1. N. I. Ahiezer and L. M. Glazman, Theory of linear operators in Hilbert space. I, II, Ungar, New York, 1961.
2. Singular points of functional equations;Bartle, Robert G.;Trans. Amer. Math. Soc.,1953
3. Functional analysis and Galerkin’s method;Cesari, Lamberto;Michigan Math. J.,1964
4. \bysame, A nonlinear problem in potential theory, Report No. 2, NSF Research Project GP-57.
5. Functional analysis and periodic solutions of nonlinear differential equations;Cesari, Lamberto;Contributions to Differential Equations,1963