Rational connectedness modulo the non-nef locus

Abstract

It is well known that a smooth projective Fano variety is rationally connected. Recently Zhang [Z2] (and later Hacon and McKernan [HM] as a special case of their work on the Shokurov RC-conjecture) proved that the same conclusion holds for a klt pair $(X,\Delta )$ such that $-(K_X+\Delta )$ is big and nef. We prove here a natural generalization of the above result by dropping the nefness assumption. Namely we show that a klt pair $(X,\Delta )$ such that $-(K_X+\Delta )$ is big is rationally connected modulo the non-nef locus of $-(K_X+\Delta )$. This result is a consequence of a more general structure theorem for arbitrary pairs $(X,\Delta )$ with $-(K_X+\Delta )$ pseff.