[1] Borůvka O.: 
Linear Differential Transformations of the Second Order. The English Univ. Press, London 1971. 
MR 0463539[2] Fialka M.: On identity of the differential equation y" - q(t)y = r(t) with its associated equation. Acta Univ. Palackianae Olomucensis, FRN 91 (1988), (to appeaг).
[3] Hošek J.: 
Über einige Eigenschaften der Separation der Nullstellen von Lösungen der Gleichung y" = Q(t)y und der sie begleitenden Gleichungen. Acta Univ. Palackianae Olomucensis, FRN 61 (1979), 43-50. 
MR 0589846[4] Laitoch M.: 
L’équation associée dans la théorie des transformations des équations différentielles du second ordre. Acta Univ. Palackianae Olomucensis, FRN 12 (1963), 45-62. 
MR 0276527 | 
Zbl 0256.34005[5] Laitoch M.: 
A Modification of the Sturm’s theorem on separating zeros of solutions of a linear differential equation of the 2nd order. Acta Univ. Palackianae Olomucensis, FRN 57 (1978), 27-33. 
MR 0554033 | 
Zbl 0429.34011[6] Palát J.: Priloženie teoremy Šturma. Acta Univ.Palackianae Olomucensis, FRN 69 (1981), 77-83.
[7] Pavlíková E.: 
A remark on the differential eqution of the second order y´´ +  q(x)y = r(x). Čas. pěst. mat. 109 (1984), 86-92. 
MR 0741211[8] Stěpanov V.V.: A Course of Differential Equations. (in Czech). Příгodov. nakl., Praha 1952.