The square of an even number is always even.

The difference of two square numbers is a square number.

If $p$ is a perfect square, then $2p$ is also a perfect square.

If $m$ is a perfect square, then $3m$ cannot be a perfect square.

If $x$ is a perfect square, then $x^2$ is also a perfect square.

The square of an odd number is an _____ number.