@article{700221,
     title = {Algebraic elements in the transformation theory of 2nd order linear oscillatory differential equations},
     booktitle = {Differential Equations and Their Applications},
     series = {GDML\_Books},
     publisher = {Slovensk\'e pedagogick\'e nakladate\v lstvo},
     address = {Bratislava},
     year = {1967},
     pages = {27-36},
     zbl = {0218.34005},
     url = {http://dml.mathdoc.fr/item/700221}
}

Borůvka, O. Algebraic elements in the transformation theory of 2nd order linear oscillatory differential equations, dans Differential Equations and Their Applications, GDML_Books,  (1967), pp. 27-36. http://gdmltest.u-ga.fr/item/700221/