Spectra of self-adjoint extensions and applications to solvable   Schroedinger operators
  We give a self-contained presentation of the theory of self-adjoint extensions using the technique of boundary triples. A description of the spectra of self-adjoint extensions in terms of the corresponding Krein maps (Weyl functions) is given. Applications include quantum graphs, point interactions, hybrid spaces, singular perturbations. 
Jochen Bruening, Vladimir Geyler, Konstantin Pankrashkin