(1) Let $A=\{x \mid x=2 n, n \in \mathbf{N}\}, B=\{x \mid x=3 n, n \in \mathbf{N}\}$, $C=\left\{x \mid x=n^{2}, n \in \mathbf{N}\right\}$, then $A \cap B \cap C=$ $\qquad$ .
1 1 $\left\{x \mid x=36 n^{2}, n \in \mathbf{N}\right\}$ Hint: Note that the numbers in $A \cap B \cap C$ are all perfect squares, and are divisible by 2 and 3, thus they are divisible by 4 and 9, so the answer should be $\{x \mid x=$ $\left.36 n^{2}, n \in \mathbf{N}\right\}$.