Krein's resolvent formula and perturbation theory
The difference between the resolvents of two selfadjoint extensions of a certain symmetric operator A is described by Krein's resolvent formula. We will prove an analog of Krein's formula in a general framework, apply it to extensions theory, and give a straightforward proof of Krein's formula including the case that A is not necessarily densely defined. We will also present a modification of Krei