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Algebraic extension

In abstract algebra, a field extension L/K is called algebraic if every element of L is algebraic over K, i.e. if every element of L is a root of some non-zero polynomial with coefficients in K. Field extensions which are not algebraic, i.e. which contain transcendental elements, are called transcendental.

For example, the field extension R/Q is transcendental, while the field extensions C/R and Q(√2)/Q are algebraic.

If L is regarded as a vector space over K, one can consider its dimension as such. This dimension is also called the degree of the extension. The extension L/K can then be further classified as a finite or infinite extension according to this dimension. All transcendental extensions are of infinite degree. This in turn implies that all finite extensions are algebraic.

The converse is not true however: there are infinite extensions which are algebraic. For instance, the field of all algebraic numbers is an infinite algebraic extension of the rational numbers.

If a is algebraic over K, then K[a], the set of all polynomials in a with coefficients in K, is a field. It is an algebraic field extension of K which has finite degree over K. In the special case where K=Q is the field of rational numbers, Q[a] is an example of an algebraic number field.

A field with no proper algebraic extensions is called algebraically closed. An example is the field of complex numbers. Every field has an algebraic extension which is algebraically closed (called its algebraic closure), but proving this in general requires some form of the axiom of choice.

Generalizations

Model theory generalizes the notion of algebraic extension to arbitrary theories: an embedding of M into N is called an algebraic extension if for every x in N there is a formula p with variables in M, such that p(x) is true and the set {y in N | p(y)} is finite. It turns out that applying this definition to the theory of fields gives the usual definition of algebraic extension. The Galois group of N over M can again be defined as the group of automorphisms, and it turns out that most of the theory of Galois groups can be developed for the general case.