[ref] 56 Fields and Division Rings

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# 56 Fields and Division Rings

### Sections

1. Generating Fields
2. Subfields of Fields
3. Galois Action

A division ring is a ring (see Chapter Rings) in which every non-zero element has an inverse. The most important class of division rings are the commutative ones, which are called fields.

GAP supports finite fields (see Chapter Finite Fields) and abelian number fields (see Chapter Abelian Number Fields), in particular the field of rationals (see Chapter Rational Numbers).

This chapter describes the general GAP functions for fields and division rings.

If a field F is a subfield of a commutative ring C, C can be considered as a vector space over the (left) acting domain F (see Chapter Vector Spaces). In this situation, we call F the field of definition of C.

Each field in GAP is represented as a vector space over a subfield (see IsField), thus each field is in fact a field extension in a natural way, which is used by functions such as `Norm` and `Trace` (see Galois Action).

## 56.1 Generating Fields

- `IsDivisionRing(` D`) C`

  A division ring in GAP is a nontrivial associative algebra D with a multiplicative inverse for each nonzero element. In GAP every division ring is a vector space over a division ring (possibly over itself). Note that being a division ring is thus not a property that a ring can get, because a ring is usually not represented as a vector space.

  The field of coefficients is stored as `LeftActingDomain(` D`)`.

- `IsField(` D`) P`

  A field is a commutative division ring (see IsDivisionRing and IsCommutative).

      gap> IsField( GaloisField(16) );           # the field with 16 elements
      true
      gap> IsField( Rationals );                 # the field of rationals
      true
      gap> q:= QuaternionAlgebra( Rationals );;  # a noncommutative division ring
      gap> IsField( q );  IsDivisionRing( q );
      false
      true
      gap> mat:= [ [ 1 ] ];;  a:= Algebra( Rationals, [ mat ] );;
      gap> IsDivisionRing( a );   # an algebra not constructed as a division ring
      false

- `Field(` z`, ... ) F`
- `Field(` list`) F`
- `Field(` F`,` list`) F`

  `Field` returns the smallest field K that contains all the elements z , ¼, or the smallest field K that contains all elements in the list list. If no subfield F is given, K is constructed as a field over itself, i.e. the left acting domain of K is K. In the third form, `Field` constructs the field generated by the field F and the elements in the list list, as a vector space over F.

- `DefaultField(` z`, ... ) F`
- `DefaultField(` list`) F`

  `DefaultField` returns a field K that contains all the elements z , ¼, or a field K that contains all elements in the list list.

  This field need not be the smallest field in which the elements lie, cf. `Field` (see Field). For example, for elements from cyclotomic fields `DefaultField` returns the smallest cyclotomic field in which the elements lie, but the elements may lie in a smaller number field which is not a cyclotomic field.

      gap> Field( Z(4) );  Field( [ Z(4), Z(8) ] );  # finite fields
      GF(2^2)
      GF(2^6)
      gap> Field( E(9) );  Field( CF(4), [ E(9) ] ); # abelian number fields
      CF(9)
      AsField( GaussianRationals, CF(36) )
      gap> f1:= Field( EB(5) );  f2:= DefaultField( EB(5) );
      NF(5,[ 1, 4 ])
      CF(5)
      gap> f1 = f2;  IsSubset( f2, f1 );
      false
      true

- `DefaultFieldByGenerators( [` z`, ... ] ) O`

  returns the default field containing the elements z,¼. This field may be bigger than the smallest field containing these elements.

- `GeneratorsOfDivisionRing(` D`) A`

  generators with respect to addition, multiplication, and taking inverses (the identity cannot be omitted ...)

- `GeneratorsOfField(` F`) A`

  generators with respect to addition, multiplication, and taking inverses. This attribute is the same as `GeneratorsOfDivisionRing` (see GeneratorsOfDivisionRing).

- `DivisionRingByGenerators( [` z`, ... ] ) O`
- `DivisionRingByGenerators(` F`, [` z`, ... ] ) O`

  The first version returns a division ring as vector space over `FieldOverItselfByGenerators(` gens`)`.

- `AsDivisionRing(` C`) O`
- `AsDivisionRing(` F`,` C`) O`
- `AsField(` C`) O`
- `AsField(` F`,` C`) O`

  If the collection C can be regarded as a division ring then `AsDivisionRing(` C`)` is the division ring that consists of the elements of C, viewed as a vector space over its prime field; otherwise `fail` is returned.

  In the second form, if F is a division ring contained in C then the returned division ring is viewed as a vector space over F.

