1. 1. N. K. Ahiezer and I. M. Glazman, The theory of linear operators in Hilbert space, GITTL, Moscow, 1950; German transl., Akademie-Verlag, Berlin, 1954; English transl., Ungar, New York, 1961. MR 13, 358; 16, 596; 41 #9015a.
2. On a problem of Weyl in the theory of singular Sturm-Liouville equations;Aronszajn, N.;Amer. J. Math.,1957
3. On the perturbation of spectra;Donoghue, William F., Jr.;Comm. Pure Appl. Math.,1965
4. 5. R. W. Carey and J. D. Pincus, Intertwining partial isometries. II (to appear).