Unramified cohomology of degree 3 and Noether’s problem |
| |
Authors: | Emmanuel Peyre |
| |
Institution: | (1) Institut Fourier, UFR de Mathématiques, UMR 5582, Université de Grenoble I et CNRS, BP 74, 38402 Saint-Martin d’Hères CEDEX, France |
| |
Abstract: | Let G be a finite group and W be a faithful representation of G over C. The group G acts on the field of rational functions C(W). The question whether the field of invariant functions C(W)
G
is purely transcendental over C goes back to Emmy Noether. Using the unramified cohomology group of degree 2 of this field as an invariant, Saltman gave
the first examples for which C(W)
G
is not rational over C. Around 1986, Bogomolov gave a formula which expresses this cohomology group in terms of the cohomology of the group G.
In this paper, we prove a formula for the prime to 2 part of the unramified cohomology group of degree 3 of C(W)
G
. Specializing to the case where G is a central extension of an F
p
-vector space by another, we get a method to construct nontrivial elements in this unramified cohomology group. In this way
we get an example of a group G for which the field C(W)
G
is not rational although its unramified cohomology group of degree 2 is trivial.
Dedicated to Jean-Louis Colliot-Thélène. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|