English

Galois trace forms of type $A_{n}, D_{n}, E_{n}$ for odd $n$

Number Theory 2023-08-25 v2

Abstract

Let pp be an odd prime number and ζp:=exp(2πi/p)\zeta_{p} := \exp(2\pi i/p). Then, it is well-known that the Ap1A_{p-1}-root lattice can be realized as the (Hermitian) trace form of the pp-th cyclotomic extension Q(ζp)/Q\mathbb{Q}(\zeta_{p})/\mathbb{Q} restricted to the fractional ideal generated by (1ζp)(p3)/2(1-\zeta_{p})^{-(p-3)/2}. In this paper, in contrast with the case of the Ap1A_{p-1}-root lattice, we prove the following theorem: Let nn be an odd positive integer and F/QF/\mathbb{Q} be a Galois extension of degree nn. Then, there exist no fractional ideals Λ\Lambda of FF such that the restricted trace form (Λ,TrΛ×Λ)(\Lambda, \mathrm{Tr}|_{\Lambda \times \Lambda}) is of type An,Dn,EnA_{n}, D_{n}, E_{n}. The proof is done by the prime ideal factorization of fractional ideals of FF with care of certain 2-adic obstruction. Additionally, we prove that every cyclic cubic field contains infinitely many distinct sub Z\mathbb{Z}-lattices of type A3A_{3} (i.e., normalized face centered cubic lattices) with normal Z\mathbb{Z}-bases. The latter fact is in contrast with another fact that among quadratic fields only Q(±3)\mathbb{Q}(\sqrt{\pm3}) contain sub Z\mathbb{Z}-lattices of type A2A_{2}.

Keywords

Cite

@article{arxiv.2307.06612,
  title  = {Galois trace forms of type $A_{n}, D_{n}, E_{n}$ for odd $n$},
  author = {Riku Higa and Yoshinosuke Hirakawa},
  journal= {arXiv preprint arXiv:2307.06612},
  year   = {2023}
}