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jirilebl committed May 21, 2019
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Expand Up @@ -6746,7 +6746,7 @@ \section{Harmonic, subharmonic, and plurisubharmonic functions}
is similar to the solution of the Dirichlet
problem using the Poisson kernel.

First $f$ is bounded above on compact sets as it is upper semicontinuous.
The function $f$ is bounded above on compact sets as it is upper semicontinuous.
If $f$ is not bounded below, we replace $f$ with $\max \bigl\{ f ,
\frac{-1}{\epsilon}
\bigr\}$, which is still plurisubharmonic. Therefore, without loss of generality
Expand All @@ -6763,13 +6763,13 @@ \section{Harmonic, subharmonic, and plurisubharmonic functions}
The two forms of the integral follow easily via change of variables.
We are perhaps abusing notation a bit as $f$ is only defined on $U$,
but it is not a problem as long as $z \in
U_\epsilon$ as $g_\epsilon$ is then zero when $f$ is undefined.
U_\epsilon$, because $g_\epsilon$ is then zero when $f$ is undefined.
By differentiating the first form under the integral, we find that
$f_\epsilon$ is smooth.

Let us show that $f_\epsilon$ is plurisubharmonic. We restrict to a
line $\xi \mapsto a+b\xi$.
We wish to test subharmonicity by the sub-mean-value property using a disc
We wish to test subharmonicity by the sub-mean-value property using a circle
of radius $r$ around $\xi = 0$:
\begin{equation*}
\begin{split}
Expand Down Expand Up @@ -6856,8 +6856,9 @@ \section{Harmonic, subharmonic, and plurisubharmonic functions}
\end{split}
\end{equation*}
The second equality above
follows because integral of $g_\epsilon$ only needs to be
done over the polydisc of radius $\epsilon$.
follows as $g_\epsilon$ is zero
outside the polydisc of radius $\epsilon$.
For the inequalities, we again needed that $g_\epsilon \geq 0$.
The penultimate equality follows from the fact that
$2\pi = \int_0^{2\pi}d \theta$.

Expand Down Expand Up @@ -6900,24 +6901,25 @@ \section{Harmonic, subharmonic, and plurisubharmonic functions}
\begin{exercise}
\begin{exparts}
\item
Show that for a subharmonic function $\int_0^{2\pi} f(a+re^{i\theta}) \,
Show that for a subharmonic function, $\int_0^{2\pi} f(a+re^{i\theta}) \,
d\theta$ is a monotone function of $r$ (Hint: Try a $C^2$ function first and
use Green's theorem).
\item
Use this
fact to show that $f_\epsilon(z)$ from \thmref{thm:subharlim} are monotone
fact to show that the $f_\epsilon(z)$ from \thmref{thm:subharlim} is monotone
decreasing in $\epsilon$.
\end{exparts}
\end{exercise}

\begin{exercise}
Let $U \subset \C^n$
and $V \subset \C^m$ be open.
If $g \colon U \to V$ is holomorphic and $f
Prove that
if $g \colon U \to V$ is holomorphic and $f
\colon V \to \R$ is a $C^2$ plurisubharmonic function, then
$f \circ g$ is plurisubharmonic.
Then use this to show that
this holds for all plurisubharmonic functions (Hint: monotone convergence).
Then use it to prove the same fact
for all plurisubharmonic functions (Hint: monotone convergence).
\end{exercise}

\begin{exercise}
Expand All @@ -6935,7 +6937,7 @@ \section{Harmonic, subharmonic, and plurisubharmonic functions}

\begin{exercise}
Let the $f$ in \thmref{thm:subharlim} be continuous and suppose $K \subset
\subset U$, in particular for small enough $\epsilon >0$, $K \subset U_\epsilon$.
\subset U$. For small enough $\epsilon >0$, $K \subset U_\epsilon$.
Show that $f_\epsilon$ converges uniformly to $f$ on $K$.
\end{exercise}

Expand Down Expand Up @@ -7169,7 +7171,7 @@ \section{Hartogs pseudoconvexity}
whenever $[x_\alpha,y_\alpha] \subset U$
is a collection of straight line segments such that
$\bigcup_{\alpha} \{ x_\alpha,y_\alpha \} \subset \subset U$
then
implies
$\bigcup_{\alpha} [ x_\alpha,y_\alpha ] \subset \subset U$.
\end{exercise}
\end{exbox}
Expand Down

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