  `AsField` is just a synonym for `AsDivisionRing`.

  ## 56.2 Subfields of Fields

- `Subfield(` F`,` gens`) F`
- `SubfieldNC(` F`,` gens`) F`

  Constructs the subfield of F generated by gens.

- `FieldOverItselfByGenerators( [` z`, ... ] ) O`

  This operation is needed for the call of `Field` or `FieldByGenerators` without explicitly given subfield, in order to construct a left acting domain for such a field.

- `PrimitiveElement(` D`) A`

  is an element of D that generates D as a division ring together with the left acting domain.

- `PrimeField(` D`) A`

  The prime field of a division ring D is the smallest field which is contained in D. For example, the prime field of any field in characteristic zero is isomorphic to the field of rational numbers.

- `IsPrimeField(` D`) P`

  A division ring is a prime field if it is equal to its prime field (see PrimeField).

- `DegreeOverPrimeField(` F`) A`

  is the degree of the field F over its prime field (see PrimeField).

- `DefiningPolynomial(` F`) A`

  is the defining polynomial of the field F as a field extension over the left acting domain of F. A root of the defining polynomial can be computed with `RootOfDefiningPolynomial` (see RootOfDefiningPolynomial).

- `RootOfDefiningPolynomial(` F`) A`

  is a root in the field F of its defining polynomial as a field extension over the left acting domain of F. The defining polynomial can be computed with `DefiningPolynomial` (see DefiningPolynomial).

- `FieldExtension(` F`,` poly`) O`

  is the field obtained on adjoining a root of the irreducible polynomial poly to the field F.

- `Subfields(` F`) A`

  is the set of all subfields of the field F.

  ## 56.3 Galois Action

  Let L > K be a field extension of finite degree. Then to each element a Î L, we can associate a K-linear mapping ja on L, and for a fixed K-basis of L, we can associate to a the matrix Ma (over K) of this mapping.

  The norm of a is defined as the determinant of Ma, the trace of a is defined as the trace of Ma, the minimal polynomial ma and the trace polynomial ca of a are defined as the minimal polynomial (see MinimalPolynomial!over a field) and the characteristic polynomial (see CharacteristicPolynomial and TracePolynomial) of Ma. (Note that ma depends only on K whereas ca depends on both L and K.)

  Thus norm and trace of a are elements of K, and ma and ca are polynomials over K, ca being a power of ma, and the degree of ca equals the degree of the field extension L > K.

  The conjugates of a in L are those roots of ca (with multiplicity) that lie in L; note that if only L is given, there is in general no way to access the roots outside L.

  Analogously, the Galois group of the extension L > K is defined as the group of all those field automorphisms of L that fix K pointwise.

  If L > K is a Galois extension then the conjugates of a are all roots of ca (with multiplicity), the set of conjugates equals the roots of ma, the norm of a equals the product and the trace of a equals the sum of the conjugates of a, and the Galois group in the sense of the above definition equals the usual Galois group,

  Note that `MinimalPolynomial(` F`,` z`)` is a polynomial over F, whereas `Norm(` F`,` z`)` is the norm of the element z in F w.r.t. the field extension F > `LeftActingDomain``(` F `)`.

- `GaloisGroup(` F`) A`

  The Galois group of a field F is the group of all field automorphisms of F that fix the subfield K = `LeftActingDomain``(` F `)` pointwise.

  Note that the field extension F > K need not be a Galois extension.

      gap> g:= GaloisGroup( AsField( GF(2^2), GF(2^12) ) );;
      gap> Size( g );  IsCyclic( g );
      6
      true
      gap> h:= GaloisGroup( CF(60) );;
      gap> Size( h );  IsAbelian( h );
      16
      true

- `MinimalPolynomial(` F`,` z`[,` ind`] ) O`

  returns the minimal polynomial of z over the field F. This is a generator of the ideal in F [x] of all polynomials which vanish on z. (This definition is consistent with the general definition of `MinimalPolynomial` for rings, see MinimalPolynomial.)

      gap> MinimalPolynomial( Rationals, E(8) );
      1+x_1^4
      gap> MinimalPolynomial( CF(4), E(8) );
      -E(4)+x_1^2
      gap> MinimalPolynomial( CF(8), E(8) );
      -E(8)+x_1

- `TracePolynomial(` L`,` K`,` z`[,` inum`] ) O`

  returns the polynomial that is the product of (X - c) where c runs over the conjugates of z in the field extension L over K. The polynomial is returned as a univariate polynomial over K in the indeterminate number inum (defaulting to 1).

