of operations under which a domain could be closed gives a
  notion  of generation. So each group has group generators, and since it is a
  monoid, one can also ask for monoid generators of a group.[133X
  
  [33X[0;0YNote that one cannot simply ask for [21Xthe generators of a domain[121X, it is always
  necessary  to  specify  what  notion  of  generation is meant. Access to the
  different  generators  is  provided  by  functions  with  names  of the form
  [10XGeneratorsOfSomething[110X.    For    example,    [2XGeneratorsOfGroup[102X   ([14XReference:
  GeneratorsOfGroup[114X)  denotes group generators, [2XGeneratorsOfMonoid[102X ([14XReference:
  GeneratorsOfMonoid[114X)  denotes  monoid  generators,  and  so on. The result of
  [2XGeneratorsOfVectorSpace[102X ([14XReference: GeneratorsOfVectorSpace[114X) is of course to
  be  understood  relative  to  the  field  of  scalars of the vector space in
  question.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfVectorSpace( GF(4)^2 );[127X[104X
    [4X[28X[ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2), Z(2)^0 ] ][128X[104X
    [4X[25Xgap>[125X [27Xv:= AsVectorSpace( GF(2), GF(4)^2 );;[127X[104X
    [4X[25Xgap>[125X [27XGeneratorsOfVectorSpace( v );[127X[104X
    [4X[28X[ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2), Z(2)^0 ], [ Z(2^2), 0*Z(2) ], [128X[104X
    [4X[28X  [ 0*Z(2), Z(2^2) ] ][128X[104X
  [4X[32X[104X
  
  
  [1X7.4 [33X[0;0YDomain Constructors[133X[101X
  
  [33X[0;0YA  group  can  be constructed from a list of group generators [3Xgens[103X by [10XGroup(
  [3Xgens[103X[10X  )[110X,  likewise  one  can construct rings and algebras with the functions
  [2XRing[102X ([14XReference: Ring[114X) and [2XAlgebra[102X ([14XReference: Algebra[114X).[133X
  
  [33X[0;0YNote  that  it  is  not  always or completely checked that [3Xgens[103X is in fact a
  valid list of group generators, for example whether the elements of [3Xgens[103X can
  be  multiplied  or  whether  they are invertible. This means that [5XGAP[105X trusts
  you, at least to some extent, that the desired domain [10XSomething( [3Xgens[103X[10X )[110X does
  exist.[133X
  
  
  [1X7.5 [33X[0;0YForming Closures of Domains[133X[101X
  
  [33X[0;0YBesides  constructing domains from generators, one can also form the closure
  of  a  given  domain  with an element or another domain. There are different
  notions  of  closure, one has to specify one according to the desired result
  and  the  structure  of  the given domain. The functions to compute closures
  have  names  such  as [10XClosureSomething[110X. For example, if [3XD[103X is a group and one
  wants  to  construct  the group generated by [3XD[103X and an element [3Xg[103X then one can
  use [10XClosureGroup( [3XD[103X[10X, [3Xg[103X[10X )[110X.[133X
  
  
  [1X7.6 [33X[0;0YChanging the Structure[133X[101X
  
  [33X[0;0YThe  same  set  of  elements  can  have  different algebraic structures. For
  example,  it may happen that a monoid [22XM[122X does in fact contain the inverses of
  all of its elements, and thus [22XM[122X is equal to the group formed by the elements
  of [22XM[122X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xm:= Monoid( mats );;[127X[104X
    [4X[25Xgap>[125X [27Xm = Group( mats );[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsGroup( m );[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  last result in the above example may be surprising. But the monoid [10Xm[110X is
  not  regarded as a group in [5XGAP[105X, and moreover there is no way to turn [10Xm[110X into
  a group. Let us formulate this as a rule:[133X
  
  [33X[0;0Y[13XThe  set  of  operations  under  which  the domain is closed is fixed in the
  construction of a domain, and cannot be changed later.[113X[133X
  
  [33X[0;0Y(Contrary  to  this, a domain [13Xcan[113X acquire knowledge about properties such as
  whether the multiplication is associative or commutative.)[133X
  
  [33X[0;0YIf one needs a domain with a different structure than the given one, one can
  construct  a  new  domain with the required structure. The functions that do
  these  constructions  have  names  such as [10XAsSomething[110X, they return a domain
  that  has  the  same  elements as the argument in question but the structure
  [10XSomething[110X. In the above situation, one can use [2XAsGroup[102X ([14XReference: AsGroup[114X).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xg:= AsGroup( m );;[127X[104X
    [4X[25Xgap>[125X [27Xm = g;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsGroup( g );[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YIf  it  is  impossible  to  construct  the  desired  domain, the [10XAsSomething[110X
  functions return [9Xfail[109X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XAsVectorSpace( GF(4), GF(2)^2 );[127X[104X
    [4X[28Xfail[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  functions  [2XAsList[102X  ([14XReference:  AsList[114X)  and  [2XAsSortedList[102X  ([14XReference:
  AsSortedList[114X)  mentioned  above do not return domains, but they fit into the
  general  pattern  in  the  sense  that  they forget all the structure of the
  argument, including the fact that it is a domain, and return a list with the
  same elements as the argument has.[133X
  
  
  [1X7.7 [33X[0;0YSubdomains[133X[101X
  
  [33X[0;0YIt  is possible to construct a domain as a subset of an existing domain. The
  respective  functions  have  names such as [10XSubsomething[110X, they return domains
  with the structure [10XSomething[110X. (Note that the second [10Xs[110X in [10XSubsomething[110X is not
  capitalized.)  For  example,  if  one wants to deal with the subgroup of the
  domain  [3XD[103X  that  is  generated by the elements in the list [3Xgens[103X, one can use
  [10XSubgroup(  [3XD[103X[10X, [3Xgens[103X[10X )[110X. It is not required that [3XD[103X is itself a group, only that
  the group generated by [3Xgens[103X must be a subset of [3XD[103X.[133X
  
  [33X[0;0YThe  superset  of a domain [3XS[103X that was constructed by a [10XSubsomething[110X function
  can be accessed as [10XParent( [3XS[103X[10X )[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xg:= SymmetricGroup( 5 );;[127X[104X
    [4X[25Xgap>[125X [27Xgens:= [ (1,2), (1,2,3,4) ];;[127X[104X
    [4X[25Xgap>[125X [27Xs:= Subgroup( g, gens );;[127X[104X
    [4X[25Xgap>[125X [27Xh:= Group( gens );;[127X[104X
    [4X[25Xgap>[125X [27Xs = h;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XParent( s ) = g;[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YMany  functions return subdomains of their arguments, for example the result
  of [10XSylowSubgroup( [3XG[103X[10X )[110X is a group with parent group [3XG[103X.[133X
  
  [33X[0;0YIf you are sure that the domain [10XSomething( [3Xgens[103X[10X )[110X is contained in the domain
  [3XD[103X  then you can also call [10XSubsomethingNC( [3XD[103X[10X, [3Xgens[103X[10X )[110X instead of [10XSubsomething(
  [3XD[103X[10X,  [3Xgens[103X[10X  )[110X.  The  [10XNC[110X stands for [21Xno check[121X, and the functions whose names end
  with [10XNC[110X omit the check of containment.[133X
  
  
  [1X7.8 [33X[0;0YFurther Information about Domains[133X[101X
  
  [33X[0;0YMore information about domains can be found in Chapter [14X'Reference: Domains'[114X.
  Many other other chapters deal with specific types of domain such as groups,
  vector spaces or algebras.[133X
  
