# Derivation of momentum expectancy

Hi,

I'm working through Griffiths' Introduction to QM, In the derivation for the expectation value of momentum, he uses that
$$\left( x \left. \left( \Psi^* \frac{\partial\Psi}{\partial x} - \frac{\partial \Psi^*}{\partial x}\Psi \right) \right) \right|_{-\infty}^{+\infty} = 0$$

Why is this so? It's easy to see that this is zero if the x weren't there, but I can't figure out why the limits are zero with the x in.

Thanks,
A_B

Last edited: