[ref] 65 Algebraic extensions of fields

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# 65 Algebraic extensions of fields

### Sections

1. Creation of Algebraic Extensions
2. Elements in Algebraic Extensions

If we adjoin a root a of an irreducible polynomial f Î K[x] to the field K we get an algebraic extension K(a), which is again a field. We call K the base field of K(a).

By Kronecker's construction, we may identify K(a) with the factor ring K[x]/(f), an identification that also provides a method for computing in these extension fields.

It is important to note that different extensions of the same field are entirely different (and its elements lie in different families), even if mathematically one could be embedded in the other one.

Currently GAP only allows extension fields of fields K, when K itself is not an extension field.

## 65.1 Creation of Algebraic Extensions

- `AlgebraicExtension(` K`,` f`) O`

  constructs an extension L of the field K by one root of the irreducible polynomial f, using Kronecker's construction. L is a field whose `LeftActingDomain` is K. The polynomial p is the `DefiningPolynomial` of L and the attribute `RootOfDefiningPolynomial` of L holds a root of f in L (see RootOfDefiningPolynomial).

      gap> x:=Indeterminate(Rationals,"x");;
      gap> p:=x^4+3*x^2+1;;
      gap> e:=AlgebraicExtension(Rationals,p);
      <field in characteristic 0>
      gap> IsField(e);
      true
      gap> a:=RootOfDefiningPolynomial(e);
      (a)

- `IsAlgebraicExtension(` obj`) C`

  is the category of algebraic extensions of fields.

      gap> IsAlgebraicExtension(e);
      true
      gap> IsAlgebraicExtension(Rationals);
      false

  ## 65.2 Elements in Algebraic Extensions

  According to Kronecker's construction, the elements of an algebraic extension considered to be polynomials in the primitive element. The elements of the base field are represented as polynomials of degree 0. GAP therefore displays elements of an algebraic extension as polynomials in an indeterminate ``a'', which is a root of the defining polynomial of the extension. Polynomials of degree 0 are displayed with a leading exclamation mark to indicate that they are different from elements of the base field.

  The usual field operations are applicable to algebraic elements.

      gap> a^3/(a^2+a+1);
      (-1/2*a+1/2*a^2-1/2*a^3)
      gap> a*(1/a);
      !1

  The external representation of algebraic extension elements are the polynomial coefficients in the primitive element `a`, the operations `ExtRepOfObj` and `ObjByExtRep` can be used for conversion.

      gap> ExtRepOfObj(One(a));
      [ 1, 0, 0, 0 ]
      gap> ExtRepOfObj(a^3+2*a-9);
      [ -9, 2, 0, 1 ]
      gap> ObjByExtRep(FamilyObj(a),[3,19,-27,433]);
      (3+19*a-27*a^2+433*a^3)

  GAP does not embed the base field in its algebraic extensions and therefore lists which contain elements of the base field and of the extension are not homogeneous and thus cannot be used as polynomial coefficients or to form matrices. The remedy is to multiply the list(s) with the `One` of the extension which will embed all entries in the extension.

      gap> m:=[[1,a],[0,1]];
      [ [ 1, (a) ], [ 0, 1 ] ]
      gap> IsMatrix(m);
      false
      gap> m:=m*One(e);
      [ [ !1, (a) ], [ !0, !1 ] ]
      gap> IsMatrix(m);
      true
      gap> m^2;
      [ [ !1, (2*a) ], [ !0, !1 ] ]

- `IsAlgebraicElement(` obj`) C`

  is the category for elements of an algebraic extension.

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  GAP 4 manual
  May 2002