A Note on Weinstein's Variational Method
Walter Kohn · Physical Review · 1947
Weinstein's modification of the Ritz principle is used to derive (1) a lower bound for the $n$-th energy-level of a quantum mechanical system if a lower bound for the $(n+1)\ensuremath{-}\mathrm{s}\mathrm{t}$ level is known; and (2) an upper bound for the $n$-th level if an upper bound for the $(n\ensuremath{-}1)\ensuremath{-}\mathrm{s}\mathrm{t}$ level is known.