Étale topos

In mathematics, the étale topos of a scheme X is the category of all étale sheaves on X. An étale sheaf is a sheaf on the étale site of X.

Definition

Let X be a scheme. An étale covering of X is a family $\{{\phi }_i:U_i\to X{\}}_{i\in I}$, where each ${\phi }_i$ is an étale morphism of schemes, such that the family is jointly surjective that is $X=\bigcup _{i\in I}{\phi }_i(U_i)$. The category Ét(X) is the category of all étale schemes over X. The collection of all étale coverings of a étale scheme U over X i.e. an object in Ét(X) defines a Grothendieck pretopology on Ét(X) which in turn induces a Grothendieck topology, the étale topology on X. The category together with the étale topology on it is called the étale site on X. The étale topos $X^{\text{ét}}$ of a scheme X is then the category of all sheaves of sets on the site Ét(X). Such sheaves are called étale sheaves on X. In other words, an étale sheaf ${ℱ}$ is a (contravariant) functor from the category Ét(X) to the category of sets satisfying the following sheaf axiom: For each étale U over X and each étale covering $\{{\phi }_i:U_i\to U\}$ of U the sequence

$0\to {ℱ}(U)\to \prod _{i\in I}{ℱ}(U_i)\genfrac{}{}{0ex}{}{\genfrac{}{}{0ex}{}{}{⟶}}{\genfrac{}{}{0ex}{}{⟶}{}}\prod _{i,j\in I}{ℱ}(U_{ij})$

is exact, where $U_{ij}=U_i{\times }_U{U}_j$.