nLab field extension

This page is about adjunctions of a set to a field in field theory. For the notion of adjunction in 2-category theory, see adjunction.

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Idea

If a subset $k$ of a field $K$ is a subfield, then we call the larger field $K$ an extension of the smaller field $k$.

More generally, if $k \to K$ is any ring homomorphism between fields, then it must be an injection, so we may treat it as a field extension.

Adjunctions of a set to a field

Let $k$ be a field, let $K$ be a field extension of $k$, and let $S \subseteq K$ be a subset of $K$. $k \subseteq K$ is also a subset of $K$. Then the adjunction of $S$ to $k$, or the field generated by $S$ over $k$, is the initial subfield $k(S) \subseteq K$ such that $k \subseteq k(S)$ and $S \subseteq k(S)$.

Properties

• Galois theory

• class field theory

Factorization system

Every field extension can be factorized as a purely transcendental extension followed by an algebraic extension. Indeed, by Zorn's lemma, we may construct a transcendence basis (i.e. maximal algebraically independent set) $B$, and the purely transcendental part is the subfield generated by $B$.

Unfortunately, this does not yield an orthogonal factorization system: given a field $K$, we may form the field $K (x)$ of rational functions over $K$, which is a purely transcendental extension of $K$, and we may form the algebraic closure $\overline{K (x)}$, which is an algebraic extension of $K (x)$; but we have the following commutative diagram,

$$\array{ K & \to & K (x^2) \\ \downarrow & & \downarrow \\ K (x) & \to & \overline{K (x)} }$$

where $K (x^2)$ is the subfield of $K (x)$ generated by $x^2$, and $K (x^2) \to \overline{K (x)}$ is algebraic, yet there is no homomorphism $K (x) \to K (x^2)$ making both evident triangles commute.

Examples

• cyclotomic field extension

• anti-cyclotomic field extension

Related concepts

• transcendence degree

• separable field extension

• regular field extension

• ramified extension

• Schur index

• group extension

• algebraic extension

References

• Wikipedia, Field extension