[1] Vlček V.: 
The first conjugate point of solution of the N-th order iterated differential equation. Acta UP Olom., F. R. N., Tom 73, 1982. 
MR 0702611 | 
Zbl 0522.34029 
[2] Borůvka O.: 
Lineare Differentialtransformationen 2. Ordnung. VEB Deutscher Verlag der Wissenschaften, Berlin, 1967. 
MR 0236448 
[3] Sherman T. L.: 
Properties of solutions of Nth order linear differential equations. Pacific J. Math., Vol. 15, No. 3, 1965. 
MR 0185185 | 
Zbl 0132.31204 
[4] Vlček V.: 
Conjugate points of solutions of a fourth-order iterated linear differential equation. Acta UP Olom., F. R. N., Tom 53, 1977. 
MR 0594951 
[5] 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