On the K-stability of blow-ups of projective bundles

TL;DR

This paper studies the K-stability of blow-ups of projective bundles over Fano varieties, establishing conditions under which the blow-up is K-stable, K-semistable, or K-unstable based on the properties of the base and divisor.

Contribution

It provides explicit criteria linking the K-stability of blow-ups to the stability of associated log Fano pairs, extending understanding of stability in algebraic geometry.

Findings

K-semistability and K-polystability are equivalent to those of a log Fano pair when B ~ 2L.

Blow-ups are K-unstable when B ~ lL with l ≠ 2.

Explicit stability conditions depend on the proportionality coefficient of B to L.

Abstract

We investigate the K-stability of certain blow-ups of $\mathbb{P}^1$-bundles over a Fano variety $V$, where the $\mathbb{P}^1$-bundle is the projective compactification of a line bundle $L$ proportional to $-K_V$ and the center of the blow-up is the image along a positive section of a divisor $B$ also proportional to $L$. When $V$ and $B$ are smooth, we show that, for $B \sim_{\mathbb{Q}} 2L$, the K-semistability and K-polystability of the blow-up is equivalent to the K-semistability and K-polystability of the log Fano pair $(V,aB)$ for some coefficient $a$ explicitly computed. We also show that, for $B \sim_{\mathbb{Q}} l L$, $l \neq 2$, the blow-up is K-unstable.