Homework Statement
Use
[H_{0},r_{j}]=\frac{i\hbar}{\mu}p_{j}
for the Hydrogen atom (where the j's denote the jth components in Cartesian coordinates) to prove that
<n_{f},l_{f},m_{l,f}|p_{j}|n_{i},l_{i},m_{l,i}>=-i\mu\omega<n_{f},l_{f},m_{l,f}|r_{j}|n_{i},l_{i},m_{l,i}>
Homework...