Writing n and En as power series in , we have
Here : En1 is the first-order correction to the nth eigenvalue, n1 is the first-order correction to the nth eigenfunction; En2 and n2 are the second-order corrections, and so on.
To first order (1),
To second order (2),
and so on. We’re done with , now — it was just a device to keep track of the different orders — so crank it up to 1.
The right side is a known function, so this amounts to an inhomogeneous differential equation for n1. Now, the unperturbed wave functions constitute a complete set, so n1 (like any other function) can be expressed as a linear combination of them:
but unless we are very lucky, we’re unlikely to be able to solve the Schrö dinger equation exactly, for this more complicated potential. Perturbation theory is a systematic procedure for obtaining approximate solutions to the perturbed problem by building on the known exact solutions to the unperturbed case.