In algebraic topology, the fundamental group π1 of a pointed topological space is defined as the group of homotopy classes of loops based at x. 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 are the appropriate analogue of covering spaces. Unfortunately, an algebraic variety X often fails to have a "universal cover" that is finite over X, so one must consider the entire category of finite étale coverings of X. One can then define the étale fundamental group as an inverse limit of finite automorphism groups.
Formal definition
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 functorrepresented 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. The functor is now a functor from to the category of finite and continuous -sets, and establishes an equivalence of categories between and the category of finite and continuous -sets.
Examples and theorems
The most basic example of a fundamental group is π1, the fundamental group of a fieldk. Essentially by definition, the fundamental group of k can be shown to be isomorphic to the absolute Galois group Gal. More precisely, the choice of a geometric point of Spec is equivalent to giving a separably closedextension fieldK, and the fundamental group with respect to that base point identifies with the Galois group Gal. This interpretation of the Galois group is known as Grothendieck's Galois theory. More generally, for any geometrically connected variety X over a field k there is an exact sequence of profinite groups
For a scheme X that is of finite type over C, the complex numbers, there is a close relation between the étale fundamental group of X and the usual, topological, fundamental group of X, the complex analytic space attached to X. The algebraic fundamental group, as it is typically called in this case, is the profinite completion of π1. This is a consequence of the Riemann existence theorem, which says that all finite étale coverings of X stem from ones of X. In particular, as the fundamental group of smooth curves over C is well understood; this determines the algebraic fundamental group. More generally, the fundamental group of a proper scheme over any algebraically closed field of characteristic zero is known, because an extension of algebraically closed fields induces isomorphic fundamental groups.
For an algebraically closed field k of positive characteristic, the results are different, since Artin–Schreier coverings exist in this situation. For example, the fundamental group of the affine line is not topologically finitely generated. The tame fundamental group of some scheme U is a quotient of the usual fundamental group of U which takes into account only covers that are tamely ramified along D, where X is some compactification and D is the complement of U in X. For example, the tame fundamental group of the affine line is zero.
It turns out that every affine scheme is a -space, in the sense that the etale homotopy type of is entirely determined by its etale homotopy group. Note where is a geometric point.
have introduced a variant of the étale fundamental group called the pro-étale fundamental group. It is constructed by considering, instead of finite étale covers, maps which are both étale and satisfy the valuative criterion of properness. For geometrically unibranch schemes, the two approaches agree, but in general the pro-étale fundamental group is a finer invariant: its profinite completion is the étale fundamental group.
