@article{700129,
     title = {Linear perturbations of general disconjugate equations},
     booktitle = {Equadiff 6},
     series = {GDML\_Books},
     publisher = {J. E. Purkyn\v e University, Department of Mathematics},
     address = {Brno},
     year = {1986},
     pages = {[181]-186},
     mrnumber = {MR0877121},
     zbl = {0611.34024},
     url = {http://dml.mathdoc.fr/item/700129}
}

Trench, William F. Linear perturbations of general disconjugate equations, dans Equadiff 6, GDML_Books,  (1986), pp. [181]-186. http://gdmltest.u-ga.fr/item/700129/