@misc{indiciae333600b4e022,
  title = {Useful extremum principle for the variational calculation of matrix elements},
  author = {Gerjuoy, E. and Rau, A. R. P. and Rosenberg, L. and Spruch, L.},
  year = {1974},
  url = {https://ntrs.nasa.gov/citations/19740040954},
  note = {Source identifier: 19740040954}
}