Laitoch, Miroslav:
Homogene lineare zu sich selbst begleitende Differentialgleichung zweiter Ordnung.
(German) [Second order homogeneous linear differential equations that are associated to themselves].Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica-Physica-Chemica,
vol. 11
(1971),
issue 1,
pp. 61-72