# Hensel ring

A commutative local ring to which the Hensel lemma applies, or, according to another definition, to which the implicit function theorem applies. For a local ring with maximal ideal the former definition means that for any unitary polynomial and a simple solution of the equation ( ) (i.e. and ) there exists an such that and ( ).
Examples of Hensel rings include complete local rings, rings of convergent power series (and, in a general sense, analytic rings, cf. Analytic ring), and the ring of algebraic power series (i.e. series from which are algebraic over ). A local ring that is integral over a Hensel ring is a Hensel ring; in particular, a quotient ring of a Hensel ring is a Hensel ring. For any local ring there exists a general construct — a local Hensel -algebra such that for any local Hensel -algebra there exists a unique homomorphism of -algebras . The algebra of a local ring is a strictly-flat -module, will be a maximal ideal of , the residue fields of and are canonically isomorphic, and the completions of and (in the topologies of the local rings) coincide. Thus, the ring of algebraic power series in is a Hensel -algebra for . If is a Noetherian (or, respectively, reduced, normal, regular, excellent) ring, so is . Conversely, if is an integral ring, need not be integral; more exactly, there exists a bijective correspondence between the maximal ideals of the integral closure of and the minimal prime ideals of .
A Hensel ring with a separably-closed residue field is called strictly local (or strictly Henselian), owing to the locality of its spectrum in the étale topology of schemes; in a manner similar to the construction of the Hensel -algebra there is a strict Hensel -algebra functor . The concept of a Hensel ring may be introduced for a semi-local ring and even, in a more-general sense, for the pair ring–ideal.
A Hensel ring may be described as a ring over which any finite algebra is a direct sum of local rings. Hensel rings were introduced in ; the general theory of Hensel rings and the construction of Hensel -algebras are developed in .
In the theory of étale morphisms and étale topology a Hensel -algebra is understood to be the inductive limit of étale extensions of the ring. In a commutative algebra a Hensel -algebra often replaces the operation of completion, which plays an important role in local studies of objects.