|
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 , where each is an étale morphism of schemes, such that the family is jointly surjective that is . 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'' 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 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 of ''U'' the sequence : is exact, where . 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Étale topos」の詳細全文を読む スポンサード リンク
|