1. Gustave Choquet (with the assistance of J. Marsden, T. Lance, and S. Gelbart) Lectures on Analysis, W. A. Benjamin, New York, 1969
2. Earl A. Coddington and Normal Levinson Theory of Ordinary Differential Equations, McGraw-Hill, New York, 1955.
3. Philip Hartman Ordinary Differential Equations, Wiley, New York, 1964.