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@article{PORMA_1955__14_3_114755, author = {Barrett, John H.}, title = {Matrix systems of second order differential equations}, journal = {Portugaliae mathematica}, pages = {79--89}, publisher = {mathdoc}, volume = {14}, number = {3}, year = {1955}, mrnumber = {0080219}, zbl = {0066.33703}, language = {en}, url = {http://geodesic.mathdoc.fr/item/PORMA_1955__14_3_114755/} }
Barrett, John H. Matrix systems of second order differential equations. Portugaliae mathematica, Tome 14 (1955) no. 3, pp. 79-89. http://geodesic.mathdoc.fr/item/PORMA_1955__14_3_114755/