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In [[algebraic geometry]], an '''étale morphism''' ({{IPA-fr|eˈtal|pron}}) is a morphism of [[Scheme (mathematics)|schemes]] that is [[formally étale]] and locally of finite presentation. This is an algebraic analogue of the notion of a local isomorphism in the complex analytic topology.  They satisfy the hypotheses of the [[implicit function theorem]], but because open sets in the [[Zariski topology]] are so large, they are not necessarily local isomorphisms.  Despite this, étale maps retain many of the properties of local analytic isomorphisms, and are useful in defining the [[algebraic fundamental group]] and the [[étale topology]].

The word ''étale'' is a French [[adjective]], which means "slack", as in "slack tide", or, figuratively, calm, immobile, something left to settle.<ref>''[[:fr: Trésor de la langue française informatisé]]'', [http://www.cnrtl.fr/definition/%C3%A9tale "étale" article]</ref>

== Definition ==

Let <math>\phi : R \to S</math> be a [[ring homomorphism]].  This makes <math>S</math> an <math>R</math>-algebra.  Choose a [[monic polynomial]] <math>f</math> in <math>R[x]</math> and a polynomial <math>g</math> in <math>R[x]</math> such that the [[Formal derivative|derivative]] <math>f'</math> of <math>f</math> is a unit in <math>(R[x]/fR[x])_g</math>.  We say that <math>\phi</math> is ''standard étale'' if <math>f</math> and <math>g</math> can be chosen so that <math>S</math> is isomorphic as an <math>R</math>-algebra to <math>(R[x]/fR[x])_g</math> and <math>\phi</math> is the canonical map.

Let <math>f : X \to Y</math> be a [[Ringed space#Morphisms|morphism of schemes]].  We say that <math>f</math> is ''étale'' if it has any of the following equivalent properties:
# <math>f</math> is [[flat morphism|flat]] and [[Glossary of scheme theory#Unramified morphism|unramified]].<ref name="Corollaire">EGA IV<sub>4</sub>, Corollaire 17.6.2.</ref>
# <math>f</math> is a [[smooth morphism]] and unramified.<ref name="Corollaire" />
# <math>f</math> is flat, [[Glossary of scheme theory#Locally of finite presentation|locally of finite presentation]], and for every <math>y</math> in <math>Y</math>, the fiber <math>f^{-1}(y)</math> is the disjoint union of points, each of which is the spectrum of a finite separable field extension of the residue field <math>\kappa(y)</math>.<ref name="Corollaire" />
# <math>f</math> is flat, locally of finite presentation, and for every <math>y</math> in <math>Y</math> and every algebraic closure <math>k'</math> of the residue field <math>\kappa(y)</math>, the geometric fiber <math>f^{-1}(y) \otimes_{\kappa(y)} k'</math> is the disjoint union of points, each of which is isomorphic to <math>\mbox{Spec } k'</math>.<ref name="Corollaire" />
# <math>f</math> is a [[smooth morphism]] of relative dimension zero.<ref>EGA IV<sub>4</sub>, Corollaire 17.10.2.</ref>
# <math>f</math> is a smooth morphism and a locally [[quasi-finite morphism]].<ref>EGA IV<sub>4</sub>, Corollaire 17.6.2 and Corollaire 17.10.2.</ref>
# <math>f</math> is locally of finite presentation and is locally a standard étale morphism, that is,
#:For every <math>x</math> in <math>X</math>, let <math>y = f(x)</math>.  Then there is an open affine neighborhood {{nowrap|Spec ''R''}} of <math>y</math> and an open affine neighborhood {{nowrap|Spec ''S''}} of <math>x</math> such that {{nowrap|''f''(Spec ''S'')}} is contained in {{nowrap|Spec ''R''}} and such that the ring homomorphism {{nowrap|''R'' &rarr; ''S''}} induced by <math>f</math> is standard étale.<ref>Milne, ''Étale cohomology'', Theorem 3.14.</ref>
# <math>f</math> is locally of finite presentation and is [[formally étale morphism|formally étale]].<ref name="Corollaire" />
# <math>f</math> is locally of finite presentation and is formally étale for maps from local rings, that is:
#:Let ''A'' be a local ring and ''J'' be an ideal of ''A'' such that {{nowrap|''J''<sup>2</sup> {{=}} 0}}.  Set {{nowrap|''Z'' {{=}} Spec ''A''}}  and {{nowrap|''Z''<sub>0</sub> {{=}} Spec ''A''/''J''}}, and let {{nowrap|''i'' : ''Z''<sub>0</sub> &rarr; ''Z''}} be the canonical closed immersion.  Let ''z'' denote the closed point of ''Z''<sub>0</sub>.  Let {{nowrap|''h'' : ''Z'' &rarr; ''Y''}} and {{nowrap|''g''<sub>0</sub> : ''Z''<sub>0</sub> &rarr; ''X''}} be morphisms such that {{nowrap|''f''(''g''<sub>0</sub>(''z'')) {{=}} ''h''(''i''(''z''))}}.  Then there exists a unique ''Y''-morphism {{nowrap|''g'' : ''Z'' &rarr; ''X''}} such that {{nowrap|''gi'' {{=}} ''g''<sub>0</sub>}}.<ref>EGA IV<sub>4</sub>, Corollaire 17.14.1.</ref>

