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5.36.4 Smooth remainders after resolution70049B9396E7

For arguments on a modification, the bounded remainder in an analytic presentation is sometimes insufficient. The following version of regularization gives a smooth remainder after resolution.

Theorem 5.36.5 (Demailly regularization).88038BD901A0 Let $\alpha $ be a smooth real $(1,1)$-form and let $u\in \mathrm{PSH}(X,\alpha )$. There are numbers $\delta _ j\downarrow 0$ and functions $\widetilde u_ j\in \mathrm{PSH}(X,\alpha +\delta _ j\omega )$ decreasing to $u$ with the following properties. Each $\widetilde u_ j$ is smooth outside an analytic pole set, and there is a modification $\pi _ j:Y_ j\to X$, obtained by a finite sequence of blowups with smooth centers, such that locally on $Y_ j$

\[ \pi _ j^*\widetilde u_ j=\sum _\ell a_\ell \log |s_\ell |^2+b, \qquad a_\ell >0,\quad b\in C^\infty . \]

Here the $s_\ell $ are local holomorphic defining functions of divisors, the $a_\ell >0$ are real numbers, and $b$ is a smooth real function. The divisors may be taken to have simple normal crossings.

This classical theorem is applied to the closed current $\mathrm{dd}^{\mathrm{c}}u$ with the smooth lower bound $-\alpha $; see section 25.3 for its source. The lower bound need not be closed, and $X$ need not be Kähler. What matters here is the smooth remainder $b$ after resolution; an analytic type, whose remainder is merely bounded, would not suffice. The sequence $(\widetilde u_ j)$ need not be the multiplier-ideal sequence of theorem 5.36.1, and we do not claim that the conclusions of the two theorems hold for one and the same sequence. If $\theta +\mathrm{dd}^{\mathrm{c}}u\geq \varepsilon \omega $ for some $\varepsilon >0$, applying the theorem with $\alpha =\theta -\varepsilon \omega $ and discarding the terms with $\delta _ j\geq \varepsilon $ gives approximants with $\theta +\mathrm{dd}^{\mathrm{c}}\widetilde u_ j\geq (\varepsilon -\delta _ j)\omega >0$; thus strict positivity is preserved.

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