English

Rationality problem of three-dimensional monomial group actions

Algebraic Geometry 2011-01-18 v3 Number Theory

Abstract

Let KK be a field of characteristic not two and K(x,y,z)K(x,y,z) the rational function field over KK with three variables x,y,zx,y,z. Let GG be a finite group of acting on K(x,y,z)K(x,y,z) by monomial KK-automorphisms. We consider the rationality problem of the fixed field K(x,y,z)GK(x,y,z)^G under the action of GG, namely whether K(x,y,z)GK(x,y,z)^G is rational (that is, purely transcendental) over KK or not. We may assume that GG is a subgroup of GL(3,Z)andtheproblemisdetermineduptoconjugacyin\mathrm{GL}(3,\mathbb{Z}) and the problem is determined up to conjugacy in \mathrm{GL}(3,\mathbb{Z}).Thereare73conjugacyclassesof. There are 73 conjugacy classes of Gin in \mathrm{GL}(3,\mathbb{Z}).ByresultsofEndoMiyata,Voskresenski\i˘,Lenstra,Saltman,Hajja,KangandYamasaki,8conjugacyclassesof2groupsin. By results of Endo-Miyata, Voskresenski\u\i, Lenstra, Saltman, Hajja, Kang and Yamasaki, 8 conjugacy classes of 2-groups in \mathrm{GL}(3,\mathbb{Z})havenegativeanswerstotheproblemundercertainmonomialactionsoversomebasefield have negative answers to the problem under certain monomial actions over some base field K,andthenecessaryandsufficientconditionfortherationalityof, and the necessary and sufficient condition for the rationality of K(x,y,z)^Gover over Kisgiven.Inthispaper,weshowthatthefixedfield is given. In this paper, we show that the fixed field K(x,y,z)^Gundermonomialactionof under monomial action of Gisrationalover is rational over Kexceptforpossiblynegative8casesof2groupsandunknownonecaseofthealternatinggroupofdegreefour.Moreoverwegiveexplicittranscendentalbasesofthefixedfieldsover except for possibly negative 8 cases of 2-groups and unknown one case of the alternating group of degree four. Moreover we give explicit transcendental bases of the fixed fields over K.Forunknowncase,weobtainanaffirmativesolutiontotheproblemundersomeconditions.Inparticular,weshowthatif. For unknown case, we obtain an affirmative solution to the problem under some conditions. In particular, we show that if Kisquadraticallyclosedfieldthen is quadratically closed field then K(x,y,z)^Gisrationalover is rational over K$. We also give an application of the result to 4-dimensional linear Noether's problem.

Keywords

Cite

@article{arxiv.0912.5259,
  title  = {Rationality problem of three-dimensional monomial group actions},
  author = {Akinari Hoshi and Hidetaka Kitayama and Aiichi Yamasaki},
  journal= {arXiv preprint arXiv:0912.5259},
  year   = {2011}
}

Comments

54 pages

R2 v1 2026-06-21T14:29:00.831Z