On numerical nonvanishing for generalized log canonical pairs

Abstract

The nonvanishing conjecture for projective log canonical pairs plays a key role in the minimal model program of higher dimensional algebraic geometry. The numerical nonvanishing conjecture considered in this paper is a weaker version of the usual nonvanishing conjecture, but valid in the more general setting of generalized log canonical pairs. We confirm it in dimension two. Under some necessary conditions we obtain effective versions of numerical nonvanishing for surfaces. Several applications are also discussed. In higher dimensions, we mainly consider the conjecture for generalized klt pairs $(X, B+\mathbf{M})$, and reduce it to lower dimensions when $K_X+\mathbf{M}_X$ is not pseudo-effective. Up to scaling the nef part, we prove the numerical nonvanishing for pseudo-effective generalized lc threefolds with rational singularities.