Nech� $u \in \Cal C^2(\Bbb R ^2)$ a $u^{\star}(\rho,\varphi) =
u(\rho \cos \varphi, \rho \sin \varphi).$ Spo�t�te
$\displaystyle \frac{\partial^2
u^{\star}}{\partial \rho \partial \varphi}(1,\pi),$ jestli�e
$\displaystyle \frac{\partial u}{\partial y}(-1,0) = \frac{\partial^2
u}{\partial y \partial x}(-1,0) = 3.$
