### 58 Fields and Division Rings

A division ring is a ring (see Chapter 56) 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 59) and abelian number fields (see Chapter 60), in particular the field of rationals (see Chapter 17).

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 61). 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 (58.1-2)), thus each field is in fact a field extension in a natural way, which is used by functions such as Norm (58.3-4) and Trace (58.3-5) (see 58.3).

#### 58.1 Generating Fields

##### 58.1-1 IsDivisionRing
 ‣ IsDivisionRing( D ) ( category )

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 the value of the attribute LeftActingDomain (57.1-11) of D.

##### 58.1-2 IsField
 ‣ IsField( D ) ( filter )

A field is a commutative division ring (see IsDivisionRing (58.1-1) and IsCommutative (35.4-9)).

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


##### 58.1-3 Field
 ‣ Field( z, ... ) ( function )
 ‣ Field( [F, ]list ) ( function )

Field returns the smallest field K that contains all the elements z, ..., 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. Called with a field F and a list list, Field constructs the field generated by F and the elements in list, as a vector space over F.

##### 58.1-4 DefaultField
 ‣ DefaultField( z, ... ) ( function )
 ‣ DefaultField( list ) ( function )

DefaultField returns a field K that contains all the elements z, ..., 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 (58.1-3). 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


##### 58.1-5 DefaultFieldByGenerators
 ‣ DefaultFieldByGenerators( [z, ...] ) ( operation )

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

##### 58.1-6 GeneratorsOfDivisionRing
 ‣ GeneratorsOfDivisionRing( D ) ( attribute )

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

##### 58.1-7 GeneratorsOfField
 ‣ GeneratorsOfField( F ) ( attribute )

generators with respect to addition, multiplication, and taking inverses. This attribute is the same as GeneratorsOfDivisionRing (58.1-6).

##### 58.1-8 DivisionRingByGenerators
 ‣ DivisionRingByGenerators( [F, ]gens ) ( operation )
 ‣ FieldByGenerators( [F, ]gens ) ( operation )

Called with a field F and a list gens of scalars, DivisionRingByGenerators returns the division ring over F generated by gens. The unary version returns the division ring as vector space over FieldOverItselfByGenerators( gens ).

FieldByGenerators is just a synonym for DivisionRingByGenerators.

##### 58.1-9 AsDivisionRing
 ‣ AsDivisionRing( [F, ]C ) ( operation )
 ‣ AsField( [F, ]C ) ( operation )

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.

#### 58.2 Subfields of Fields

##### 58.2-1 Subfield
 ‣ Subfield( F, gens ) ( function )
 ‣ SubfieldNC( F, gens ) ( function )

Constructs the subfield of F generated by gens.

##### 58.2-2 FieldOverItselfByGenerators
 ‣ FieldOverItselfByGenerators( [z, ...] ) ( operation )

This operation is needed for the call of Field (58.1-3) or FieldByGenerators (58.1-8) without explicitly given subfield, in order to construct a left acting domain for such a field.

##### 58.2-3 PrimitiveElement
 ‣ PrimitiveElement( D ) ( attribute )

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

##### 58.2-4 PrimeField
 ‣ PrimeField( D ) ( attribute )

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.

##### 58.2-5 IsPrimeField
 ‣ IsPrimeField( D ) ( property )

A division ring is a prime field if it is equal to its prime field (see PrimeField (58.2-4)).

##### 58.2-6 DegreeOverPrimeField
 ‣ DegreeOverPrimeField( F ) ( attribute )

is the degree of the field F over its prime field (see PrimeField (58.2-4)).

##### 58.2-7 DefiningPolynomial
 ‣ DefiningPolynomial( F ) ( attribute )

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 (58.2-8).

##### 58.2-8 RootOfDefiningPolynomial
 ‣ RootOfDefiningPolynomial( F ) ( attribute )

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 (58.2-7).

##### 58.2-9 FieldExtension
 ‣ FieldExtension( F, poly ) ( operation )

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

##### 58.2-10 Subfields
 ‣ Subfields( F ) ( attribute )

is the set of all subfields of the field F.

#### 58.3 Galois Action

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

The norm of α is defined as the determinant of M_α, the trace of α is defined as the trace of M_α, the minimal polynomial μ_α and the trace polynomial χ_α of α are defined as the minimal polynomial (see 58.3-2) and the characteristic polynomial (see CharacteristicPolynomial (24.13-1) and TracePolynomial (58.3-3)) of M_α. (Note that μ_α depends only on K whereas χ_α depends on both L and K.)

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

The conjugates of α in L are those roots of χ_α (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 α are all roots of χ_α (with multiplicity), the set of conjugates equals the roots of μ_α, the norm of α equals the product and the trace of α equals the sum of the conjugates of α, 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 ).

The default methods for field elements are as follows. MinimalPolynomial (66.8-1) solves a system of linear equations, TracePolynomial (58.3-3) computes the appropriate power of the minimal polynomial, Norm (58.3-4) and Trace (58.3-5) values are obtained as coefficients of the characteristic polynomial, and Conjugates (58.3-6) 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 (58.3-6) returns the sorted list of images (with multiplicity) of the element under the Galois group, Norm (58.3-4) computes the product of the conjugates, Trace (58.3-5) computes the sum of the conjugates, TracePolynomial (58.3-3) and MinimalPolynomial (66.8-1) compute the product of linear factors x - c with c ranging over the conjugates and the set of conjugates, respectively.

##### 58.3-1 GaloisGroup
 ‣ GaloisGroup( F ) ( attribute )

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


##### 58.3-2 MinimalPolynomial
 ‣ MinimalPolynomial( F, z[, ind] ) ( operation )

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 (66.8-1) for rings.)

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


##### 58.3-3 TracePolynomial
 ‣ TracePolynomial( L, K, z[, inum] ) ( operation )

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 (24.13-1) that call TracePolynomial in the case of field extensions.

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


##### 58.3-4 Norm
 ‣ Norm( [L, [K, ]]z ) ( attribute )

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.

##### 58.3-5 Traces of field elements and matrices
 ‣ Trace( [L, [K, ]]z ) ( attribute )
 ‣ Trace( mat ) ( attribute )

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.

##### 58.3-6 Conjugates
 ‣ Conjugates( [L, [K, ]]z ) ( attribute )

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, or of the default field of z, respectively. 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 ]


##### 58.3-7 NormalBase
 ‣ NormalBase( F[, elm] ) ( attribute )

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 (58.3-1)) 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 [Art73].

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) ]
gap> NormalBase( GF(3^6) );
[ Z(3^6)^2, Z(3^6)^6, Z(3^6)^18, Z(3^6)^54, Z(3^6)^162, Z(3^6)^486 ]
gap> NormalBase( GF( GF(8), 2 ) );
[ Z(2^6), Z(2^6)^8 ]


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