View source for Hilbert–Speiser theorem

{{short description|Result on cyclotomic fields, characterising those with a normal integral basis}}
In [[Mathematics|mathematics]], the '''Hilbert–Speiser theorem''' is a result on [[Cyclotomic field|cyclotomic field]]s, characterising those with a normal integral basis. More generally, it applies to any finite [[Abelian extension|abelian extension]] of {{math|'''Q'''}}, which by the [[Kronecker–Weber theorem]] are isomorphic to subfields of cyclotomic fields.

:'''Hilbert–Speiser Theorem.''' A finite abelian extension {{math|''K''/'''Q'''}} has a normal integral basis if and only if it is tamely ramified over {{math|'''Q'''}}.

This is the condition that it should be a subfield of {{math|'''Q'''(''ζ<sub>n</sub>'')}} where {{mvar|n}} is a [[Square-free integer|squarefree]] [[Odd number|odd number]]. This result was introduced by  {{harvs|txt|last=Hilbert|year1=1897|loc1=Satz 132|year2=1998|loc2=theorem 132}} in his [[Zahlbericht]] and by {{harvs|txt|authorlink=Andreas Speiser|last=Speiser|year=1916|loc=corollary to proposition 8.1}}.

In cases where the theorem states that a normal integral basis does exist, such a basis may be constructed by means of [[Gaussian period]]s. For example if we take {{mvar|n}} a prime number {{math|''p'' > 2}}, {{math|'''Q'''(''ζ<sub>p</sub>'')}} has a normal integral basis consisting of all the {{math|''p''}}-th roots of unity other than {{math|1}}. For a field {{mvar|K}} contained in it, the [[Field trace|field trace]] can be used to construct such a basis in {{mvar|K}} also (see the article on [[Gaussian period]]s). Then in the case of {{mvar|n}} squarefree and odd, {{math|'''Q'''(''ζ<sub>n</sub>'')}} is a compositum of subfields of this type for the primes {{mvar|p}} dividing {{mvar|n}} (this follows from a simple argument on ramification). This decomposition can be used to treat any of its subfields.

{{harvs|txt | last1=Greither | first1=Cornelius |  last2=Replogle | first2=Daniel R. | last3=Rubin | first3=Karl |  last4=Srivastav | first4=Anupam |year=1999}} proved a converse to the Hilbert–Speiser theorem:

:Each finite tamely ramified [[Abelian extension|abelian extension]] {{mvar|K}} of a fixed number field {{mvar|J}} has a relative normal integral basis if and only if {{math|''J'' {{=}}'''Q'''}}.

There is an elliptic analogue of the theorem proven by  {{harvs|txt | last1=Srivastav | first1=Anupam  | last2=Taylor | first2=Martin J. | year=1990}}. 
It is now called the Srivastav-Taylor theorem {{harvs|txt |year=1996}}.

==References==
<references/>
*{{Citation | last1=Agboola | first1=A. | title= Torsion points on elliptic curves and Galois module structure | year=1996 | journal=Invent Math | volume=123 |
 pages= 105–122 | doi=10.1007/BF01232369
 }}
*{{Citation | last1=Greither | first1=Cornelius | last2=Replogle | first2=Daniel R. | last3=Rubin | first3=Karl | last4=Srivastav | first4=Anupam | title=Swan modules and Hilbert–Speiser number fields | journal=Journal of Number Theory | volume=79 | pages=164–173 | doi=10.1006/jnth.1999.2425 | year=1999| doi-access=free }}
*{{Citation | last1=Hilbert | first1=David |  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 |  title=The theory of algebraic number fields | url=https://books.google.com/books?id=_Q2h83Bm94cC | publisher=[[Physics:Springer-Verlag|Springer-Verlag]] | location=Berlin, New York | isbn=978-3-540-62779-1 |mr=1646901 | year=1998}}
*{{Citation | last1=Speiser | first1=A. | title=Gruppendeterminante und Körperdiskriminante | year=1916 | journal=[[Mathematische Annalen]] | issn=0025-5831 | volume=77 | issue=4 | pages=546–562 | doi=10.1007/BF01456968| url=https://zenodo.org/record/1510602 }}
*{{Citation | last1=Srivastav | first1=Anupam |last2=Taylor | first2=Martin J. | title=Elliptic curves with complex multiplication and Galois module structure | year=1990 | journal=Invent Math | volume=99 |
 pages= 165–184 | doi=10.1007/BF01234415
 }}



{{DEFAULTSORT:Hilbert-Speiser theorem}}
[[Category:Cyclotomic fields]]
[[Category:Theorems in algebraic number theory]]

{{Sourceattribution|Hilbert–Speiser theorem}}