English

$h^1\ne h_1$ for Anderson t-motives

Number Theory 2021-01-05 v3

Abstract

Let MM be an Anderson t-motive of dimension nn and rank rr. Associated are two Fq[T]\Bbb F_q[T]-modules H1(M)H^1(M), H1(M)H_1(M) of dimensions h1(M)h^1(M), h1(M)rh_1(M)\le r - analogs of H1(A,Z)H^1(A,\Bbb Z), H1(A,Z)H_1(A,\Bbb Z) for an abelian variety AA. There is a theorem (Anderson): h1(M)=r    h1(M)=rh^1(M)=r \iff h_1(M)=r; in this case MM is called uniformizable. It is natural to expect that always h1(M)=h1(M)h^1(M)=h_1(M). Nevertheless, we explicitly construct a counterexample. Further, we answer a question of D.Goss: is it possible that two Anderson t-motives that differ only by a nilpotent operator NN are of different uniformizability type, i.e. one of them is uniformizable and other not? We give an explicit example that this is possible.

Keywords

Cite

@article{arxiv.1807.08675,
  title  = {$h^1\ne h_1$ for Anderson t-motives},
  author = {Aleksandr Grishkov and Dmitry Logachev},
  journal= {arXiv preprint arXiv:1807.08675},
  year   = {2021}
}

Comments

31 pages. Minor improvements