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Proposition 13.24.820027E32C14E

Proposition 13.24.8.20027E32C14E Let $v\in \mathrm{PSH}(X,\theta )$. The conditions

\[ \nu (v\circ \pi ,E)=b(E)\text{ for every }\pi ,E, \qquad v\sim _{\mathcal I}u \]

are equivalent. Under either condition,

\[ q=P_\theta [v]_{\mathcal I}=P_\theta [u]_{\mathcal I}\sim u, \]

and every $w\in \mathrm{PSH}(X,\theta )$ with $w\sim q$ satisfies

027648EDCB33 13.24.9
\begin{equation} \int _ X\mathrm{MA}^{\mathrm{ext}}_\theta (w) =\underline{\operatorname {vol}}_\theta (q) =\overline{\operatorname {vol}}_\theta (q) =\int _ XR^2>0. \end{equation}

The background is big and $X$ has the bounded mass property.

Proof. Ideal profile and envelope. Choose an integer $C>A$ with $C\kappa \geq \theta $. Both $u$ and $v$ are $C\kappa $-plurisubharmonic. The projective valuative characterization of all-weight ideal types applies in this closed background: generic orders on all smooth projective modifications recover point Lelong numbers by further point blowups, and equality of those numbers is equivalent to ideal equivalence. The stated profile therefore implies $v\sim _{\mathcal I}u$. Conversely, ideal equivalence pulls back under every compact modification by lemma 5.43.23; the local valuative criterion theorem 5.37.3 then gives equality of generic coefficients on every such modification. The common factor between growth against $\log |z|$ and the book's $\log |z|^2$ convention does not affect these equalities.

The potential $u$ has complex Hessian $S-A\kappa $. By corollary 18.4.9, its ordinary and ideal envelopes agree and are ordinarily equivalent to $u$. Ideal equivalence identifies the defining families for the two ideal envelopes in the statement, proving the assertion about $q$. The inequality $\theta +\mathrm{dd}^{\mathrm{c}}u\geq R>0$ proves bigness. For any Hermitian form $h$ on $X$, choose $D$ with $D\kappa \geq h$. Bounded $h$-plurisubharmonic potentials are $D\kappa $-plurisubharmonic, and positivity of bounded mixed products bounds their masses by $\int _ X(D\kappa )^2=D^2$. This proves the bounded mass property.

The bounded residual. Fix $w\sim u$ in $\mathrm{PSH}(X,\theta )$ and put $T=C\kappa +\mathrm{dd}^{\mathrm{c}}w$. Its base divisorial coefficients are $a_ j$, so divisorial removal and passage through increasing finite sums give

\[ K=T-S=(C-A)\kappa +\mathrm{dd}^{\mathrm{c}}h\geq 0,\qquad h=w-u. \]

The distributional difference has a canonical quasi-plurisubharmonic representative, which is bounded by ordinary equivalence and almost-everywhere order. Thus $K$ has bounded local potentials, and its trace charges no pluripolar set. The current $S$ is carried by the actual pole union $\bigcup _ jH_ j\subset \{ w=-\infty \} $. Thus $\langle T\rangle =K$. For every smooth real $(1,1)$-form $L$ with $\mathrm{dd}^{\mathrm{c}}L=0$, distributional Stokes gives

\[ \int _ X L\wedge \langle T\rangle =(C-A)\int _ XL\wedge \kappa . \]

Closed top mass and smooth correction. Put $T_ u=C\kappa +\mathrm{dd}^{\mathrm{c}}u=S+(C-A)\kappa $. For the finite partial sums $u_ N=\sum _{j\leq N}a_ j\ell _ j$, the closed non-pluripolar mass is $(C-\sum _{j\leq N}a_ j)^2$: outside the finite divisor union the current is the smooth residual, and the measure gives that union zero mass. Since $u_0\leq u_ N$, closed-background mass monotonicity, the closed case of theorem 13.17.1, gives

\[ \int _ X\langle T_ u^2\rangle \leq \left(C-\sum _{j\leq N}a_ j\right)^2. \]

For the reverse inequality, locally write $S=\mathrm{dd}^{\mathrm{c}}b$ with $b$ plurisubharmonic. The currents $(C-A)\kappa +\mathrm{dd}^{\mathrm{c}}\max (b,-k)$ have bounded local potentials and dominate $(C-A)\kappa $. Their squared products, restricted to $\{ b>-k\} $, therefore dominate $(C-A)^2\kappa ^2$ on that set. Letting $k$ increase and using the local truncation definition gives $\langle T_ u^2\rangle \geq (C-A)^2\kappa ^2$. The two bounds prove equality of total masses. Closed mass invariance under bounded changes then gives

\[ \int _ X\langle T^2\rangle =(C-A)^2. \]

Set $L=C\kappa -\theta \geq 0$, so $\mathrm{dd}^{\mathrm{c}}L=0$. Expanding the bounded products of $\max (w,-k)$ in the two backgrounds and restricting to $\{ w>-k\} $ gives, in the limit,

\[ \mathrm{MA}^{\mathrm{ext}}_\theta (w) =\langle T^2\rangle -2L\wedge \langle T\rangle +L^2. \]

All correction measures are finite: the closed top measure has the mass just computed, the degree-one limit is $K$, and the smooth measure $L^2$ gives the pole set zero mass. The same truncations are used in both backgrounds, so this identity is an equality of Borel measures. Integrating yields

\[ \int _ X\mathrm{MA}^{\mathrm{ext}}_\theta (w) =(C-A)^2-2(C-A)\int _ XL\wedge \kappa +\int _ XL^2 =\int _ X(\theta -A\kappa )^2. \]

This holds throughout the ordinary type, proving both singularity-volume identities and their strict positivity. $\square$

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