@book{NeidhardtZagrebnov1994,
  author    = {Neidhardt, Hagen and Zagrebnov, Valentin A.},
  title     = {Mathematical results in quantum mechanics : international conference in Blossin (Germany), May 17 - 21, 1993},
  series = {Operator Theory},
  volume    = {70},
  journal   = {Operator Theory},
  editor    = {Demuth, Michael and Exner, Pavel},
  publisher = {Birkh{\"a}user},
  address   = {Basel},
  pages     = {X, 356 S.},
  year      = {1994},
  language  = {en}
}
@article{NeidhardtZagrebnov1994,
  author    = {Neidhardt, Hagen and Zagrebnov, Valentin A.},
  title     = {Singular perturbations, regularization and extension theory},
  year      = {1994},
  language  = {en}
}
@article{BrascheNeidhardt1994,
  author    = {Brasche, Johannes F. and Neidhardt, Hagen},
  title     = {Some remarks to Krein's extension theory},
  year      = {1994},
  language  = {en}
}
@article{BrascheKoshmanenkoNeidhardt1994,
  author    = {Brasche, Johannes F. and Koshmanenko, V. D. and Neidhardt, Hagen},
  title     = {Some new aspects of Krein's extension theory},
  year      = {1994},
  language  = {en}
}