Prokhorov–Shokurov's effective b-semiampleness conjecture

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Fix an integer dd. For every lc-trivial fibration f ⁣:X→Yf\colon X\to Y of relative dimension dd, choose a birational diagram

(X′,Δ′)→ϕ(X,Δ)f′↓↓fY′→ψY.\begin{CD} (X',\Delta') @>{\phi}>> (X,\Delta)\\ @V{f'}VV @VV{f}V\\ Y' @>{\psi}>> Y. \end{CD}

Here ϕ\phi and ψ\psi are birational, and MY′\mathbb M_{Y'} denotes the trace of the moduli b-divisor.

Effective b-semiampleness conjecture. There is an integer I=I(d)I=I(d) such that one can choose the diagram with KX′+Δ′=ϕ∗(KX+Δ)K_{X'}+\Delta'=\phi^*(K_X+\Delta), IMY′I\mathbb M_{Y'} Cartier and free, and M{\bf M} descending to MY′\mathbb M_{Y'}.

This is the effective, uniform form of the b-semiampleness expectation: the index depends only on the relative dimension. The conjecture is open in the stated generality.

References

Primary source

Philip Engel, Stefano Filipazzi, François Greer, Mirko Mauri and Roberto Svaldi, “Boundedness of some fibered K-trivial varieties”, arXiv:2507.00973 (2025).

Additional references

8 papers in this index state this conjecture (2012–2025). The statement above is taken from the most recent of them; the others are arXiv:2405.05489, arXiv:2203.11460, arXiv:2111.03373, arXiv:2005.02613, arXiv:1907.10490, arXiv:1808.00717, arXiv:1207.4070.

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