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@article{PORMA_1970__29_2_115117, author = {Jones, John jr.}, title = {On oscillatory solutions of second order differential systems}, journal = {Portugaliae mathematica}, pages = {125--133}, publisher = {mathdoc}, volume = {29}, number = {2}, year = {1970}, mrnumber = {0324138}, zbl = {0222.34035}, language = {en}, url = {http://geodesic.mathdoc.fr/item/PORMA_1970__29_2_115117/} }
Jones, John jr. On oscillatory solutions of second order differential systems. Portugaliae mathematica, Tome 29 (1970) no. 2, pp. 125-133. http://geodesic.mathdoc.fr/item/PORMA_1970__29_2_115117/