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Let $B=\left\{ \left(x,y\right)\in\mathbb{R}^{2}:x^{2}+y^{2}<1\right\} $ be the unit ball in $\mathbb{R}^{2}.$

Can we construct a subharmonic function $f:B\rightarrow\left[-\infty,0\right]$ such that $$ 0<\int_{\widetilde{B}}\left(1-x^{2}-y^{2}\right)^{-2}dV<\infty, $$ where $\widetilde{B}=\left\{ \left(x,y\right)\in B:-1<f\left(x,y\right)\right\} $? Here $dV$ is the standard Lebesgue measure on $\mathbb{R}^{2}$.

I thought that in order to answer this we need to control (understand) the growth on sublevel sets of a subharmonic function.

The question was already asked on mathstackexchange and received no answers.

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  • $\begingroup$ I'm not sure I understand your comment on the key ' the growth on sublevel sets'. Could you elaborate on the approach you had in mind? The way I see it, the question hinges upon whether $\{ f > -1 \}$ can meet the boundary $\partial B$ along a cusp. $\endgroup$
    – Leo Moos
    Commented Jun 29, 2021 at 10:41
  • $\begingroup$ @Leo It was just my vague comment. The integrand is singular w.r.t. the distance function and the domain can be seen as a union of consecutive level sets of $f$. $\endgroup$ Commented Jun 29, 2021 at 11:02

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The answer is "yes". Let $E$ be some Jordan region in the unit disk on which $$\int_E(1-x^2-y^2)^{-2}dxdy<\infty,$$ and such that $E$ contains $[0,1)$, and the closure of $E$ is contained in the open unit disk, except the point $1$. Let $\phi$ be a conformal map of $E$ onto the right half-plane, such that $\phi(1)=\infty$.

Let $u=\Re \phi$. Then $u$ is positive and harmonic n $E$, zero on $\partial E\backslash\{1\}$. Extend $u$ to the whole unit disk by setting is equal to $0$ outside $E$. Then take $f=u-2.$

As you see from this example, the weight $(1-|z|^2)^{-2}$ is not very relevant. You can replace it by anything you like.

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  • $\begingroup$ @Eremenko Does the existence of such a conformal map $\phi$ follow from Riemann mapping theorem? $\endgroup$ Commented Jun 29, 2021 at 14:48
  • $\begingroup$ @Hana_a_student: yes, it does. $\endgroup$ Commented Jun 30, 2021 at 3:21
  • $\begingroup$ @Hana_a_stuent: unless you work a bit harder and find such a set E for which the map is an elementary function. $\endgroup$ Commented Jun 30, 2021 at 4:04

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