Solution
= Solution
The zero value at $x=0$ removes the even factor in $x$, and the zero value at $y=0$ removes the even factor in $y$. Up to multiplicative constants, the nonzero separated modes are
$$
\boxed{\sinh(kx)\sin(ky),\qquad \sin(kx)\sinh(ky),\quad k>0.}
$$
The zero-separation case also supplies $\boxed{xy}$. Each is a <harmonic function> and vanishes on both axes.