Pome

Theorem 15.13.2F71978C42841

Theorem 15.13.2.F71978C42841 Suppose $u,v\in \mathrm{PSH}(X,\theta )$ satisfy, for constants $C_0,C_1\geq 0$,

\[ \min (u,v)\geq \rho -C_0,\qquad v\leq u+C_1. \]

If, for some $0\leq c<1$,

\[ \mathbf1_{\{ u<v\} }\mathrm{MA}_\theta (u)\leq c\, \mathbf1_{\{ u<v\} }\mathrm{MA}_\theta (v), \]

then $u\geq v$.

Proof. We first modify the barrier. Let $r=a\rho -A$, where $a>1$ is sufficiently close to one and $A$ is sufficiently large. Since

\[ \theta +\mathrm{dd}^{\mathrm{c}}(a\rho )=a\theta _\rho +(1-a)\theta , \]

we can arrange $\theta _ r\geq \omega /2$ and $r-v\leq -1$, and moreover $u-r\to +\infty $ at $Z$, because $u-r\geq (a-1)(-\rho )+A-C_0$. When $Z$ is empty, the last property is not needed.

Choose $0<\delta <1$ with $(1-\delta )^ n>c$, and put $v_\delta =(1-\delta )v+\delta r$. Then

\[ u-v_\delta \geq \delta (u-r)-(1-\delta )C_1. \]

Suppose that the infimum $m$ of $u-v_\delta $ on $\Omega $ were negative. Since $u-v_\delta \to +\infty $ at $Z$, a sequence $x_ j$ along which $u-v_\delta $ tends to $m$ has a subsequence converging to a point $x_0\in \Omega $. Choose a coordinate ball $B\Subset \Omega $ centered at $x_0$, identify it with $\{ |z|<1\} $, and take

\[ \eta (z)=b(1-|z|^2),\qquad 0<b<1, \qquad \omega /2+\mathrm{dd}^{\mathrm{c}}\eta \geq 0. \]

The function $w=v_\delta +\delta \eta +m$ is $\theta $-plurisubharmonic on $B$, and $\liminf _{\partial B}(u-w)\geq 0$ because $\eta $ vanishes on $\partial B$. Also

\[ w-v=\delta (r-v+\eta )+m<0. \]

Thus $\{ u<w\} \subset \{ u<v\} $, and positivity of mixed products gives on this set

\[ \mathrm{MA}_\theta (u)\leq c\mathrm{MA}_\theta (v)\leq (1-\delta )^ n\mathrm{MA}_\theta (v)\leq \mathrm{MA}_\theta (w). \]

By lemma 15.13.1, $u\geq w$ on $B$. Evaluating at $x_ j$ gives $u(x_ j)-v_\delta (x_ j)\geq m+\delta \eta (x_ j)$, which contradicts $\eta (x_0)>0$ in the limit. Hence $u\geq v_\delta $ on $\Omega $. Letting $\delta \downarrow 0$ along values with $(1-\delta )^ n>c$, we obtain $u\geq v$ on $\Omega $, and lemma 5.24.3 extends this inequality to $X$. $\square$

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