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In mathematics, the '''Golod–Shafarevich theorem''' was proved in 1964 by two [[Russia]]n mathematicians, [[Evgeny Golod]] and [[Igor Shafarevich]]. It is a result in non-commutative [[homological algebra]] which has consequences in various branches of [[algebra]].
 
==The inequality==
Let ''A'' = ''K''<''x''<sub>1</sub>, ..., ''x''<sub>''n''</sub>> be the [[free algebra]] over a [[field (mathematics)|field]] ''K''  in ''n'' = ''d''&nbsp;+&nbsp;1 non-commuting variables ''x''<sub>''i''</sub>.
 
Let ''J'' be the 2-sided ideal of ''A'' generated by homogeneous elements ''f''<sub>''j''</sub> of ''A'' of degree ''d''<sub>''j''</sub> with
:2 ≤ ''d''<sub>1</sub> ≤ ''d''<sub>2</sub> ≤ ...
 
where ''d''<sub>''j''</sub> tends to infinity. Let ''r''<sub>''i''</sub> be the number of ''d''<sub>''j''</sub> equal to ''i''.
 
Let ''B''=''A''/''J'', a [[graded algebra]]. Let ''b''<sub>''j''</sub> = dim ''B''<sub>''j''</sub>.  
 
The ''fundamental inequality'' of Golod and Shafarevich states that
 
:: <math> b_j\ge nb_{j-1} -\sum_{i=2}^{j} b_{j-i} r_i.</math>
 
As a consequence:
 
* ''B'' is infinite-dimensional if ''r''<sub>''i''</sub> ≤ ''d''<sup>2</sup>/4 for all ''i''
 
* if ''B'' is finite-dimensional, then ''r''<sub>''i''</sub> > ''d''<sup>2</sup>/4 for some ''i''.
 
==Applications==
This result has important applications in [[combinatorial group theory]]:
 
* If ''G'' is a nontrivial finite [[p-group]], then ''r'' > ''d''<sup>2</sup>/4  where ''d'' = dim&nbsp;''H''<sup>1</sup>(''G'','''Z'''/''p'''''Z''') and ''r'' = dim&nbsp;''H''<sup>2</sup>(''G'','''Z'''/''p'''''Z''') (the mod ''p'' [[group cohomology|cohomology groups]] of ''G'').  In particular if ''G'' is a finite [[p-group]] with minimal number of generators ''d'' and has ''r'' relators in a given presentation, then ''r'' > ''d''<sup>2</sup>/4.
 
* For each prime ''p'', there is an infinite group ''G'' generated by three elements in which each element has order a power of ''p''. The group ''G'' provides a counterexample to the [[Burnside's problem|generalised Burnside conjecture]]: it is a [[Finitely generated group|finitely generated]] infinite [[torsion group]], although there is no uniform bound on the order of its elements.
 
In [[class field theory]], the '''class field tower''' of a [[number field]] ''K'' is created by iterating the [[Hilbert class field]] construction. Another consequence of the construction is that such [[tower of fields|towers]] may be [[infinite tower of fields|infinite]] (in other words, do not always terminate in a field equal to its [[David Hilbert|Hilbert]] class field).  Specifically,
 
* Let ''K'' be an imaginary quadratic field whose [[Discriminant of an algebraic number field|discriminant]] has at least 6 prime factors.  Then the maximal unramified 2-extension of ''K'' has infinite degree.
 
More generally, a number field with sufficiently many prime factors in the discriminant has an infinite class field tower.
 
