Singularities on Fano fibrations and beyond

Abstract: In this paper we investigate singularities on fibrations and related topics. We prove a conjecture of Shokurov and McKernan on singularities on Fano type fibrations and a conjecture of the author on singularities on log Calabi-Yau fibrations. From these we derive a variant of a conjecture of McKernan and Prokhorov on rationally connected varieties with nef anti-canonical divisor. We discuss other applications including existence of bounded klt complements and gonality of fibres of del Pezzo fibrations. We prove a general result on controlling multiplicities of fibres of certain fibrations (not necessarily of Fano type) which is the key ingredient of the proofs of the above results.