# Problem understanding Green's function equality in Messiah QM II

1. Sep 28, 2012

### tobe

Hi,

It's about green's function in the book Messiah - Quantum Mechanics II - Chapter 16.3.2
(see http://books.google.de/books?id=OJ1...dq=messiah+quantenmechanik+kapitel+16.3&hl=de). The book actually is in german, but I guess that doesn't matter understanding the formulas.
I don't understand the last equality between eq. (16.60) & (16.61). Why is there in the first term's nominator a 1 (instead of "z - H_o - lV")? Can somebody help me out?

Cheers
Tobe

2. Sep 28, 2012

### George Jones

Staff Emeritus
Because the first term is

$$\frac{1}{z-H_0} \left( z - H_0 -\lambda V \right) \frac{1}{z - H_0 -\lambda V} = \frac{1}{z-H_0}.$$

3. Sep 28, 2012

### tobe

oh dammit! :D

thanks man!