[1] Kaňovský V.: Global transformations of linear differential equations and quadratic functionals, I. Arch. Math. Brno 3, 1983, 161-171. MR 0725199 | Zbl 0548.34039

[2] Borůvka O.: Lineare Differentialtransformationen 2. Ordnung. VEB Deutscheг Verlag der Wissenschaften Berlín, 1967. In English: Linear Differential Transformation of the second Order. The English Universities Press. Ltd 1971, London.

[3] Kurzweil J.: Obyčejné diferenciální rovnice. SNTL Praha 1978. MR 0617010

[4] Neuman F.: Extremal Property of the Equation $y = -k^2 y$. Arch. Math. (Brno) 3, 1967, 161-164. MR 0236465 | Zbl 0217.40202

[5] Neuman F.: A survey of global properties of linear differential equations of the n-th order. Ordinary and Partial Differential Equations-Proceedings, Dundee, Scotland 1982, Lecture Notes in Mathematics 964, Springer-Berlin, 548-563. MR 0693139 | Zbl 0501.34003

[6] Leighton W., Martin A. D.: Quadratic functionals with a singular endpoint. Tгans. Ameг. Math. Soc. vol. 78, 1955, 98-128. MR 0066570

[7] Kгbila J.: Quadratic functionals with Euler's equation. Acta Univ. Palac. 45, 1974, 31-43.