Semi-ampleness and Kodaira-dimension conjecture for the moduli part of maximal-moduli fibrations

Let (X/Z,B)(X/Z,B) be a GLC fibration, meaning that (X,B)(X,B) is a log pair and f ⁣:XZf\colon X\to Z is a projective surjective morphism with connected fibres between normal quasi-projective varieties, with (X,B)(X,B) log canonical above the generic point of ZZ. Assume that (X,B)(X,B) is log canonical, B0B\geq 0, (X/Z,B)(X/Z,B) has maximal moduli, MXM_X is ff-nef, and MXM_X is BP stable. Here MXM_X denotes the moduli part of the fibration, and κ(X/Z,B)\kappa(X/Z,B) and var(X/Z,B){\rm var}(X/Z,B) denote the relative Kodaira dimension and variation, respectively.

Variant of Shokurov's b-semi-ampleness conjecture. For all m0m\gg 0 and any ϵQ>0\epsilon\in\mathbb Q_{>0}, the divisor

MX+ϵfdetfO(mMX)M_X+\epsilon f^*\operatorname{det} f_*\mathcal O(mM_X)

is semi-ample; moreover,

κ(MX)κ(X/Z,B)+var(X/Z,B).\kappa(M_X)\geq\kappa(X/Z,B)+{\rm var}(X/Z,B).

The statement is a variant of Shokurov's conjecture on semi-ampleness of the moduli part for generically log canonical fibrations. The preceding discussion explains that ordinary semi-ampleness can fail for general GLC fibrations, while the conjectured perturbation by a divisor pulled back from the moduli space of fibres is intended to address this obstruction. The source provides no resolution status for this variant.

Sources & referencesView supporting material

Primary source

Stefano Filipazzi and Calum Spicer, “On semi-ampleness of the moduli part”, arXiv:2212.03736 (2025).

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