Let $U\subseteq \mathbb{C}$ be an open set and let $u(z)=-\log(\mathrm{dist}(z,\partial U))$.
I need to show that $u$ is subharmonic on $U$?
$\partial U$ it means the boundary of $U$.
Let $U\subseteq \mathbb{C}$ be an open set and let $u(z)=-\log(\mathrm{dist}(z,\partial U))$.
I need to show that $u$ is subharmonic on $U$?
$\partial U$ it means the boundary of $U$.