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

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Let (X/Z,B)(X/Z,B) be a GLC fibration, meaning that (X,B)(X,B) is a log pair and f ⁣:X→Zf\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, B≥0B\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 m≫0m\gg 0 and any ϵ∈Q>0\epsilon\in\mathbb Q_{>0}, the divisor

MX+ϵf∗det⁡f∗O(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.

References

Primary source

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

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