Point contacts and boundary triples
  We suggest an abstract approach for point contact problems in the framework of boundary triples. Using this approach we obtain the perturbation series for a simple eigenvalue in the discrete spectrum of the model self-adjoint extension with weak point coupling. An example of a two-level quantum model is provided. 
Vladimir Lotoreichik, Hagen Neidhardt, and Igor Yu. Popov
math-ph math.MP math.SP