In algebraic geometry, an étale morphism is a morphism of 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.

Definition

Let be a ring homomorphism. This makes S an R-algebra. Choose a monic polynomial f in R[x] and a polynomial g in R[x] such that the derivative f' of f is a unit in. We say that \phi is standard étale if f and g can be chosen so that S is isomorphic as an R-algebra to and \phi is the canonical map. Let f : X \to Y be a morphism of schemes. We say that f is étale if and only if it has any of the following equivalent properties: Assume that Y is locally noetherian and f is locally of finite type. For x in X, let y = f(x) and let be the induced map on completed local rings. Then the following are equivalent: If in addition all the maps on residue fields are isomorphisms, or if \kappa(y) is separably closed, then f is étale if and only if for every x in X, the induced map on completed local rings is an isomorphism.

Examples

Any open immersion is étale because it is locally an isomorphism. Covering spaces form examples of étale morphisms. For example, if d \geq 1 is an integer invertible in the ring R then is a degree d étale morphism. Any ramified covering \pi:X \to Y has an unramified locus which is étale. Morphisms induced by finite separable field extensions are étale — they form arithmetic covering spaces with group of deck transformations given by. Any ring homomorphism of the form, where all the f_i are polynomials, and where the Jacobian determinant is a unit in S, is étale. For example the morphism is etale and corresponds to a degree n covering space of with the group of deck transformations. Expanding upon the previous example, suppose that we have a morphism f of smooth complex algebraic varieties. Since f is given by equations, we can interpret it as a map of complex manifolds. Whenever the Jacobian of f is nonzero, f 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 f : X\to Y be a dominant morphism of finite type with X, Y locally noetherian, irreducible and Y normal. If f is unramified, then it is étale. 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 where \bar K is the separable closure of the field K and the right hand side is a finite direct sum, all of whose summands are \bar K. This characterization of etale K-algebras is a stepping stone in reinterpreting classical Galois theory (see Grothendieck's Galois theory).

Properties

Inverse function theorem

Étale morphisms 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 spaces is an isomorphism. This is in turn precisely the condition needed to ensure that a map between manifolds is a local diffeomorphism, i.e. for any point y ∈ Y, there is an 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 to the y-axis. This morphism is étale at every point except the origin (0, 0), because the differential is given by 2x, which does not vanish at these points. However, there is no (Zariski-)local inverse of f, just because the square root is not an 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 f : X\to Y is étale and finite, then for any point y lying in Y, 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 (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 f : X\to Y of relative dimension n, étale-locally in X and in Y, f is an open immersion into an affine space. This is the étale analogue version of the structure theorem on submersions.