$$u = \log ( x ^ { 2 } + y ^ { 2 } ) \frac { \partial u } { \partial y } =$$
$u=0$
$y\neq 0\text{ or }x\neq 0$
$\left\{\begin{matrix}x\in \mathrm{R}\text{, }&u=0\text{ and }y\neq 0\\x\neq 0\text{, }&\left(y\neq 0\text{ or }x\neq 0\right)\text{ and }u=0\end{matrix}\right.$
