- Profinite group
In

mathematics ,**profinite groups**aretopological group s that are in a certain sense assembled fromfinite group s; 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 theinverse limit of aninverse system of discretefinite group s. 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"nZ → Z/"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 offield extension s of infinite degree gives rise naturally to Galois groups that are profinite. Specifically, if "L"/"K" is aGalois 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**afterWolfgang 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 whichfinite group s occur as Galois groups over "K". This is theinverse 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 analgebraic variety . Thefundamental group s ofalgebraic 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 thefactor group "G"/"N" is profinite; the topology arising from the profiniteness agrees with thequotient topology .

*Since every profinite group "G" is compact Hausdorff, we have aHaar 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 andDan Segal , in any topologically finitely-generated profinite group (that is, a profinite group that has a densefinitely-generated subgroup ) the subgroups of finite index are open. This generalizes an earlier analogous result ofJean-Pierre Serre for topologically finitely-generatedpro-p group s. The proof uses theclassification 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 thenormal subgroup s 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 followinguniversal 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 thedirect limit of an inductive system of finite groups. The usual terminology is different: a group "G" is called locally finite if every finitely-generatedsubgroup 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 abeliantorsion group s.**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.

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