1. N. Aronszajn,Approximation Methods for Eigenvalues of Completely Continuous Symmetric Operators, Proc. Symp. Spectral Theory and Differential Problems, Stillwater, Okla., 179–202 (1951).
2. R. G. Bartle,Singular Points of Functional Equations, Trans. Am. Math. Soc.75, 366–384 (1953).
3. N. Bazley,Lower Bounds for Eigenvalues, J. Math. Mech.10, 289–309 (1961).
4. N. Bazley andD. W. Fox,Truncations in the Method of Intermediate Problems for Lower Bounds to Eigenvalues, J. Res. Natl. Bur. Std.65B, 105–111 (1961).
5. N. Bazley andD. W. Fox,Error Bounds for Expectation Values, Rev. Mod. Phys.35, 712–715 (1963).