1. M. Abramowitz and I. Stegun, Handbook of Mathematical Functions, Dover, New York, 1965.
2. G. Boole, Differential Equations (1859), 5th ed., Chelsea, New York, 1865.
3. G. Doetsch, Theorie und Anwendung der Laplace-Transformation, Springer, Berlin, 1937.
4. E. L. Ince, Ordinary Differential Equations (1926), Dover, New York, 1965.