Rational connectedness modulo the Non-nef locus

TL;DR

This paper generalizes the rational connectedness of certain algebraic varieties by showing that pairs with big anti-canonical divisors are rationally connected outside their non-nef locus, extending previous results that required nefness.

Contribution

It proves that klt pairs with big anti-canonical divisors are rationally connected modulo the non-nef locus, removing the nefness condition from earlier theorems.

Findings

Rational connectedness holds outside the non-nef locus for pairs with big anti-canonical divisors.

General structure theorem for pairs with pseudo-effective anti-canonical divisor.

Extension of known results from nef to big divisors in rational connectedness.

Abstract

It is well known that a smooth projective Fano variety is rationally connected. Recently Zhang (and later Hacon and McKernan as a special case of their work on the Shokurov RC-conjecture) proved that the same conclusion holds for a klt pair $(X,\D)$ such that $-(K_X+\D)$ 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 $(X,\D)$ such that $-(K_X+\D)$ is big is rationally connected modulo the non-nef locus of $-(K_X+\D)$. This result is a consequence of a more general structure theorem for arbitrary pairs $(X,\D)$ with $-(K_X+\D)$ pseff.