@article{GDZPPN002473909, title = {Some Properties of Solutions of u'' + a(t) f(u) = 0.}, journal = {Monatshefte f\"ur Mathematik}, volume = {67}, year = {1963}, pages = {50-54}, zbl = {0116.29401}, url = {http://dml.mathdoc.fr/item/GDZPPN002473909} }
Waltman, P. Some Properties of Solutions of u'' + a(t) f(u) = 0.. Monatshefte für Mathematik, Tome 67 (1963) pp. 50-54. http://gdmltest.u-ga.fr/item/GDZPPN002473909/