1. Rotation of eigenvectors by a perturbation;Davis;J. Math. Anal. Appl.,1963
2. Separation of two linear subspaces;Davis;Acta Sci. Math. Szeged.,1958
3. Approximate proper vectors;Berberian,1962
4. Introduction to Hilbert Space and the Theory of Spectral Multiplicity;Halmos,1951
5. Upper and lower bounds of eigenvalues;Kato;J. Phys. Soc. Japan,1949