@misc{indiciaecbe9e8f87719,
  title = {Upper and lower bounds to eigenvalues from variational functionals.},
  author = {FM Fernández and EA Castro},
  year = {1987},
  doi = {10.1103/physreva.35.4861},
  url = {https://pubmed.ncbi.nlm.nih.gov/9898101/},
  note = {Source identifier: 9898101}
}