1. Existence, convergence and equivalence of plates and membranes;Aronszjan,1941
2. Bounds for eigenvalues and the method of intermediate problems;Weinstein,1961
3. Methods of Intermediate Problems for Eigenvalues;Weinstein,1972
4. Variational Methods of Eigenvalue Problems;Gould,1966
5. Approximation of bounds for eigenvalues and eigenvectors using the Intermediate-Problem technique;Dame,1972