A relation T with domain and range in a Hilbert space is said to be accretive if the
transformation $ (T − \lambda)/(T + \bar \lambda\ ) $
with domain and range in the Hilbert space is contractive for some, and hence every,
complex number $\lambda$ in the right half–plane.


How does some $\lambda$ imply for all $\lambda$? De Branges states this in his "proof" of RH (in fact uses it quite extensively). Also can anyone suggest references for such operators.



definition of accretive operator
Check Kato: [Perturbation theory...](http://poncelet.sciences.univ-metz.fr/~gnc/bibliographie/Functional%20Analysis/Perturbation_Theory_for_Linear_Operators__Second_Edition__Classics_in_Mathematics_%283%29.pdf), Section V.3.10. 


