Watch this videoLaplacian in 3D and tell me whether $C_{n,m}$ missing y in the argument of $\cosh{\sqrt{(\frac{n\pi}{a})^2+(\frac{m\pi}{a})^2}}$ in the denominator. So the final answer is $V_{(x,y,z)}=\frac{32V_0}{\pi^2}\displaystyle\sum_{n=1,3,5...}^\infty \displaystyle\sum_{m=1,3,5...}^\infty\frac{\sin{(\frac{n\pi x}{a})\sin{(\frac{m\pi z}{a})}}}{m n}$
So what is your opinion about this? I think math professor in the video forgot to write y in the argument of $\cosh$