@article{NeidhardtZagrebnov1997,
  author    = {Neidhardt, Hagen and Zagrebnov, Valentin A.},
  title     = {Fractional powers of self-adjoint operators and Trotter-Kato product formula},
  year      = {1997},
  language  = {en}
}
@article{NeidhardtZagrebnov1997,
  author    = {Neidhardt, Hagen and Zagrebnov, Valentin A.},
  title     = {On the right Hamiltonian for the singular perturbations : general theory},
  year      = {1997},
  language  = {en}
}