Trivial sample of Schroedinger equation solvable exactly by the separation of variables. MR2575622 Zhang, Min-Cang; Sun, Guo-Hua; Dong, Shi-Hai Exactly complete solutions of the Schrödinger equation with a spherically harmonic oscillatory ring-shaped potential. Phys. Lett. A 374 (2010), no. 5, 704--708. 81Q05