1. The problem statement, all variables and given/known data In the derivation of the momentum operator: Why is this equal? 2. Relevant equations [PLAIN]http://img268.imageshack.us/img268/4575/momentumoperatorderivat.png [Broken] 3. The attempt at a solution I thought maybe the integral is executed, but the integral sign stays. One p vanishes, the momentum operator itself shows up. You would really expect this in a derivation of p, but dp stays.