On a conjecture of Tian

Abstract

We study Tian's $\alpha$-invariant in comparison with the ${\alpha }_1$-invariant for pairs $(S_d,H)$ consisting of a smooth surface $S_d$ of degree $d$ in the projective three-dimensional space and a hyperplane section $H$. A conjecture of Tian asserts that $\alpha (S_d,H)={\alpha }_1(S_d,H)$. We show that this is indeed true for $d=4$ (the result is well known for $d⩽3$), and we show that $\alpha (S_d,H)<{\alpha }_1(S_d,H)$ for $d⩾8$ provided that $S_d$ is general enough. We also construct examples of $S_d$, for $d=6$ and $d=7$, for which Tian's conjecture fails. We provide a candidate counterexample for $S_5$.

arXiv commentFinal version. To appear in Mathematische Zeitschrift