F. V. Atkinson, A. B. Mingarelli, Asymptotics of the number of zeros and the eigenvalues of general weighted Sturm-Liouville problems, J. Reine angev. Math. 375 (1987), 380-393.
 M. Duman, Asymptotics for the Sturm-Liouville problem by homotopy perturbation method, Applied Mathematics and Computation, 216 (2010), 492-496.
 J. H. He, Homotopy perturbation technique, computer Methods in Applied mathematics and Engineering, 178 (1999), 257-262.
 J. H. He, Homotopy perturbation method: a new nonlinear technique, Applied Mathematics and Computation, 135 (2003), 73-79.
 H. Hochstsdt, Differential equations, Dover, New york, 1957.
 E. L. Ince, Ordinary differential equations, Dover, New york,1956.
 B. M. Levitan, G. Gasymov, Determination of a differential equation by its spectra, Russian Math. Surveys. 19 (1964), 1-63.
 B. M. Levitan, G. Gasymov, Introduction to spectral theory, Translation of Math. Amer. Math. Society (1975).
 A. B. Mingarelli, A survey of the regular weighted Sturm-Liouville problem, The non-definite case, P. Fuquan, X. Shutie(Eds.), Applied Differential Equations, World scientific, Singapoure and Philadelphia(1986), 109-137.
 Z. M. Odibat, A new modification of the homotopy perturbation method for linear and nonlinear operators, Applied Mathematics and Computation, 189 (2007), 746-753.
 J. I. Ramos, Piecewise homotopy methods for nonlinear ordinary differential equations, Appled Mathematics and Computation, 198 (2008), 92-116.
 F. G. Tricomi, Differential equations, Hofner, New york, 1961.
 E. yusufoglu, A homotopy perturbation algorithm to solve a system of Fredholm-Volterra type integral equations, Mathematical and Computer Modeling, 47 (2008), 1099-1107.