@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/}
}