A three-dimensional harmonic oscillator moves in the potential $P(x, y, z)=\frac{1}{2} m \omega^{2}\left(x^{2}+y^{2}+z^{2}\right) .$ Separate the Schrödinger equation and show that the allowed energies are given by $E=\left(n+\frac{3}{2}\right) \hbar \omega .$