==References==
*{{citation | last1=Golod| first1= E.S|author1-link=Evgeny Golod|last2=Shafarevich|first2=I.R.|author2-link=Igor Shafarevich|title=On the class field tower|journal=Izv. Akad. Nauk SSSSR|volume=28|year=1964|pages=261–272}} (in [[Russian language|Russian]])  {{MathSciNet|id=0161852}}
*{{citation | last=Golod| first= E.S|authorlink=Evgeny Golod| title=On nil-algebras and finitely approximable p-groups.|journal=Izv. Akad. Nauk SSSSR|volume=28|year=1964|pages=273–276}} (in [[Russian language|Russian]])  {{MathSciNet|id=0161878}}
* {{cite book | last=Herstein | first=I.N. | year=1968 | title=Noncommutative rings | series=Carus Mathematical Monographs | publisher=MAA | isbn=0-88385-039-7 }} See Chapter 8.
* Johnson, D.L. (1980). "Topics in the Theory of Group Presentations" (1st ed.). [[Cambridge University Press]]. ISBN 0-521-23108-6.  See chapter VI.
* {{cite book | first=Helmut | last=Koch | title=Algebraic Number Theory | publisher=[[Springer-Verlag]] | year=1997 | isbn=3-540-63003-1 | zbl=0819.11044 | series=Encycl. Math. Sci. | volume=62 | edition=2nd printing of 1st | page=180 }}
* {{cite book | last=Roquette | first=P.  | year=1967 | chapter=On class field towers | pages=231–249 | title=Algebraic number theory, Proceedings of the instructional conference held at the University of Sussex, Brighton, September 1–17, 1965 | editor1-first=J. W. S. | editor1-last=Cassels | editor1-link=J. W. S. Cassels | editor2-first=A. | editor2-last=Fröhlich | editor2-link=Albrecht Fröhlich | edition=Reprint of the 1967 original | publisher=[[Academic Press]] | location=London | year=1986 | isbn=0-12-163251-2 }}
* [[Jean-Pierre Serre|Serre, J.-P.]] (2002), "Galois Cohomology," [[Springer-Verlag]]. ISBN 3-540-42192-0. See Appendix 2. (Translation of ''Cohomologie Galoisienne'', Lecture Notes in Mathematics '''5''', 1973.)
 
{{DEFAULTSORT:Golod-Shafarevich theorem}}
[[Category:Group theory]]
[[Category:Class field theory]]
[[Category:Theorems in abstract algebra]]

Revision as of 04:11, 21 January 2014

In mathematics, the Golod–Shafarevich theorem was proved in 1964 by two Russian mathematicians, Evgeny Golod and Igor Shafarevich. It is a result in non-commutative homological algebra which has consequences in various branches of algebra.

The inequality

Let A = K<x1, ..., xn> be the free algebra over a field K in n = d + 1 non-commuting variables xi.

Let J be the 2-sided ideal of A generated by homogeneous elements fj of A of degree dj with

2 ≤ d1d2 ≤ ...

where dj tends to infinity. Let ri be the number of dj equal to i.

Let B=A/J, a graded algebra. Let bj = dim Bj.

The fundamental inequality of Golod and Shafarevich states that

bjnbj1i=2jbjiri.

As a consequence:

  • B is infinite-dimensional if rid2/4 for all i
  • if B is finite-dimensional, then ri > d2/4 for some i.

Applications

This result has important applications in combinatorial group theory:

  • If G is a nontrivial finite p-group, then r > d2/4 where d = dim H1(G,Z/pZ) and r = dim H2(G,Z/pZ) (the mod p cohomology groups of G). In particular if G is a finite p-group with minimal number of generators d and has r relators in a given presentation, then r > d2/4.
  • For each prime p, there is an infinite group G generated by three elements in which each element has order a power of p. The group G provides a counterexample to the generalised Burnside conjecture: it is a finitely generated infinite torsion group, although there is no uniform bound on the order of its elements.

In class field theory, the class field tower of a number field K is created by iterating the Hilbert class field construction. Another consequence of the construction is that such towers may be infinite (in other words, do not always terminate in a field equal to its Hilbert class field). Specifically,

  • Let K be an imaginary quadratic field whose discriminant has at least 6 prime factors. Then the maximal unramified 2-extension of K has infinite degree.

More generally, a number field with sufficiently many prime factors in the discriminant has an infinite class field tower.

References

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    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

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  • Johnson, D.L. (1980). "Topics in the Theory of Group Presentations" (1st ed.). Cambridge University Press. ISBN 0-521-23108-6. See chapter VI.
  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

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  • 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  • Serre, J.-P. (2002), "Galois Cohomology," Springer-Verlag. ISBN 3-540-42192-0. See Appendix 2. (Translation of Cohomologie Galoisienne, Lecture Notes in Mathematics 5, 1973.)