Problem

甲、簡答題：共 8 題，每題 8 分，共 64 分。請在答案卷上列出題號依序作答。

5. Find the values of $p$ for which the function

$$f(x,y)= \begin{cases} \dfrac{(xy)^p}{x^4+y^4}, & \text{if } (x,y)\ne(0,0),\\ 0, & \text{if } (x,y)=(0,0) \end{cases}$$

is discontinuous at $(0,0)$.