@misc{indiciae41e742631f2e,
  title = {Studies in perturbation theory. part 9 - 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/19680066077},
  note = {Source identifier: 19680066077}
}