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

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=OJ1X...tel+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
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 Mentor Because the first term is $$\frac{1}{z-H_0} \left( z - H_0 -\lambda V \right) \frac{1}{z - H_0 -\lambda V} = \frac{1}{z-H_0}.$$
 oh dammit! :D thanks man!