In mathematics, a Henselian ring (or Hensel ring) is a local ring in which Hensel's lemma holds. They were defined by Azumaya (1951), who named them after Kurt Hensel.
Some standard references for Hensel rings are (Nagata 1962, Chapter VII), (Raynaud 1970), and (Grothendieck 1967, Chapter 18).
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In this article Henselian rings will be assumed to be commutative, though there is also a theory of non-commutative Henselian rings.
A commutative local ring R with maximal ideal m is called Henselian if Hensel's lemma holds. This means that if P is a monic polynomial in R, then any factorization of its image P in R/m into a product of coprime monic polynomials can be lifted to a factorization in R.
A local ring is Henselian if and only if every finite ring extension is a product of local rings.
A Henselian ring is called strict, if its residue field is separably closed.
Henselian rings are the local rings of "points" with respect to the Nisnevich topology, so the spectra of these rings do not admit non-trivial connected coverings with respect to the Nisnevich topology. Likewise strict Henselian rings are the local rings of "points" of the étale topology.
For any local ring A there is a universal Henselian ring B generated by A, called the Henselization of A, such that any local homomorphism from A to a Henselian ring can be extended uniquely to B. The Henselization of A is unique up to unique isomorphism. The Henselization of A is an algebraic substitute for the completion of A. The Henselization of A has the same completion and residue field as A and is a flat module over A. If A is Noetherian, reduced, normal, regular, or excellent then so is its Henselization.
Similarly there is a strictly Henselian ring generated by A, called the strict Henselization of A (which is not quite universal: it is unique, but only up to non-unique isomorphism).
Example. The Henselization of the ring of polynomials k[x,y,...] localized at the point (0,0,...) is the ring of algebraic formal power series (the formal power series satisfying an algebraic equation). This can be thought of as the "algebraic" part of the completion.
Example A strict Henselization of the field of p-adic numbers is given by the maximal unramified extension, generated by all roots of unity of order prime to p. It is not "universal" as it has non-trivial automorphisms.