@Isaac0427 @A. Neumaier :
Taking Isaac's wave function,
$$I = \int^\infty_{-\infty} dx ~ \psi^*_{k'}(x) \psi_k(x) = I_> + I_<,$$ where
$$I_> = \int^\infty_{0} dx ~ \psi^*_{k'}(x) \psi_k(x), \quad I_< = \int^0_{-\infty} dx \psi^*_{k'}(x) \psi_k(x) .$$ Substituting the expressions,
$$I_> = N^2_k...