Shokurov's conjecture on singularities of bases of Fano type fibrations

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Assume d∈Nd\in\mathbb N and ϵ∈R>0\epsilon\in\mathbb R^{>0}. An ϵ\epsilon-lc pair (X,B)(X,B) has log discrepancies at least ϵ\epsilon; let f ⁣:X→Zf\colon X\to Z be a contraction with dim⁡Z>0\dim Z>0 such that KX+B∼R0/ZK_X+B\sim_{\mathbb R}0/Z and −KX-K_X is big over ZZ.

Shokurov's conjecture. There is δ∈R>0\delta\in\mathbb R^{>0} such that one can write

KX+B∼Rf∗(KZ+BZ+MZ)K_X+B\sim_{\mathbb R}f^*(K_Z+B_Z+M_Z)

such that (Z,BZ+MZ)(Z,B_Z+M_Z) is δ\delta-lc, where BZB_Z and MZM_Z are the discriminant and moduli parts of adjunction.

The conjecture is known when dim⁡X−dim⁡Z≤1\dim X-\dim Z\leq 1 and when the general fibre together with the restricted boundary belongs to a bounded family. It remains open in higher dimension without that boundedness condition.

References

Primary source

Caucher Birkar, “Birational geometry of algebraic varieties”, arXiv:1801.00013 (2017).

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