Varieties with free tangent sheaves

Abstract

We coin the term \emph{$T$-trivial varieties} to denote smooth proper schemes over ground fields $k$ whose tangent sheaf is free. Over the complex numbers, this are precisely the abelian varieties. However, Igusa observed that in characteristic $p\le 3$ certain bielliptic surfaces are $T$-trivial. We show that $T$-trivial varieties $X$ separably dominated by abelian varieties $A$ can exist only for $p\le 3$. Furthermore, we prove that every $T$-trivial variety, after passing to a finite étale covering, is fibered in $T$-trivial varieties with Betti number $b_1=0$. We also show that if some $n$-dimensional $T$-trivial $X$ lifts to characteristic zero and $p\ge 2n+2$ holds, it admits a finite étale covering by an abelian variety. Along the way, we establish several results about the automorphism group of abelian varieties, and the existence of relative Albanese maps.