PAQ

2013-12-28T13:09:37Z

89.204.139.20: /* PAQ derivations */ cite web error (missing url=)

In [[number theory]], the '''Hasse norm theorem''' states that if L/K is a [[cyclic extension]] of [[number field]]s, then if a nonzero element of K is a local norm everywhere, then it is a global norm.
Here to be a global norm means to be an element ''k'' of K such that there is an element ''l'' of L with <math>\mathbf{N}_{L/K}(l) = k</math>; in other words ''k'' is a relative norm of some element of the extension field L. To be a local norm means that for some prime '''p''' of K and some prime '''P''' of L lying over K, then ''k'' is a norm from L'''P'''; here the "prime" '''p''' can be an archimedean valuation, and the theorem is a statement about completions in all valuations, archimedean and non-archimedean.

The theorem is no longer true in general if the extension is abelian but not cyclic. A counter-example is given by the field <math>{\mathbf Q}(\sqrt{13},\sqrt{17})/{\mathbf Q}</math> where every rational square is a local norm everywhere but <math>5^2</math> is not a global norm.

This is an example of a theorem stating a [[local-global principle]], and is due to [[Helmut Hasse]].

The Hasse norm theorem can be deduced from the theorem that an element of the Galois cohomology group H2(''L''/''K'') is trivial if it is trivial locally everywhere, which is in turn equivalent to the deep theorem that the first cohomology of the idele class group vanishes. This is true for all finite Galois extensions of number fields, not just cyclic ones. For cyclic extensions the group H2(''L''/''K'') is isomorphic to the Tate cohomology group H0(''L''/''K'') which describes which elements are norms, so for cyclic extensions it becomes Hasse's theorem that an element is a norm if it is a local norm everywhere.

==See also==

*[[Grunwald–Wang theorem]], about when an element that is a power everywhere locally is a power.

==References==

* H. Hasse, "A history of class field theory", in [[J.W.S. Cassels]] and [[A. Frohlich]] (edd), ''Algebraic number theory'', [[Academic Press]], 1973. Chap.XI.
* G. Janusz, ''Algebraic number fields'', Academic Press, 1973. Theorem V.4.5, p. 156

[[Category:Class field theory]]
[[Category:Theorems in algebraic number theory]]

Ideal lattice cryptography

2012-06-16T13:42:57Z

89.204.139.226: /* Ideal-LWE */

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PAQ

Ideal lattice cryptography