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Theorem 10.30.428C8F709D1CC

Theorem 10.30.4.28C8F709D1CC The family defined by

\[ \mathbb B_ Z(u;Y,\theta ) =\{ \pi ^*\theta -S_ Z(u;Y)\} \in \mathscr A_ Z(\theta ) \]

is a pseudoeffective b-divisor class over $\theta $. It is unchanged by ambient multiplier-ideal equivalence and, after the resulting affine spaces are identified, by a smooth $\mathrm{dd}^{\mathrm{c}}$ change of the restricted background.

Proof. The currents $S_ Z$ are closed, so the classes belong to the indicated affine spaces, and their compatibility follows from theorem 20.4.5, proposition 10.30.2. Take $v=\mathcal T_\theta (u;Y)$, the trace envelope of (20.3.25). By proposition 20.4.8,

\[ \pi ^*\theta +\mathrm{dd}^{\mathrm{c}}(v\circ \pi )-S_ Z(u;Y)\geq 0 \]

on every model, which proves pseudoeffectivity in the original restricted background. The currents $S_ Z$ depend only on the ambient multiplier ideals. For a smooth real function $f$ on $Y$, the change $\theta \mapsto \theta +\mathrm{dd}^{\mathrm{c}}f$ adds $\mathrm{dd}^{\mathrm{c}}(f\circ \pi )$ to every representative, which is zero in the quotient. This proves both invariances. $\square$

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