1. Spectral concentration for self-adjoint operators;Riddell;Pacific J. Math.,1967
2. Spectral concentration II—general theory;Conley,1966
3. On the convergence of the perturbation method;Kato;J. Fac. Sci. Univ. Tokyo Sect. I,1951
4. Perturbation theory for linear operators;Kato,1966
5. Espaces intermédiaires entre espaces hilbertiens et applications;Lions;Bull. Math. Soc. Sci. Math. Phys., R. P. Roumaine,1958