On the numerically and cohomologically trivial automorphisms of elliptic surfaces II: $χ(S)>0$ (2412.17033v2)

Abstract: In this second part we study first the group $Aut_{\mathbb{Q}}(S)$ of numerically trivial automorphisms of a properly elliptic surface $S$, that is, of a minimal surface with Kodaira dimension $\kappa(S)=1$, in the case $\chi(S) \geq 1$. Our first surprising result is that, against what has been believed for over 40 years, we have nontrivial such groups for $p_g(S) >0$. Indeed, we show even that there is no absolute upper bound for their cardinalities $|Aut_{\mathbb{Q}}(S)|$. At any rate, we give explicit and nearly optimal upper bounds for $|Aut_{\mathbb{Q}}(S)|$ in terms of the numerical invariants of $S$, as $\chi(S)$, or the irregularity $q(S)$, or the bigenus $P_2(S)$. Moreover, we come quite close to a complete description of the possible groups $Aut_{\mathbb{Q}}(S)$ as 2-generated finite abelian groups, and we give an effective criterion for surfaces to have trivial $Aut_{\mathbb{Q}}(S)$. Our second surprising results concern the group $Aut_{\mathbb{Z}}(S)$ of cohomologically trivial automorphisms; we are able to give the explicit upper bounds for $|Aut_{\mathbb{Z}}(S)|$ in special cases: $9$ when $p_g(S) =0$, and the sharp upper bound $3$ when $S$ (i.e., the pluricanonical elliptic fibration) is isotrivial. We produce also non isotrivial examples where $Aut_{\mathbb{Z}}(S)$ is a cyclic group of order $2$ or $3$.