1. Bellman, R.,Perturbation Techniques in Mathematics, Physics, and Engineering, Holt, Rinehart, and Winston, New York, New York, 1964.
2. Courant, R.,Differential and Integral Calculus, Vol. 2, Interscience Publishers, New York, New York, 1961.
3. Ford, L. R.,Differential Equations, McGraw-Hill, New York, New York, 1955.
4. Ince, E. L.,Ordinary Differential Equations, Dover, New York, New York, 1956.
5. Murphy, G. M.,Ordinary Differential Equations and Their Solutions, D. Van Nostrand, New York, New York, 1960.