<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% --> <!-- %% --> <!-- %A fldabnum.xml GAP documentation Thomas Breuer --> <!-- %% --> <!-- %% --> <!-- %Y (C) 1998 School Math and Comp. Sci., University of St Andrews, Scotland --> <!-- %Y Copyright (C) 2002 The GAP Group --> <!-- %% -->
<Chapter Label="Abelian Number Fields">
<Heading>Abelian Number Fields</Heading>
<#Include Label="[1]{fldabnum}">
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<Section Label="Construction of Abelian Number Fields">
<Heading>Construction of Abelian Number Fields</Heading>
Besides the usual construction using
<Ref Func="Field" Label="for several generators"/> or
<Ref Func="DefaultField" Label="for cyclotomics"/>
(see <Ref Func="DefaultField" Label="for cyclotomics"/>),
abelian number fields consisting of cyclotomics can be created with
<Ref Func="CyclotomicField" Label="for (subfield and) conductor"/>
and <Ref Func="AbelianNumberField"/>.
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<Section Label="Operations for Abelian Number Fields">
<Heading>Operations for Abelian Number Fields</Heading>
For operations for elements of abelian number fields, e.g.,
<Ref Attr="Conductor" Label="for a cyclotomic"/> or
<Ref Attr="ComplexConjugate"/>,
see Chapter <Ref Chap="Cyclotomic Numbers"/>.
<ManSection>
<Meth Name="Factors" Arg='F'
Label="for polynomials over abelian number fields"/>
<Description>
Factoring of polynomials over abelian number fields consisting of cyclotomics
works in principle but is not very efficient if the degree of the field
extension is large.
<P/>
<Example><![CDATA[
gap> x:= Indeterminate( CF(5) );
x_1
gap> Factors( PolynomialRing( Rationals ), x^5-1 );
[ x_1-1, x_1^4+x_1^3+x_1^2+x_1+1 ]
gap> Factors( PolynomialRing( CF(5) ), x^5-1 );
[ x_1-1, x_1+(-E(5)), x_1+(-E(5)^2), x_1+(-E(5)^3), x_1+(-E(5)^4) ]
]]></Example>
</Description>
</ManSection>
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<Section Label="Integral Bases of Abelian Number Fields">
<Heading>Integral Bases of Abelian Number Fields</Heading>
<#Include Label="[2]{fldabnum}">
<#Include Label="ZumbroichBase">
<#Include Label="LenstraBase"> <!-- %T missing: <C>IsIntegralBasis</C>, <C>NormalBasis</C>, <C>IsNormalBasis</C>, --> <!-- %T rings of integers in abelian number fields -->
</Section>
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<Section Label="Galois Groups of Abelian Number Fields">
<Heading>Galois Groups of Abelian Number Fields</Heading>
<Index Subkey="Galois group">abelian number fields</Index>
<Index Subkey="Galois group">number fields</Index>
<Index Subkey="of number fields">automorphism group</Index>
The field automorphisms of the cyclotomic field <M>&QQ;_n</M>
(see Chapter <Ref Chap="Cyclotomic Numbers"/>)
are given by the linear maps <M>*k</M> on <M>&QQ;_n</M>
that are defined by <C>E</C><M>(n)^{{*k}} = </M><C>E</C><M>(n)^k</M>,
where <M>1 \leq k < n</M> and <C>Gcd</C><M>( n, k ) = 1</M> hold
(see <Ref Oper="GaloisCyc" Label="for a cyclotomic"/>).
Note that this action is <E>not</E> equal to exponentiation of cyclotomics,
i.e., for general cyclotomics <M>z</M>, <M>z^{{*k}}</M> is different from
<M>z^k</M>.
<P/>
(In &GAP;, the image of a cyclotomic <M>z</M> under <M>*k</M> can be
computed as <C>GaloisCyc( </C><M>z, k</M><C> )</C>.)
