1. Multiplicity of boundary values of inner functions;Aleksandrov;Izv. Akad. Nauk Arm. SSR,1987
2. Discrete and Continuous Boundary Problems;Atkinson,1964
3. On the location of the Weyl circles;Atkinson;Proc. Roy. Soc. Edinburgh Sect. A,1981
4. G. Borg, Uniqueness theorems in the spectral theory of y″+(λ−q(x))y=0, Proceedings of the 11th Scandinavian Congress of Mathematicians, Trondheim, 1949, Johan Grundt Tanums Forlag, Oslo, 1952, pp. 276–287.
5. Theory of Ordinary Differential Equations;Coddington,1955