Effective b-semiampleness conjecture

Let dd be a nonnegative integer and let Γ[0,1]\Gamma\subset [0,1] be a rational dcc subset. For a 00-contraction (X,D)Z(X,D)\to Z under the assumptions cited in the source, write Dmod\mathcal D_{\rm mod} for its moduli b-divisor. A b-divisor is b-free if it is the closure M\overline M of a base point free divisor MM on some model ZZ' of ZZ.

Effective b-semiampleness conjecture. There exists a positive integer I=I(d,Γ)I=I(d,\Gamma) such that, for every such 00-contraction with dimX=d\dim X=d and DhΓD^{\mathrm h}\in\Gamma, IDmodI\mathcal D_{\rm mod} is b-free:

IDmod=M.I\mathcal D_{\rm mod}=\overline M.

Here MM is a base point free divisor on some model ZZ' of ZZ. Equivalently, (Z,D÷+Dmod)(Z,D_{\rm \div}+\mathcal D_{\rm mod}) is an Alexeev log pair of index II. The same is expected for 00-contractions of Alexeev pairs of index mm, with I(d,Γ,m)I(d,\Gamma,m) depending also on mm.

This conjecture would provide effective semiampleness for the moduli part in the adjunction framework and is intended to support reductions to finite-type and Calabi–Yau cases. The source attributes it to PSh08 and leaves it open.

Sources & referencesView supporting material

Primary source

V. V. Shokurov, “Existence and boundedness of n-complements”, arXiv:2012.06495 (2020).

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