# Perturbation theory

1. Feb 18, 2012

### PineApple2

time-independent, non-degenerate. I am referring to the following text, which I am reading:
http://www.pa.msu.edu/~mmoore/TIPT.pdf
On page 4, it represents the results of the 2nd order terms. In Eqs. (32), (33) and (34) I don't understand the second equality, i.e. basing on which formula he has turned the potential terms into a sum.
For example, in (32) how he got from $\langle n^{(0)}|V|n^{(1)} \rangle$ to $-\sum_{m\neq 0}\frac{|V_{nm}|^2}{E_{mn}}$

2. Feb 18, 2012

### genericusrnme

You substitute in the expression you found for the second order coefficients in the expansion.
The sum is something along the lines of

$E_n^2 = \sum_{m \ne n} V_{n,m} c^1_m$

and in the first approximation you find that $c^1_m = \frac{V_n,m}{E_{m,n}}$ and you realise that V is hermitian and you multiply them together and get what you have, the negative sign comes from interchanging the m and n in the $E_{m,n} = -E_{n,m}$ term after you hermitian conjugate $c^1_m$