Bounded fiber multiplicities for Fano fibrations over curves

Let dd be a positive integer and ϵ\epsilon a positive real number. Let (X,B)(X,B) be a log pair of dimension dd and let π:XC\pi:X\to C be a contraction, where XX is of Fano type over the curve CC and KX+BR,C0K_X+B\sim_{\mathbb{R},C}0. Assume that (X,B)(X,B) is ϵ\epsilon-lc. For a closed point zCz\in C, write

πz=imiFi,\pi^*z=\sum_i m_iF_i,

where the FiF_i are the irreducible components of πz\pi^*z. Bounded fiber multiplicity conjecture. There exists a positive integer mm, depending only on dd and ϵ\epsilon, such that mimm_i\le m for every ii and every closed point zCz\in C. This conjecture is presented as a direct corollary of the McKernan–Shokurov conjecture and is not resolved in general.

Sources & referencesView supporting material

Primary source

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

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