@misc{indiciae773d223bb800,
  title = {Studies in perturbation theory. ix - connection between various approaches in the recent development - evaluation of upper bounds to energy eigenvalues in schroedinger's perturbation theory.},
  author = {Loewdin, P.-O.},
  year = {1965},
  url = {https://ntrs.nasa.gov/citations/19650050736},
  note = {Source identifier: 19650050736}
}