A Frobenius Version of Tians Alpha-Invariant

Abstract: For a pair (X,L) consisting of a projective variety X over a perfect field of characteristic p>0 and an ample line bundle L on X, we introduce and study a positive characteristic analog of the $\alpha$-invariant introduced by Tian, which we call the $\alpha_F$-invariant. We utilize the theory of F-singularities in positive characteristics, and our approach is based on replacing klt singularities with the closely related notion of global F-regularity. We show that the $\alpha_F$-invariant of a pair (X,L) can be understood in terms of the global Frobenius splittings of the linear systems |mL|, for m>0. We establish inequalities relating the $\alpha_F$-invariant with the F- signature, and use that to prove the positivity of the $\alpha_F$-invariant for all globally F-regular projective varieties (with respect to any ample L on X). When X is a Fano variety and L is $-K_X$, we prove that the $\alpha_F$-invariant of X is always bounded above by 1/2 and establish tighter comparisons with the F-signature. We also show that for toric Fano varieties, the $\alpha_F$-invariant matches with the usual (complex) $\alpha$-invariant. Finally, we prove that the $\alpha_F$-invariant is lower semicontinuous in a family of globally F-regular Fano varieties.