Profinite group


Profinite group

In mathematics, profinite groups are topological groups that are in a certain sense assembled from finite groups; they share many properties with their finite quotients.

Definition

Formally, a profinite group is a Hausdorff, compact, and totally disconnected topological group.Equivalently, one can define a profinite group to be a topological group that is isomorphic to the inverse limit of an inverse system of discrete finite groups. In categorical terms, this is a special case of a (co)filtered limit construction.

Examples

* Finite groups are profinite, if given the discrete topology.

* The group of "p"-adic integers Z"p" under addition is profinite (in fact procyclic). It is the inverse limit of the finite groups Z/"p""n"Z where "n" ranges over all natural numbers and the natural maps Z/"p"nZZ/"p""m"Z ("n"≥"m") are used for the limit process. The topology on this profinite group is the same as the topology arising from the p-adic valuation on Z"p".

* The Galois theory of field extensions of infinite degree gives rise naturally to Galois groups that are profinite. Specifically, if "L"/"K" is a Galois extension, we consider the group "G" = Gal("L"/"K") consisting of all field automorphisms of "L" which keep all elements of "K" fixed. This group is the inverse limit of the finite groups Gal("F"/"K"), where "F" ranges over all intermediate fields such that "F"/"K" is a "finite" Galois extension. For the limit process, we use the restriction homomorphisms Gal("F"1/"K") → Gal("F"2/"K"), where "F"2 ⊆ "F"1. The topology we obtain on Gal("L"/"K") is known as the Krull topology after Wolfgang Krull. Waterhouse showed that "every" profinite group is isomorphic to one arising from the Galois theory of "some" field "K"; but one cannot (yet) control which field "K" will be in this case. In fact, for many fields "K" one does not know in general precisely which finite groups occur as Galois groups over "K". This is the inverse Galois problem for a field "K". (For some fields "K" the inverse Galois problem is settled, such as the field of rational functions in one variable over the complex numbers.)

* The fundamental groups considered in algebraic geometry are also profinite groups, roughly speaking because the algebra can only 'see' finite coverings of an algebraic variety. The fundamental groups of algebraic topology, however, are in general not profinite.

Properties and facts

*Every product of (arbitrarily many) profinite groups is profinite; the topology arising from the profiniteness agrees with the product topology.
*Every closed subgroup of a profinite group is itself profinite; the topology arising from the profiniteness agrees with the subspace topology. If "N" is a closed normal subgroup of a profinite group "G", then the factor group "G"/"N" is profinite; the topology arising from the profiniteness agrees with the quotient topology.
*Since every profinite group "G" is compact Hausdorff, we have a Haar measure on "G", which allows us to measure the "size" of subsets of "G", compute certain probabilities, and integrate functions on "G".
* A subgroup of a profinite group is open if and only if it is closed and has finite index.
*According to a theorem of Nikolay Nikolov and Dan Segal, in any topologically finitely-generated profinite group (that is, a profinite group that has a dense finitely-generated subgroup) the subgroups of finite index are open. This generalizes an earlier analogous result of Jean-Pierre Serre for topologically finitely-generated pro-p groups. The proof uses the classification of finite simple groups.
*As an easy corollary of the Nikolov-Segal result above, "any" surjective discrete group homomorphism φ: "G" → "H" between profinite groups "G" and "H" is continuous as long as "G" is topologically finitely-generated. Indeed, any open set of "H" is of finite index, so its preimage in "G" is also of finite index, hence it must be open.
*Suppose "G" and "H" are topologically finitely-generated profinite groups which are isomorphic as discrete groups by an isomorphism ι. Then ι is bijective and continuous by the above result. Furthermore, ι−1 is also continuous, so ι is a homeomorphism. Therefore the topology on a topologically finitely-generated profinite group is uniquely determined by its "algebraic" structure.

Profinite completion

Given an arbitrary group "G", there is a related profinite group "G"^, the profinite completion of "G". It is defined as the inverse limit of the groups "G"/"N", where "N" runs through the normal subgroups in "G" of finite index (these normal subgroups are partially ordered by inclusion, which translates into an inverse system of natural homomorphisms between the quotients). There is a natural homomorphism η : "G" → "G"^, and the image of "G" under this homomorphism is dense in "G"^. The homomorphism η is injective if and only if the group "G" is residually finite (i.e.,cap N = 1, where the intersection runs through all normal subgroups of finite index). The homomorphism η is characterized by the following universal property: given any profinite group "H" and any group homomorphism "f" : "G" → "H", there exists a unique continuous group homomorphism "g" : "G"^ → "H" with "f" = "g"η.

