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In [[abstract algebra]], '''Hilbert's Theorem 90''' (or '''Satz 90''') refers to an important result on [[cyclic extension]]s of [[field (mathematics)|field]]s (or to one of its generalizations) that leads to [[Kummer theory]]. In its most basic form, it states that if ''L''/''K'' is a cyclic extension of fields with Galois group ''G'' =''Gal''(''L''/''K'')
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generated by an element '''s''' and if a is an element of ''L'' of [[relative norm]] 1, then there exists b in ''L'' such that
 
:a = '''s'''(b)/b.
 
The theorem takes its name from the fact that it is the 90th theorem in [[David Hilbert]]'s famous [[Zahlbericht]] {{harvs|last=Hilbert|year=1897|year2=1998}}, although it is originally due to {{harvs|txt|authorlink=Ernst Kummer|last=Kummer|year1=1855|loc1=p.213|year2=1861}}. Often a more general theorem due to {{harvs|txt|authorlink=Emmy Noether|first=Emmy|last=Noether|year=1933}} is given the name, stating that if ''L''/''K'' is a finite Galois extension of fields with Galois group ''G'' =''Gal''(''L''/''K''), then the first cohomology group is trivial:
:''H''<sup>1</sup>(''G'', ''L''<sup>×</sup>) = {1}
 
== Examples ==
Let ''L/K'' be the [[quadratic extension]] <math> \mathbb{Q}(i) / \mathbb{Q} </math>. The Galois group is cyclic of order 2, its generator s acting via conjugation:
:<math> s:\, \, a+bi\mapsto a-bi\ . </math>
An element <math> x=a+bi</math> in ''L'' has norm <math> xx^{s}=a^2 +b^2</math>. An element of norm one corresponds to a rational solution of the equation <math>a^2+b^2=1</math> or in other words, a point with rational coordinates on the [[unit circle]]. Hilbert's Theorem 90 then states that every element ''y'' of norm one can be parametrized (with integral ''c,d'') as
:<math> y={{c+di}\over{c-di}}={{c^2-d^2}\over{c^2+d^2}}+{2dc\over{c^2+d^2}}i
</math>
which may be viewed as a rational parametrization of the rational points on the unit circle. Rational points <math>\, (x,y)=(a/c,b/c) </math> on the unit circle <math> x^2+y^2=1 </math> correspond to [[Pythagorean triple]]s, i.e. triples <math>\,(a,b,c)</math>  of integers satisfying <math>\, a^2+b^2=c^2 </math>.
 
==Cohomology==
The theorem can be stated in terms of [[group cohomology]]: if ''L''<sup>×</sup> is the [[multiplicative group]] of any (not necessarily finite) Galois extension ''L'' of a field ''K'' with corresponding Galois group ''G'', then
 
:''H''<sup>1</sup>(''G'', ''L''<sup>×</sup>) = {1}.
 
A further generalization using [[Non-abelian cohomology|non-abelian group cohomology]] states that if ''H'' is either the [[general linear group|general]] or [[special linear group]] over ''L'', then
 
:''H''<sup>1</sup>(''G'',''H'') = {1}.
 
This is a generalization since ''L''<sup>×</sup> = GL<sub>1</sub>(''L'').
 
Another generalization is <math>H^1_{\acute{e}t}(X,\mathbf{G}_m) = H^1(X,\mathcal{O}_X^\times) = \mathrm{Pic}(X)</math> for ''X'' a scheme, and another one to [[Milnor K-theory]] plays a role in [[Vladimir Voevodsky|Voevodsky's]] proof of the [[Milnor conjecture]].
 
==References==
 
*{{Citation | last1=Hilbert | first1=David | author1-link=David Hilbert | title=Die Theorie der algebraischen Zahlkörper | url=http://resolver.sub.uni-goettingen.de/purl?GDZPPN002115344 | language=German | year=1897 | journal=Jahresbericht der Deutschen Mathematiker-Vereinigung | issn=0012-0456 | volume=4 | pages=175–546 }}
*{{Citation | last1=Hilbert | first1=David | author1-link=David Hilbert | title=The theory of algebraic number fields | url=http://books.google.com/books?id=_Q2h83Bm94cC | publisher=[[Springer-Verlag]] | location=Berlin, New York | isbn=978-3-540-62779-1 | id={{MR|1646901}} | year=1998}}
*{{Citation | last1=Kummer | first1=Ernst Eduard | title=Über eine besondere Art, aus complexen Einheiten gebildeter Ausdrücke. | url=http://resolver.sub.uni-goettingen.de/purl?GDZPPN002148927 | language=German | doi=10.1515/crll.1855.50.212 | year=1855 | journal=[[Journal für die reine und angewandte Mathematik]] | issn=0075-4102 | volume=50 | pages=212–232}}
*{{Citation | last1=Kummer | first1=Ernst Eduard | title=Zwei neue Beweise der allgemeinen Reciprocitätsgesetze unter den Resten und Nichtresten der Potenzen, deren Grad eine Primzahl ist | language=German | id=Reprinted in volume 1 of his collected works, pages 699–839 | year=1861 | journal=Abdruck aus den Abhandlungen der Kgl. Akademie der Wissenschaften zu Berlin}}
*Chapter II of J.S. Milne, ''Class Field Theory'', available at his website [http://www.jmilne.org/math].
*{{Neukirch et al. CNF}}
*{{Citation | last1=Noether | first1=Emmy | author1-link=Emmy Noether | title=Der Hauptgeschlechtssatz für relativ-galoissche Zahlkörper. | language=German | doi=10.1007/BF01452845 | id={{Zbl|0007.29501}} | year=1933 | journal=[[Mathematische Annalen]] | issn=0025-5831 | volume=108 | issue=1 | pages=411–419}}
 
[[Category:Theorems in algebraic number theory]]
[[Category:Theorems in abstract algebra]]

Latest revision as of 13:23, 22 April 2014

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