The alpha invariant $\alpha(X)$ of a Fano manifold $X$ of dimension $n$ is defined as the infimum of log canonical thresholds of (effective) $\mathbb{Q}$-divisors $D\sim\_{\mathbb{Q}}-K\_X$. Similarly, for $G\subset Aut(X)$ a compact subgroup of the automorphism group, one defines $\alpha\_G(X)$ considering only $G$-invariant divisors. The alpha invariant has a corresponding analytic definition involving complex singularity exponents of singular hermitian metrics [2, Appendix].


Tian introduced this invariant and proved that the lower bound $\alpha\_G(X)>\frac{n}{n+1}$ implies the existence of a Kähler-Einstein metric [1] (in fact, even today it is one of the few sufficient conditions which is checkable in practice). I'd like to know if this theorem is sharp? That is:


**Question:** Are there examples Fano manifolds such that $\alpha\_G(X)=\frac{n}{n+1}$ but without a Kähler-Einstein metric?


I'd also be interested in any partial results in the positive direction.


An example I know of with $\alpha(X)=\frac{n}{n+1}$ is a del Pezzo surface of degree $4$ (this is due to Cheltsov [3]), however by Tian's classification of Kähler-Einstein metrics on del Pezzo surfaces [4], such surfaces are known to admit Kähler-Einstein metrics.


References:


[1] G. Tian. On Kahler-Einstein metrics on certain K ̈ahler manifolds with $c\_1(M)>0$.


[2] I. Cheltsov, C. Shramov, Appendix by J. P. Demailly. Log canonical thresholds of smooth Fano threefolds.


[3] I. Cheltsov. Log canonical thresholds of del Pezzo surfaces.


[4] Tian, G. On Calabi’s conjecture for complex surfaces with positive first Chern class.



Are there examples of Fano manifolds such that Tian's alpha invariant satisfies $\alpha_G(X)=\frac{n}{n+1}$ but without a Kähler-Einstein metric?
I see. THis is more subtle. There is no known example. 
I think it will be impossible or very hard to create one.
Vanya