Ind-finite groups

There is a notion of ind-finite group, which is the concept dual to profinite groups; i.e. a group "G" is ind-finite if it is the direct limit of an inductive system of finite groups. The usual terminology is different: a group "G" is called locally finite if every finitely-generated subgroup is finite. This is equivalent, in fact, to being 'ind-finite'.

By applying Pontryagin duality, one can see that abelian profinite groups are in duality with locally finite discrete abelian groups. The latter are just the abelian torsion groups.

ee also

*Locally cyclic group
*Pro-p group
*Residual property (mathematics)

References

*Nikolay Nikolov and Dan Segal. "On finitely generated profinite groups I: strong completeness and uniform bounds.". 2006, [http://arxiv.org/abs/math.GR/0604399 online version] .
*Nikolay Nikolov and Dan Segal. "On finitely generated profinite groups II, products in quasisimple groups". 2006, [http://arxiv.org/abs/math.GR/0604400 online version] .
* Hendrik Lenstra: Profinite Groups, talk given in Oberwolfach, November 2003. [http://websites.math.leidenuniv.nl/algebra/Lenstra-Profinite.pdf online version] .
* Alexander Lubotzky: review of several books about profinite groups. Bulletin of the American Mathematical Society, 38 (2001), pages 475-479. [http://www.ams.org/bull/2001-38-04/S0273-0979-01-00914-4/home.html online version] .
* | year=1994 | volume=5
*William C. Waterhouse. "Profinite groups are Galois groups". Proc. Amer. Math. Soc. 42 (1973), pp. 639–640.


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Group theory — is a mathematical discipline, the part of abstract algebra that studies the algebraic structures known as groups. The development of group theory sprang from three main sources: number theory, theory of algebraic equations, and geometry. The… …   Wikipedia

  • Group (mathematics) — This article covers basic notions. For advanced topics, see Group theory. The possible manipulations of this Rubik s Cube form a group. In mathematics, a group is an algebraic structure consisting of a set together with an operation that combines …   Wikipedia

  • Group cohomology — This article is about homology and cohomology of a group. For homology or cohomology groups of a space or other object, see Homology (mathematics). In abstract algebra, homological algebra, algebraic topology and algebraic number theory, as well… …   Wikipedia

  • Absolute Galois group — In mathematics, the absolute Galois group GK of a field K is the Galois group of K sep over K , where K sep is a separable closure of K . Alternatively it is the group of all automorphisms of the algebraic closure of K that fix K . The absolute… …   Wikipedia

  • Glossary of group theory — A group ( G , •) is a set G closed under a binary operation • satisfying the following 3 axioms:* Associativity : For all a , b and c in G , ( a • b ) • c = a • ( b • c ). * Identity element : There exists an e ∈ G such that for all a in G , e •… …   Wikipedia

  • Compact group — In mathematics, a compact (topological, often understood) group is a topological group whose topology is compact. Compact groups are a natural generalisation of finite groups with the discrete topology and have properties that carry over in… …   Wikipedia

  • Pro-p group — In mathematics, a pro p group (for some prime number p) is a profinite group G such that for any open normal subgroup the quotient group G / N is a p group. Note that, as profinite groups are compact, the open subgroup must be of finite index, so …   Wikipedia

  • Topological group — Concepts in group theory category of groups subgroups, normal subgroups group homomorphisms, kernel, image, quotient direct product, direct sum semidirect product, wreath product …   Wikipedia

  • Finite group — In mathematics, a finite group is a group which has finitely many elements. Some aspects of the theory of finite groups were investigated in great depth in the twentieth century, in particular the local theory, and the theory of solvable groups… …   Wikipedia

  • Galois group — In mathematics, a Galois group is a group associated with a certain type of field extension. The study of field extensions (and polynomials which give rise to them) via Galois groups is called Galois theory. The name is for Évariste Galois.For a… …   Wikipedia