[1] G. Birkhoff G.-C. Rota: 
Ordinary Differential Equations. Ginn and Company, Boston 1962. 
MR 0138810 
[2] O. Borůvka: 
Lineare Differentialtransformationen 2. Ordnung. VEB Deutscher Verlag der Wissenschaften, Berlin 1967. 
MR 0236448 
[3] E. A. Coddington N. Levinson: 
Theory of Ordinary Differential Equations. McGraw-Hill Book Company, Inc. New York 1955. 
MR 0069338