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.
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