Regular extension

In field theory, a branch of algebra, a field extension $L/k$ is said to be regular if k is algebraically closed in L (i.e., $k=\hat{k}$ where $\hat{k}$ is the set of elements in L algebraic over k) and L is separable over k, or equivalently, $L{\otimes }_k\bar{k}$ is an integral domain when $\bar{k}$ is the algebraic closure of $k$ (that is, to say, $L,\bar{k}$ are linearly disjoint over k).

Properties

• Regularity is transitive: if F/E and E/K are regular then so is F/K.
• If F/K is regular then so is E/K for any E between F and K.
• The extension L/k is regular if and only if every subfield of L finitely generated over k is regular over k.
• Any extension of an algebraically closed field is regular.
• An extension is regular if and only if it is separable and primary.
• A purely transcendental extension of a field is regular.

Self-regular extension

There is also a similar notion: a field extension $L/k$ is said to be self-regular if $L{\otimes }_{k}L$ is an integral domain. A self-regular extension is relatively algebraically closed in k. However, a self-regular extension is not necessarily regular.

References

• M. Nagata (1985). Commutative field theory: new edition, Shokado. (Japanese) [1]
• A. Weil, Foundations of algebraic geometry.