1. N.I. Akhiezer and I.M. Glazman. Theory of linear operators in Hilbert space: I and II (Ungar, New York, 1961; translated from the Russian edition).
2. A. Erdélyi et al. Higher transcendental functions: I and II (McGraw-Hill, New York, 1953).
3. Mathematical Studies;W.N. Everitt,1974
4. M.A. Naimark. Linear differential operators: II (Ungar, New York, 1968; translated from the Russian).
5. Mathematical Studies;Å. Pleijel,1976