English

Norm one tori and Hasse norm principle

Algebraic Geometry 2022-02-01 v6 Number Theory

Abstract

Let kk be a field and TT be an algebraic kk-torus. In 1969, over a global field kk, Voskresenskii proved that there exists an exact sequence 0A(T)H1(k,PicX)Sha(T)00\to A(T)\to H^1(k,{\rm Pic}\,\overline{X})^\vee\to Sha(T)\to 0 where A(T)A(T) is the kernel of the weak approximation of TT, Sha(T)Sha(T) is the Shafarevich-Tate group of TT, XX is a smooth kk-compactification of TT, X=X×kk\overline{X}=X\times_k\overline{k}, PicX{\rm Pic}\,\overline{X} is the Picard group of X\overline{X} and \vee stands for the Pontryagin dual. On the other hand, in 1963, Ono proved that for the norm one torus T=RK/k(1)(Gm)T=R^{(1)}_{K/k}(G_m) of K/kK/k, Sha(T)=0Sha(T)=0 if and only if the Hasse norm principle holds for K/kK/k. First, we determine H1(k,PicX)H^1(k,{\rm Pic}\, \overline{X}) for algebraic kk-tori TT up to dimension 55. Second, we determine H1(k,PicX)H^1(k,{\rm Pic}\, \overline{X}) for norm one tori T=RK/k(1)(Gm)T=R^{(1)}_{K/k}(G_m) with [K:k]=n15[K:k]=n\leq 15 and n12n\neq 12. We also show that H1(k,PicX)=0H^1(k,{\rm Pic}\, \overline{X})=0 for T=RK/k(1)(Gm)T=R^{(1)}_{K/k}(G_m) when the Galois group of the Galois closure of K/kK/k is the Mathieu group MnSnM_n\leq S_n with n=11,12,22,23,24n=11,12,22,23,24. Third, we give a necessary and sufficient condition for the Hasse norm principle for K/kK/k with [K:k]=n15[K:k]=n\leq 15 and n12n\neq 12. As applications of the results, we get the group T(k)/RT(k)/R of RR-equivalence classes over a local field kk via Colliot-Th\'{e}l\`{e}ne and Sansuc's formula and the Tamagawa number τ(T)\tau(T) over a number field kk via Ono's formula τ(T)=H1(k,T^)/Sha(T)\tau(T)=|H^1(k,\widehat{T})|/|Sha(T)|.

Cite

@article{arxiv.1910.01469,
  title  = {Norm one tori and Hasse norm principle},
  author = {Akinari Hoshi and Kazuki Kanai and Aiichi Yamasaki},
  journal= {arXiv preprint arXiv:1910.01469},
  year   = {2022}
}

Comments

To appear in Math. Comp., 98 pages

R2 v1 2026-06-23T11:33:44.109Z