McKernan–Shokurov conjecture for generalized pairs

Let dd be a positive integer and ϵ\epsilon a positive real number. Let (X,B+M)(X,B+\mathbf{M}) be a generalized pair of dimension dd and let π:XZ\pi:X\to Z be a contraction between projective normal varieties. Assume (X,B+M)(X,B+\mathbf{M}) is ϵ\epsilon-lc, XX is of Fano type over ZZ, and KX+B+MXR,Z0K_X+B+\mathbf{M}_X\sim_{\mathbb{R},Z}0. Let BZB_Z denote the discriminant part of the canonical bundle formula for (X,B+M)(X,B+\mathbf{M}) over ZZ. Generalized McKernan–Shokurov conjecture. There exists a positive real number δ\delta, depending only on dd and ϵ\epsilon, such that BZ1δB_Z\le 1-\delta. The source states this generalized conjecture and records that it holds in dimension 22 and for relative-dimension-one fibrations, while the general case remains open.

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Primary source

Guodu Chen and Chuyu Zhou, “On multiplicities of fibers of Fano fibrations”, arXiv:2208.10372 (2026).

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