Formally étale morphism

In commutative algebra and algebraic geometry, a morphism is called formally étale if it has a lifting property that is analogous to being a local diffeomorphism.

Formally étale homomorphisms of rings

Let A be a topological ring, and let B be a topological A-algebra. Then B is formally étale if for all discrete A-algebras C, all nilpotent ideals J of C, and all continuous A-homomorphisms, there exists a unique continuous A-algebra map such that, where is the canonical projection.
Formally étale is equivalent to formally smooth plus formally unramified.

Since the structure sheaf of a scheme naturally carries only the discrete topology, the notion of formally étale for schemes is analogous to formally étale for the discrete topology for rings. That is, a morphism of schemes is formally étale if for every affine Y-scheme Z, every nilpotent sheaf of ideals J on Z with be the closed immersion determined by J, and every Y-morphism, there exists a unique Y-morphism such that.
It is equivalent to let Z be any Y-scheme and let J be a locally nilpotent sheaf of ideals on Z.

Open immersions are formally étale.
The property of being formally étale is preserved under composites, base change, and fibered products.
If and are morphisms of schemes, g is formally unramified, and gf is formally étale, then f is formally étale. In particular, if g is formally étale, then f is formally étale if and only if gf is.
The property of being formally étale is local on the source and target.
The property of being formally étale can be checked on stalks. One can show that a morphism of rings is formally étale if and only if for every prime Q of B, the induced map is formally étale. Consequently, f is formally étale if and only if for every prime Q of B, the map is formally étale, where.

Localizations are formally étale.
Finite separable field extensions are formally étale. More generally, any flat separable A-algebra B is formally étale.