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Corollary 20.2.50B590E31078F3

Corollary 20.2.50.B590E31078F3 Assume that $n\geq 2$. Let $\varphi \in \mathrm{QPSH}(X)$. Then quasi-every $H\in \Lambda $ is connected and smooth, satisfies $\nu (\varphi ,H)=0$ and we have

\[ \operatorname{Tr}_ H(\varphi )\sim _{\mathcal{I}} \varphi |_ H. \]

Proof. First observe that the set $\{ x\in X:\nu (\varphi ,x)>0\} $ is a countable union of proper analytic subsets by Siu's semicontinuity theorem theorem 5.16.7. It follows that a very general element in $\Lambda $ is not contained in this set.

Fix an ample line bundle $L$ so that there is a smooth Hermitian metric $h_ L$ such that $c_1(L,h_ L)+\mathrm{dd}^{\mathrm{c}}\varphi $ is a Kähler current. Thanks to theorem 20.2.45 and the connectedness assumption on $\Lambda $, we can find a co-pluripolar set $\Lambda '\subseteq \Lambda $ such that each $H\in \Lambda '$ satisfies the following:

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    $H$ is smooth and connected;

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    $\nu (\varphi ,H)=0$;

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    $\mathcal{I}(k\varphi |_ H)=\operatorname{Res}_{H}(\mathcal{I}(k\varphi ))$ for all $k\in \mathbb {Z}_{>0}$.

Put $\alpha _ H\coloneqq c_1(L,h_ L)|_ H$. It follows from theorem 20.2.39 and theorem 18.1.46 that

\[ \int _ H \left( \alpha _ H+\mathrm{dd}^{\mathrm{c}}\operatorname{Tr}_ H^{c_1(L,h_ L)}(\varphi )\right)^{n-1} = \int _ H \left( \alpha _ H+\mathrm{dd}^{\mathrm{c}}P_{c_1(L,h_ L)|_ H}\left[\varphi |_ H\right]_{\mathcal{I}} \right)^{n-1}. \]

Since $\varphi |_ H\preceq _ P \operatorname{Tr}_ H(\varphi )$ by proposition 20.2.12, we also have $\varphi |_ H\preceq _{\mathcal I}\operatorname{Tr}_ H(\varphi )$. Hence

\[ P_{c_1(L,h_ L)|_ H}[\varphi |_ H]_{\mathcal I}\leq P_{c_1(L,h_ L)|_ H}[\operatorname{Tr}_ H(\varphi )]_{\mathcal I}. \]

The trace is $\mathcal I$-good, so the right-hand side agrees with its $P$-envelope. The equality of the masses above and theorem 16.3.15 then imply that the two sides have the same $P$-singularity type, hence

\[ \operatorname{Tr}_ H(\varphi )\sim _{\mathcal I}\varphi |_ H. \]
$\square$

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