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Given , $f(x,y)=y\sin\frac{1}{x}$ when $x\ne0$ and $f(0,0)=0$ . Investigate differentiability at $(0,0)$ .
I've found that it is continuous at $(0,0)$ and the partial derivatives $f_x=0$ $\forall (x,y)$ and $f_y=\begin{cases}
\sin\frac1x & \text{ if } x\ne0\\
1 & \text{ if } x= 0
\end{cases}.$...