TY  - RPRT
TI  - Upper and lower bounds to eigenvalues from variational functionals.
AU  - FM Fernández
AU  - EA Castro
PY  - 1987
DO  - 10.1103/physreva.35.4861
UR  - https://pubmed.ncbi.nlm.nih.gov/9898101/
ID  - 9898101
ER  -