Posted by haifeng on 2024-05-14 23:32:07 last update 2024-05-14 23:32:07 | Answers (1) | 收藏
\[ f(x,y)=\begin{cases} xy\sin\frac{1}{x^2+y^2}, & (x,y)\neq(0,0),\\ 0, & (x,y)=(0,0). \end{cases} \]