Assume that <math>Y</math> is locally noetherian and ''f'' is locally of finite type.  For <math>x</math> in <math>X</math>, let <math>y = f(x)</math> and let <math>\hat{\mathcal O}_{Y,y} \to \hat{\mathcal O}_{X,x}</math> be the induced map on [[completion (ring theory)|completed]] local rings.  Then the following are equivalent:
# <math>f</math> is étale.
# For every <math>x</math> in <math>X</math>, the induced map on completed local rings is formally étale for the adic topology.
# For every <math>x</math> in <math>X</math>, <math>\hat{\mathcal O}_{X,x}</math> is a free <math>\hat{\mathcal O}_{Y,y}</math>-module and the fiber <math>\hat{\mathcal O}_{X,x}/m_y\hat{\mathcal O}_{X,x}</math> is a field which is a finite separable field extension of the residue field <math>\kappa(y)</math>.  (Here <math>m_y</math> is the maximal ideal of <math>\hat{\mathcal O}_{Y,y}</math>.)
# ''f'' is formally étale for maps of local rings with the following additional properties.  The local ring ''A'' may be assumed Artinian.  If ''m'' is the maximal ideal of ''A'', then ''J'' may be assumed to satisfy {{nowrap|''mJ'' {{=}} 0}}.  Finally, the morphism on residue fields {{nowrap|&kappa;(''y'') &rarr; ''A'' / ''m''}} may be assumed to be an isomorphism.<ref>EGA IV<sub>4</sub>, Proposition 17.14.2</ref>
If in addition all the maps on residue fields <math>\kappa(y) \to \kappa(x)</math> are isomorphisms, or if <math>\kappa(y)</math> is separably closed, then <math>f</math> is étale if and only if for every <math>x</math> in <math>X</math>, the induced map on completed local rings is an isomorphism.

== Examples of étale morphisms ==

Any [[Glossary of scheme theory#Open immersion|open immersion]] is étale because it is locally an isomorphism. 

Morphisms induced by finite separable field extensions are étale.

Any ring homomorphism of the form <math>R \to S=R[x_1,\ldots,x_n]_g/(f_1,\ldots, f_n)</math>, where all the <math>f_i</math> are polynomials, and where the [[Jacobian matrix and determinant|Jacobian]] determinant <math>\det(\partial f_i/\partial x_j)</math> is a unit in <math>S</math>, is étale.

Expanding upon the previous example, suppose that we have a morphism <math>f</math> of smooth complex algebraic varieties.  Since <math>f</math> is given by equations, we can interpret it as a map of complex manifolds.  Whenever the Jacobian of <math>f</math> is nonzero, <math>f</math> is a local isomorphism of complex manifolds by the [[implicit function theorem]].  By the previous example, having non-zero Jacobian is the same as being étale.

Let <math>f : X\to Y</math> be a dominant morphism of finite type with ''X'', ''Y'' locally noetherian, irreducible and ''Y'' normal. If ''f'' is [[unramified morphism|unramified]], then it is étale.<ref>SGA1, Exposé I, 9.11</ref>

For a field ''K'', any ''K''-algebra ''A'' is necessarily flat. Therefore, ''A'' is an etale algebra if and only if it is unramified, which is also equivalent to
:<math>A \otimes_{K}\bar{K}\cong\bar{K}\oplus ...\oplus\bar{K}, </math>
where <math>\bar K</math> is the [[separable closure]] of the field ''K'' and the right hand side is a finite direct sum, all of whose summands are <math>\bar K</math>. This characterization of etale ''K''-algebras is a stepping stone in reinterpreting classical [[Galois theory]] (see [[Grothendieck's Galois theory]]).

