پ) $C = \{x^{2} | x \in \mathbb{Z}, \sqrt{-x} \in \mathbb{Z}\}$ $\sqrt{-x} \in \mathbb{Z} \Rightarrow \begin{cases} -x \geq 0 \Rightarrow x \leq 0 \\ (-x) \text{ مربع کامل است} \end{cases} \Rightarrow x = 0, -1, -4, -9, -16, \dots \Rightarrow C = \{0^{2}, (-1)^{2}, (-4)^{2}, (-9)^{2}, (-16)^{2}, \dots \} = \{0, 1, 16, 81, 256, \dots \}$