Robin-to-Robin Maps and Krein-Type Resolvent Formulas for Schr\"odinger   Operators on Bounded Lipschitz Domains
  We study Robin-to-Robin maps, and Krein-type resolvent formulas for Schr\"odinger operators on bounded Lipschitz domains in $\bbR^n$, $n\ge 2$, with generalized Robin boundary conditions. 
Fritz Gesztesy and Marius Mitrea