[1] C. D. Ahlbrand D. B. Hinton R. T. Lewis:
The effect of variable change on oscillation and disconjugacy criteria with application to spectral theory. J. Math. Anal. Appl. 81 (1981) 234-277.
MR 0618771
[2] J. H. Barret:
A Prüfer transformation for matrix differential equations. Proc. Amer. Math. Soc. 8 (1957) 510-518.
MR 0087821
[4] O. Borůvka:
Linear Differential Transformation of the Second Order. London 1971.
MR 0463539
[5] P. Hartman:
Self-adjoint, nonoscillatory systems of ordinary, second order, linear equations. Duke Math. J. 24 (1957) 25-35.
MR 0082591
[6] M. Morse: A generalization of the Sturm separation and comparison theorem in n-space. Math. Ann. 103 (1935) 72-91.
[7] J. Radon: Zum problem von Lagrange. Abh. Math. Sem. Univ. Hamburg 6 (1928) 273 - 279.
[9] R. L. Sternberg:
Variational methods and nonoscillatory theorems for systems of differential equations. Duke Math. J. 19 (1952) 311-322.
MR 0048668