Etgen, G. J., & Morris, W. L. (1977). Introduction to ordinary differential equations: With difference equations, numerical methods, and applications. Harper & Row.

Etgen, Garret J., and William L. Morris. Introduction to Ordinary Differential Equations: With Difference Equations, Numerical Methods, and Applications. New York: Harper & Row, 1977.

Etgen, Garret J., and William L. Morris. Introduction to Ordinary Differential Equations: With Difference Equations, Numerical Methods, and Applications. Harper & Row, 1977.