Êtes-vous un étudiant de l'EPFL à la recherche d'un projet de semestre?
Travaillez avec nous sur des projets en science des données et en visualisation, et déployez votre projet sous forme d'application sur GraphSearch.
Concept
Étale fundamental group
Résumé
The étale or algebraic fundamental group is an analogue in algebraic geometry, for schemes, of the usual fundamental group of topological spaces.
In algebraic topology, the fundamental group of a pointed topological space is defined as the group of homotopy classes of loops based at . This definition works well for spaces such as real and complex manifolds, but gives undesirable results for an algebraic variety with the Zariski topology.
In the classification of covering spaces, it is shown that the fundamental group is exactly the group of deck transformations of the universal covering space. This is more promising: finite étale morphisms of algebraic varieties are the appropriate analogue of covering spaces of topological spaces. Unfortunately, an algebraic variety often fails to have a "universal cover" that is finite over , so one must consider the entire category of finite étale coverings of . One can then define the étale fundamental group as an inverse limit of finite automorphism groups.
Let be a connected and locally noetherian scheme, let be a geometric point of and let be the category of pairs such that is a finite étale morphism from a scheme Morphisms in this category are morphisms as schemes over This category has a natural functor to the category of sets, namely the functor
geometrically this is the fiber of over and abstractly it is the Yoneda functor represented by in the category of schemes over . The functor is typically not representable in ; however, it is pro-representable in , in fact by Galois covers of . This means that we have a projective system in , indexed by a directed set where the are Galois covers of , i.e., finite étale schemes over such that . It also means that we have given an isomorphism of functors
In particular, we have a marked point of the projective system.
For two such the map induces a group homomorphism
which produces a projective system of automorphism groups from the projective system . We then make the following definition: the étale fundamental group of at is the inverse limit
with the inverse limit topology.
Source officielle
À propos de ce résultat
Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.
Concepts associés (10)
Étale fundamental group
The étale or algebraic fundamental group is an analogue in algebraic geometry, for schemes, of the usual fundamental group of topological spaces. In algebraic topology, the fundamental group of a pointed topological space is defined as the group of homotopy classes of loops based at . This definition works well for spaces such as real and complex manifolds, but gives undesirable results for an algebraic variety with the Zariski topology.
Glossary of algebraic geometry
This is a glossary of algebraic geometry. See also glossary of commutative algebra, glossary of classical algebraic geometry, and glossary of ring theory. For the number-theoretic applications, see glossary of arithmetic and Diophantine geometry. For simplicity, a reference to the base scheme is often omitted; i.e., a scheme will be a scheme over some fixed base scheme S and a morphism an S-morphism.
Topos (mathématiques)
En mathématiques, un topos (au pluriel topos ou topoï) est un type particulier de catégorie. La théorie des topoï est polyvalente et est utilisée dans des domaines aussi variés que la logique, la topologie ou la géométrie algébrique. Un topos peut être défini comme une catégorie pourvue : de limites et colimites finies ; d'exponentielles ; d'un . D'autres définitions équivalentes sont données plus bas.
Cours associés (6)
MATH-410: Riemann surfaces
This course is an introduction to the theory of Riemann surfaces. Riemann surfaces naturally appear is mathematics in many different ways: as a result of analytic continuation, as quotients of complex
MATH-497: Homotopy theory
We propose an introduction to homotopy theory for topological spaces. We define higher homotopy groups and relate them to homology groups. We introduce (co)fibration sequences, loop spaces, and suspen
MATH-225: Topology
On étudie des notions de topologie générale: unions et quotients d'espaces topologiques; on approfondit les notions de revêtements et de groupe fondamental,et d'attachements de cellules et on démontre