1. Variational solution of some nonlinear eigenvalue problems. I;Andrews;J. Math. Anal. Appl.,1970
2. Multiparameter spectral theory;Atkinson;Bull. Amer. Math. Soc.,1968
3. Lower bounds for eigenvalues;Bazley;J. Math. Mech.,1961
4. Truncations in the method of intermediate problems for lower bounds to eigenvalues;Bazley;J. Res. Nat. Bur. Standards Sect. B,1961
5. Numerical Ranges of Operators on Normed Spaces and of Elements of Normed Algebras;Bonsall,1971