[1] Hartmann F.: Ordinary Differential Equations. (rus.), Moskva 1970.
[2] Borůvka O.: Lineare Differentialtransformationen 2. Ordnung. VEB Deutscheг Verlag deг Wissеnschaftеn, Berlin 1967.
[3] Vlček V.:
On a distribution of zeros of solutions of a fourth-order iterated linear differential equation. Acta UP Olom., F. R. N., Tom 57 (1978).
MR 0554040
[4] Vlček V.:
On functions describing a distribution of zeros of solutions of an iterated linear differential equation of the fourth order. Acta UP Olom., F. R. N., (v tisku).
Zbl 0452.34028
[5] Kamke E.:
Differentialgleichungen I. Gewöhnliche Differentialgleichungen, Leipzig 1969.
Zbl 0194.39301