In field theory, a branch of algebra, a field extension L / k {\displaystyle L/k} is said to be regular if k is algebraically closed in L (i.e., k = k ^ {\displaystyle k={\hat {k}}} where k ^ {\displaystyle {\hat {k}}} is the set of elements in L algebraic over k) and L is separable over k, or equivalently, L ⊗ k k ¯ {\displaystyle L\otimes _{k}{\overline {k}}} is an integral domain when k ¯ {\displaystyle {\overline {k}}} is the algebraic closure of k {\displaystyle k} (that is, to say, L , k ¯ {\displaystyle L,{\overline {k}}} are linearly disjoint over k).
Properties
- Regularity is transitive: if F/E and E/K are regular then so is F/K.3
- If F/K is regular then so is E/K for any E between F and K.4
- The extension L/k is regular if and only if every subfield of L finitely generated over k is regular over k.5
- Any extension of an algebraically closed field is regular.67
- An extension is regular if and only if it is separable and primary.8
- A purely transcendental extension of a field is regular.
Self-regular extension
There is also a similar notion: a field extension L / k {\displaystyle L/k} is said to be self-regular if L ⊗ k L {\displaystyle L\otimes _{k}L} is an integral domain. A self-regular extension is relatively algebraically closed in k.9 However, a self-regular extension is not necessarily regular.
- Fried, Michael D.; Jarden, Moshe (2008). Field arithmetic. Ergebnisse der Mathematik und ihrer Grenzgebiete. 3. Folge. Vol. 11 (3rd revised ed.). Springer-Verlag. pp. 38–41. ISBN 978-3-540-77269-9. Zbl 1145.12001.
- M. Nagata (1985). Commutative field theory: new edition, Shokado. (Japanese) [1]
- Cohn, P. M. (2003). Basic Algebra. Groups, Rings, and Fields. Springer-Verlag. ISBN 1-85233-587-4. Zbl 1003.00001.
- A. Weil, Foundations of algebraic geometry.