http://en.wikipedia.org/wiki/Macbeth
So perhaps something very easy for one is very difficult for others, and vice versa.
http://en.wikipedia.org/wiki/Perturbation_theory_(quantum_mechanics)
The equation itself should not be difficult to understand even for undergraduate students, but it took me several hours to solve a simple example such as:
The problem is a harmonic oscillator (the first and second terms of the right-hand side) with a perturbation proportional to x (the third term).
If you only notice that the operator x can be replaced by the lowering and raising operators, you will immediately get:
This is because there are only two non-zero terms in the summation (the third term of the right-hand side of the first equation). Although k goes from 0 to infinity, non-zero terms appear only when k=n-1 and k=n+1.