@article{702185,
     title = {On various properties of the solutions of third- and fourth-order linear differential equations},
     booktitle = {Differential Equations and Their Applications},
     series = {GDML\_Books},
     publisher = {Publishing House of the Czechoslovak Academy of Sciences},
     address = {Prague},
     year = {1963},
     pages = {187-198},
     mrnumber = {MR0174825},
     zbl = {0128.31103},
     url = {http://dml.mathdoc.fr/item/702185}
}

Švec, M. On various properties of the solutions of third- and fourth-order linear differential equations, dans Differential Equations and Their Applications, GDML_Books,  (1963), pp. 187-198. http://gdmltest.u-ga.fr/item/702185/