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

## Main Question or Discussion Point

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=OJ1XQ5hnINwC&pg=PA200&lpg=PA202&ots=NWr6A89Mkt&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

Related Quantum Physics News on Phys.org
George Jones
Staff Emeritus
$$\frac{1}{z-H_0} \left( z - H_0 -\lambda V \right) \frac{1}{z - H_0 -\lambda V} = \frac{1}{z-H_0}.$$