  This polynomial is sometimes also called the characteristic polynomial of z w.r.t. the field extension L > K . Therefore methods are installed for `CharacteristicPolynomial` (see CharacteristicPolynomial) that call `TracePolynomial` in the case of field extensions.

      gap> TracePolynomial( CF(8), Rationals, E(8) );
      1+x_1^4
      gap> TracePolynomial( CF(16), Rationals, E(8) );
      1+2*x_1^4+x_1^8

- `Norm(` z`) A`
- `Norm(` L`,` z`) O`
- `Norm(` L`,` K`,` z`) O`

  `Norm` returns the norm of the field element z. If two fields L and K are given then the norm is computed w.r.t. the field extension L > K , if only one field L is given then `LeftActingDomain(` L`)` is taken as default for the subfield K, and if no field is given then `DefaultField(` z`)` is taken as default for L.

- `Trace(` z`) A`
- `Trace(` mat`) A`
- `Trace(` L`,` z`) O`
- `Trace(` L`,` K`,` z`) O`

  `Trace` returns the trace of the field element z. If two fields L and K are given then the trace is computed w.r.t. the field extension L > K , if only one field L is given then `LeftActingDomain(` L`)` is taken as default for the subfield K, and if no field is given then `DefaultField(` z`)` is taken as default for L.

  The trace of a matrix is the sum of its diagonal entries. Note that this is not compatible with the definition of `Trace` for field elements, so the one-argument version is not suitable when matrices shall be regarded as field elements.

- `Conjugates(` z`) A`
- `Conjugates(` L`,` z`) O`
- `Conjugates(` L`,` K`,` z`) O`

  `Conjugates` returns the list of conjugates of the field element z. If two fields L and K are given then the conjugates are computed w.r.t. the field extension L > K , if only one field L is given then `LeftActingDomain(` L`)` is taken as default for the subfield K, and if no field is given then `DefaultField(` z`)` is taken as default for L.

  The result list will contain duplicates if z lies in a proper subfield of L, respectively of the default field of z. The result list need not be sorted.

      gap> Norm( E(8) );  Norm( CF(8), E(8) );
      1
      1
      gap> Norm( CF(8), CF(4), E(8) );
      -E(4)
      gap> Norm( AsField( CF(4), CF(8) ), E(8) );
      -E(4)
      gap> Trace( E(8) );  Trace( CF(8), CF(8), E(8) );
      0
      E(8)
      gap> Conjugates( CF(8), E(8) );
      [ E(8), E(8)^3, -E(8), -E(8)^3 ]
      gap> Conjugates( CF(8), CF(4), E(8) );
      [ E(8), -E(8) ]
      gap> Conjugates( CF(16), E(8) );
      [ E(8), E(8)^3, -E(8), -E(8)^3, E(8), E(8)^3, -E(8), -E(8)^3 ]

  The default methods for field elements are as follows. `MinimalPolynomial` solves a system of linear equations, `TracePolynomial` computes the appropriate power of the minimal polynomial, `Norm` and `Trace` values are obtained as coefficients of the characteristic polynomial, and `Conjugates` uses the factorization of the characteristic polynomial.

  For elements in finite fields and cyclotomic fields, one wants to do the computations in a different way since the field extensions in question are Galois extensions, and the Galois groups are well-known in these cases. More general, if a field is in the category `IsFieldControlledByGaloisGroup` then the default methods are the following. `Conjugates` returns the sorted list of images (with multiplicity) of the element under the Galois group, `Norm` computes the product of the conjugates, `Trace` computes the sum of the conjugates, `TracePolynomial` and `MinimalPolynomial` compute the product of linear factors x - c with c ranging over the conjugates and the set of conjugates, respectively.

- `NormalBase(` F`) A`
- `NormalBase(` F`,` elm`) O`

  Let F be a field that is a Galois extension of its subfield `LeftActingDomain(` F`)`. Then `NormalBase` returns a list of elements in F that form a normal basis of F, that is, a vector space basis that is closed under the action of the Galois group (see GaloisGroup!of field) of F.

  If a second argument elm is given, it is used as a hint for the algorithm to find a normal basis with the algorithm described in Art68.

      gap> NormalBase( CF(5) );
      [ -E(5), -E(5)^2, -E(5)^3, -E(5)^4 ]
      gap> NormalBase( CF(4) );
      [ 1/2-1/2*E(4), 1/2+1/2*E(4) ]

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  GAP 4 manual
  May 2002