<P/>
<Example><![CDATA[
gap> ( E(5) + E(5)^4 )^2; GaloisCyc( E(5) + E(5)^4, 2 );
-2*E(5)-E(5)^2-E(5)^3-2*E(5)^4
E(5)^2+E(5)^3
]]></Example>
<P/>
For <C>Gcd</C><M>( n, k ) \neq 1</M>,
the map <C>E</C><M>(n) \mapsto</M> <C>E</C><M>(n)^k</M> does <E>not</E>
define a field automorphism of <M>&QQ;_n</M>
but only a <M>&QQ;</M>-linear map.
<P/>
<Example><![CDATA[
gap> GaloisCyc( E(5)+E(5)^4, 5 ); GaloisCyc( ( E(5)+E(5)^4 )^2, 5 );
2
-6
]]></Example>
<ManSection>
<Meth Name="GaloisGroup" Arg='F' Label="for abelian number fields"/>
<Description>
The Galois group <M>Gal( &QQ;_n, &QQ; )</M> of the field extension
<M>&QQ;_n / &QQ;</M> is isomorphic to the group
<M>(&ZZ; / n &ZZ;)^{*}</M>
of prime residues modulo <M>n</M>, via the isomorphism
<M>(&ZZ; / n &ZZ;)^{*} \rightarrow Gal( &QQ;_n, &QQ; )</M>
that is defined by
<M>k + n &ZZ; \mapsto ( z \mapsto z^{*k} )</M>.
<P/>
The Galois group of the field extension <M>&QQ;_n / L</M> with
any abelian number field <M>L \subseteq &QQ;_n</M> is simply the
factor group of <M>Gal( &QQ;_n, &QQ; )</M> modulo the stabilizer of <M>L</M>,
and the Galois group of <M>L / L', with L'</M> an abelian
number field contained in <M>L</M>, is the subgroup in this group that stabilizes
<M>L'.
These groups are easily described in terms of <M>(&ZZ; / n &ZZ;)^{*}</M>.
Generators of <M>(&ZZ; / n &ZZ;)^{*}</M> can be computed using
<Ref Func="GeneratorsPrimeResidues"/>.
<P/>
In &GAP;, a field extension <M>L / L' is given by the field
object <M>L</M> with <Ref Attr="LeftActingDomain"/> value <M>L'
(see <Ref Sect="Integral Bases of Abelian Number Fields"/>).
<P/>
<Example><![CDATA[
gap> f:= CF(15);
CF(15)
gap> g:= GaloisGroup( f );
<group with 2 generators>
gap> Size( g ); IsCyclic( g ); IsAbelian( g );
8
false
true
gap> Action( g, NormalBase( f ), OnPoints );
Group([ (1,6)(2,4)(3,8)(5,7), (1,4,3,7)(2,8,5,6) ])
]]></Example>
<P/>
The following example shows Galois groups of a cyclotomic field
and of a proper subfield that is not a cyclotomic field.
<P/>
<Example><![CDATA[
gap> gens1:= GeneratorsOfGroup( GaloisGroup( CF(5) ) );
[ ANFAutomorphism( CF(5), 2 ) ]
gap> gens2:= GeneratorsOfGroup( GaloisGroup( Field( Sqrt(5) ) ) );
[ ANFAutomorphism( NF(5,[ 1, 4 ]), 2 ) ]
gap> Order( gens1[1] ); Order( gens2[1] );
4
2
gap> Sqrt(5)^gens1[1] = Sqrt(5)^gens2[1];
true
]]></Example>
<P/>
The following example shows the Galois group of a cyclotomic field
over a non-cyclotomic field.
<P/>
<Example><![CDATA[
gap> g:= GaloisGroup( AsField( Field( [ Sqrt(5) ] ), CF(5) ) );
<group of size 2 with 1 generator>
gap> gens:= GeneratorsOfGroup( g );
[ ANFAutomorphism( AsField( NF(5,[ 1, 4 ]), CF(5) ), 4 ) ]
gap> x:= last[1];; x^2;
IdentityMapping( AsField( NF(5,[ 1, 4 ]), CF(5) ) )
]]></Example>
</Description>
</ManSection>
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