Counterexamples to the Ambro--Kawamata effective non-vanishing conjecture

Source abstract

Let $k$ be an algebraically closed field of characteristic $0$ or $p\neq3$. We construct a terminal projective 4-fold $X$ and an ample Cartier divisor $D$ such that $$3K_X\sim0,\qquad D-K_X\ \text{is ample},\qquad H^0(X,D)=0.$$ In char 0 this disproves the Ambro--Kawamata effective non-vanishing conjecture. From this example we construct a smooth projective 4-fold $Y$ with a semiample and big Cartier divisor $D_Y$ such that $D_Y-K_Y$ is basepoint-free and big and $H^0(Y,D_Y)=0$. We also give geometric variants and a separate terminal order 5 Jacobian quotient.