== Properties of étale morphisms ==

* Étale morphisms are preserved under composition and base change.
* Étale morphisms are local on the source and on the base. In other words, <math>f: X\to Y</math> is étale if and only if for each covering of <math>X</math> by open subschemes the restriction of <math>f</math> to each of the open subschemes of the covering is étale, and also if and only if for each cover of <math>Y</math> by open subschemes the induced morphisms <math>f_{(U)} : X \times_Y U \to U</math> is étale for each subscheme <math>U</math> of the covering. In particular, it is possible to test the property of being étale on open affines <math>V=Spec(B)\to U=Spec(A)</math>.
* The product of a finite family of étale morphisms is étale.
* Given a finite family of morphisms <math>\{f_\alpha : X_\alpha \to Y\}</math>, the disjoint union <math>\coprod f_\alpha : \coprod X_\alpha \to Y</math> is étale if and only if each <math>f_\alpha</math> is étale.
* Let <math>f : X \to Y</math> and <math>g : Y \to Z</math>, and assume that <math>g</math> is unramified and <math>gf</math> is étale.  Then <math>f</math> is étale. In particular, if <math>X</math> and <math>X'</math> are étale over <math>Y</math>, then any <math>Y</math>-morphism between <math>X</math> and <math>X'</math> is étale.
* [[Glossary of scheme theory#Notions related to the topological structure|Quasi-compact]] étale morphisms are [[quasi-finite morphism|quasi-finite]].
* A morphism <math>f : X \to Y</math> is an open immersion if and only if it is étale and [[radicial morphism|radicial]].<ref>EGA IV<sub>4</sub>, Théorème 17.9.1.</ref>
* If  <math>f : X \to Y</math> is étale and surjective, then <math>\dim X = \dim Y </math> (finite or otherwise).

==Étale morphisms and the inverse function theorem==
As said in the introduction, étale morphisms 
:''f'': ''X''&nbsp;&rarr;&nbsp;''Y''
are the algebraic counterpart of local [[diffeomorphisms]]. More precisely, a morphism between smooth varieties is étale at a point iff the differential between the corresponding [[tangent space]]s is an isomorphism. This is in turn precisely the condition needed to ensure that a map between [[manifold]]s is a local diffeomorphism, i.e. for any point ''y'' ∈ ''Y'', there is an [[open subset|open]] neighborhood ''U'' of ''x'' such that the restriction of ''f'' to ''U'' is a diffeomorphism. This conclusion does not hold in algebraic geometry, because the topology is too coarse. For example, consider the projection ''f'' of the [[parabola]] 
:''y''&nbsp;=&nbsp;''x''<sup>2</sup>
to the ''y''-axis. This morphism is étale at every point except the origin (0, 0), because the differential is given by 2''x'', which does not vanish at these points. 

However, there is no ([[Zariski topology|Zariski-]])local inverse of ''f'', just because the [[square root]] is not an [[Regular map (algebraic geometry)|algebraic map]], not being given by polynomials. However, there is a remedy for this situation, using the étale topology. The precise statement is as follows:  if <math>f : X\to Y</math> is étale and quasi-compact, then for any point ''y'' lying in ''f''(''X''), there is an étale morphism ''V'' → ''Y'' containing ''y'' in its image (''V'' can be thought of as an étale open neighborhood of ''y''), such that when we base change ''f'' to ''V'', then <math>X\times_Y V\to V</math> (the first member would be the pre-image of ''V'' by ''f'' if ''V'' were a Zariski open neighborhood) is a finite disjoint union of open subsets isomorphic to ''V''. In other words, ''étale-locally'' in ''Y'', the morphism ''f'' is a topological finite cover.

For a smooth morphism <math> f : X\to Y</math> of relative dimension ''n'', ''étale-locally'' in ''X'' and in ''Y'', ''f'' is an open immersion into an affine space <math>\mathbb A^n_Y</math>. This is the étale analogue version of the structure theorem on [[submersion (mathematics)#Local_normal_form|submersions]].

== References ==
{{reflist}}

== Bibliography ==
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*J. S. Milne (2008). ''[http://www.jmilne.org/math/CourseNotes/LEC.pdf Lectures on Etale Cohomology]''

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[[Category:Morphisms of schemes]]