1. Theory of linear operators in Hilbert space;Achieser-Glasman,1954
2. On the Perturbation of Continuous Spectra;Friedrichs;Comm. Appl. Math.,1948
3. Leçons sur l’Analyse Fonctionnelle;Riesz-Nagy,1952
4. Linear Transformations in. Hilbert Space, esp. Chapter VI and VII;Stone,1932