Automorphisms of smooth canonically polarized surfaces in positive characteristic

TL;DR

This paper explores the automorphism schemes of smooth canonically polarized surfaces over algebraically closed fields of positive characteristic, revealing conditions under which these schemes are non-smooth and describing their geometric implications.

Contribution

It characterizes when the automorphism scheme of such surfaces is non-smooth in positive characteristic and describes the geometric structure of these surfaces in that case.

Findings

Non-smooth automorphism schemes occur only in positive characteristic.

Surfaces with certain invariants tend to be uniruled and simply connected.

Such surfaces are quotients of rational or ruled surfaces by rational vector fields.

Abstract

This paper investigates the geometry of a smooth canonically polarized surface $X$ defined over an algebraically closed field of characteristic $p>0$ in the case when the automorphism scheme of $X$ is not smooth. This is a situation that appears only in positive characteristic and it is closely related to the structure of the moduli stack of canonically polarized surfaces. Restrictions on certain numerical invariants of $X$ are obtained in order for Aut(X) to be smooth or not and information is provided about the structure of the component of Aut(X) containing the identity. In particular, it is shown that a smooth canonically polarized surface X with 0< K_X^2 < 3 and non smooth automorphism scheme tends to be uniruled and simply connected. Moreover, X is the purely inseparable quotient of a ruled or rational surface by a rational vector field.