correct this: 
The pair $(\eta_n,\psi_n)$ solves the eigenvalue problem of an oscillator. According to this theory, the set of functions $\psi_n$ forms a complete, orthonormal, and normalized basis for the space of solutions of the Schrödinger equation.  Write exactly 3 paragraphs each separated with two new lines answering this question. The last paragraph must start with "difficult".
user