Bounded relative-global klt complements for Fano fibrations

Let f:XZf:X\to Z be an ϵ\epsilon-lc Fano fibration of dimension dd, meaning that XX and ZZ are normal projective varieties, fOX=OZf_*\mathcal{O}_X=\mathcal{O}_Z, XX is ϵ\epsilon-lc, and KX-K_X is ample over ZZ. A bounded relative-global klt complement is an effective Q\mathbb{Q}-divisor Λ\Lambda on XX for which N(KX+Λ)0/ZN(K_X+\Lambda)\sim 0/Z and (X,Λ)(X,\Lambda) is klt.

Bounded relative-global klt complement conjecture. For every positive integer dd and positive real number ϵ\epsilon, there exists a positive integer NN, depending only on dd and ϵ\epsilon, such that every ϵ\epsilon-lc Fano fibration f:XZf:X\to Z of dimension dd admits an effective Q\mathbb{Q}-divisor Λ\Lambda satisfying

N(KX+Λ)0/ZN(K_X+\Lambda)\sim 0/Z

and (X,Λ)(X,\Lambda) is klt.

This is a klt version of the bounded relative-global complement problem and is presented as a variant of Shokurov's conjecture on bounded local complements for Fano fibrations. The supplied abstract states that bounded relative-global klt complements are proved when the base has dimension one; the general assertion in the stated form is not resolved by the supplied text.

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Primary source

Sung Rak Choi and Chuyu Zhou, “On existence of bounded relative-global complements for Fano fibrations”, arXiv:2402.12031 (2024).

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