The Miyaoka-Yau inequality and the delta invariant for Fano varieties I -klt Fano case-

Abstract

We establish the following Miyaoka-Yau inequality for any $n$-dimensional klt Fano variety $X$, possibly K-unstable, in terms of its delta invariant: \[\left(2(n+1)\widehat{c}_2(X)-n c_1(X)^2\right)\cdot c_1(X)^{n-2} \ge -n \left(1-\min\{1,δ(X)\}\right)^2 \cdot c_1(X)^n.\] Moreover, if equality holds in this inequality, then there exist a finite quasi-étale cover $\hat{X}\to X$ and a positive integer $p$ such that $\hat{X}\cong \mathbb{P}(1,\dots ,1,p)$. We also discuss the relationship between Donaldson's conjecture and the Miyaoka-Yau inequality.

arXiv commentv3: 31 pages; Major revision with improved and additional results. Split into two papers; the second will appear separately. v2: 66 pages; Minor revision. Theorem 1.1 and Theorem B.3 have been improved. v1: 64 pages; comments are welcome