1. S. Agmon,Lectures on Elliptic Boundary Value Problems, Van Nostrand, Princeton, N.J., 1965.
2. A. G. Aslanjan -V. B. Lidskii,The spectrum of an elliptic equation, Mat. Zametki,7 (1970), pp. 495–502.
3. D. Banks,Bounds for the eigenvalues of some vibrating systems, Pacific J. Math.,10 (1960), pp. 439–474.
4. P. R. Beesack,A note on an integral inequality, Proc. Amer. Math. Soc.,8 (1957), pp. 875–879.
5. D. R. Dunninger,A Picone identity for non-self adjoint elliptic operators, Atti Accad. Naz. Lincei Rend.,48 (1970), pp. 133–139.