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Lemma 5.28.35087175B2C39

Lemma 5.28.3.5087175B2C39 Let $a\leq b$ be locally bounded quasi-plurisubharmonic functions on an $n$-dimensional complex manifold, and let $\theta $ be a smooth real $(1,1)$-form such that $\theta _ a,\theta _ b\geq 0$. For $1\leq k\leq n$, let $W$ be a product of $n-k$ positive currents with smooth backgrounds and locally bounded potentials, with $W=1$ when $k=n$. Then

\[ \mathbf1_{\{ a=b\} }\theta _ a^ k\wedge W \leq \mathbf1_{\{ a=b\} }\theta _ b^ k\wedge W. \]

Proof. The functions $m_\delta =\max (a+\delta ,b)$ satisfy $b\leq m_\delta \leq b+\delta $ and decrease to $b$ as $\delta \downarrow 0$. By lemma 5.24.1 and proposition 5.28.2, the products $\theta _{m_\delta }^ k\wedge W$ are positive, and by bounded decreasing continuity they converge weakly to $\theta _ b^ k\wedge W$. On the plurifine open set $\{ a+\delta >b\} $, which contains $\{ a=b\} $, we have $m_\delta =a+\delta $, so plurifine locality identifies the restriction of $\theta _{m_\delta }^ k\wedge W$ to this set with that of $\theta _ a^ k\wedge W$. Hence, for every compact set $K\subset \{ a=b\} $ in a coordinate chart, the upper semicontinuity of the mass of compact sets under weak convergence gives

\[ \int _ K\theta _ a^ k\wedge W =\lim _{\delta \downarrow 0}\int _ K\theta _{m_\delta }^ k\wedge W \leq \int _ K\theta _ b^ k\wedge W. \]

Since the measures are locally finite, inner regularity extends this inequality to all Borel subsets of the contact set, and a countable cover by charts proves the assertion. $\square$

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