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etale morphism of schemes

This entry is about étale morphisms between schemes. The term étale map is preferred in the context of topology and differential geometry, see étalé space for the topological version.

Context

Étale morphisms

Geometry

Contents

Idea

The notion of étale morphism of schemes is the realization of the general notion of étale morphism for maps between schemes, hence it captures roughly the idea of a map of schemes which is a local homeomorphism/local diffeomorphism.

A central use of étale morphisms of schemes is that they appear as coverings in the Grothendieck topology of the étale site. The abelian sheaf cohomology with respect to these étale covers of schemes is accordingly called étale cohomology.

Definition

Definition

A morphism of schemes is an étale morphism if the following equivalent conditions hold:

  1. it is

    1. smooth

    2. unramified

  2. it is

    1. smooth

    2. of relative dimension 00.

  3. it is

    1. flat

    2. unramified;

  4. it is

    1. formally étale;

    2. locally of finite presentation

(A number of other equivalent definitions are listed at wikipedia.)

Remark

For morphisms f:XYf \colon X \longrightarrow Y between algebraic varieties over an algebraically closed field this means that for all points pXp \in X the induced morphism on tangent cones

T pXT f(p)Y T_p X \longrightarrow T_{f(p)} Y

is an isomorphism. This is analogous to the corresponding characterization of local diffeomorphisms of smooth manifolds.

Remark

Relaxing the finiteness condition in item 4 of 1 yields the notion of weakly étale morphism.

étale morphism \Rightarrow pro-étale morphism \Rightarrow weakly étale morphism \Rightarrow formally étale morphism

Definition

A jointly surjective collection of étale morphisms {U iX}\{U_i \to X\} is called an étale cover.

Remark

Most of the pairs of conditions in def. 1 can be read as constraining the fiber of the morphism to be first suitably surjective/bundle-like (smooth, flat) and second suitably locally injective (unramified).

Specifically the first condition has an infinitesimal anlog: a formally étale morphism is a formally smooth and formally unramified morphism. These notions also have an interpretation in synthetic differential geometry and there they correspond to the statement that a local diffeomorphism is a submersion which is also an immersion of smooth manifolds.

Definition

A morphism is formally étale morphism if it is

Remark

These are sheaf-like properties, which can be formalized in the language of Q-categories (monopresheaf and epipresheaf properties on the QQ-category of nilpotent thickenings).

See at differential cohesion and at infinitesimal shape modality.

Properties

Closure properties

Proposition
  • A composite of two étale morphism is itself étale.

  • The pullback of an étale morphism is étale.

  • If f 1f 2f_1 \circ f_2 is étale and f 1f_1 is, then so is f 2f_2.

(e.g. Milne, prop. 2.11)

Proof

Use that an étale morphism is a formally étale morphism with finite fibers, and that f:XYf \colon X \to Y is formally étale precisely if the infinitesimal shape modality unit naturality square

X Π inf(X) Y Π inf(Y) \array{ X &\longrightarrow& \Pi_{inf}(X) \\ \downarrow && \downarrow \\ Y &\longrightarrow& \Pi_{inf}(Y) }

is a pullback square. Then the three properties to be shown are equivalently the pasting law for pullback diagrams.

Proposition

A smooth morphism of schemes is étale iff there is an étale cover of the base scheme by open subschemes such that the pullback of the projection to each of them is an open local isomorphism of locally ringed spaces (and in particular, the pullback of the projection morphism is an étale map of the corresponding underlying topological spaces).

Remark

This disjointness picture of étale covers make them suitable for having nontrivial cohomology in situations where Zariski covers give vanishing cohomology.

Classes of examples

Proposition

Let kk be a field. A morphism of schemes YSpeckY \to Spec k is étale precisely if YY is a coproduct Y iSpeck iY \simeq \coprod_i Spec k_i for each k ik_i a finite and separable field extension of kk.

This appears for instance as de Jong, prop. 3.1 i).

Remark

Such étale morphisms are classified by the classical Galois theory of field extensions.

Proposition

A ring homomorphism of affine varieties Spec(A)Spec(B)Spec(A) \to Spec(B) for Spec(B)Spec(B) non-singular and for AB[x 1,,x n]/(f 1,,f n)A \simeq B[x_1, \cdots, x_n]/(f_1, \cdots, f_n) with polynomials f if_i is étale precisel if the Jacobian det(f ix j)det(\frac{\partial f_i}{\partial x_j}) is invertible.

This appears for instance as (Milne, prop. 2.1).

As locally constant sheaves

Proposition

A sheaf FF on a scheme XX corresponds to an étale morphism YXY \to X precisely if there is an étale cover {U iX}\{U_i \to X\} such that each restriction

F U iLConstK i F|_{U_i} \simeq LConst K_i

is isomorphic to a constant sheaf on a set K iK_i.

A proof is in (Deligne).

Examples

Example

A finite separable field extension KLK \hookrightarrow L corresponds dually to an étale morphism SpecLSpecKSpec L \to Spec K. These are the morphisms classified by classical Galois theory.

Example

Every open immersion of schemes is an étale morphism of schemes. In particular a standard open inclusion (a cover in the Zariski topology) induced by the localization of a commutative ring

Spec(R[S 1])Spec(R) Spec(R[S^{-1}]) \longrightarrow Spec(R)

is étale.

(e.g. Stacks Project, lemma 28.37.9)

Proof

By one of the equivalent characterizations of étale morphism it is sufficient to check that the map Spec(R[S 1])Spec(R)Spec(R[S^{-1}]) \longrightarrow Spec(R) is a formally étale morphism and locally of finite presentation.

The latter is clear, since the very definition of

R[S 1]=R[s 1 1,,s n 1](s 1s 1 11,,s ns n 11) R[S^{-1}] = R[s_1^{-1}, \cdots, s_n^{-1}](s_1 s_1^{-1} - 1, \cdots , s_n s_n^{-1} - 1)

exhibits a finitely presented algebra over RR.

To see that it is formally étale we need to check that for every commutative ring TT with nilpotent ideal JJ we have a pullback diagram

Hom(R[S 1],T) Hom(R[S 1],T/J) Hom(R,T) Hom(R,T/J). \array{ Hom(R[S^{-1}], T) &\longrightarrow& Hom(R[S^{-1}],T/J) \\ \downarrow && \downarrow \\ Hom(R, T) &\longrightarrow& Hom(R, T/J) } \,.

Now by the universal property of the localization, a homomorphism R[S 1]TR[S^{-1}] \longrightarrow T is a homomorphism RTR \longrightarrow T which sends all elements in SRS \hookrightarrow R to invertible elements in TT. But no element in a nilpotent ideal can be invertible, Therefore the fiber product of the bottom and right map is the set of maps from RR to TT such that SS is taken to invertibles, which is indeed the top left set.

References

The classical references are

  • Pierre Deligne et al., Cohomologie étale , Lecture Notes in Mathematics, no. 569, Springer-Verlag, 1977.

Lecture notes include

Discussion of etale morphisms between E-infinity rings/spectral schemes is in

and generally in E-∞ geometry in

Revised on December 5, 2013 02:47:41 by Urs Schreiber (82.113.99.73)