[1] J. H. Barrett: A Prüfer transformation for matrix differential equations. Proc. Amer. Math. Soc. 8 (1957) 510-518. MR 0087821 | Zbl 0079.10603

[2] O. Borůvka: Lineare Differentialtransformationen 2. Ordnung. VEB Deutscher Verlag der Wissenschaften, Berlin 1967. MR 0236448

[3] W. A. Coppel: Disconjugacy. Lectures Notes in Mathematics 220, Springer Verlag, Berlin-New York-Heidelberg 1971. MR 0460785 | Zbl 0224.34003

[4] O. Došlý: A phase matrix of linear differential systems. (to appear). MR 0796568

[5] J. Radon: Zum problem von Lagrange. Abh. Math. Sem. Univ. Hamburg 6 (1928) 237-299.

[6] W. T. Reid: Ordinary Differential Equations. John Willey, New York 1971. MR 0273082 | Zbl 0212.10901