@article{PORMA_1991__48_2_115748,
     author = {Rubio, G. and J\'odar, L.},
     title = {Approximate solutions and error bounds for second order operator differential equations},
     journal = {Portugaliae mathematica},
     pages = {165--177},
     publisher = {mathdoc},
     volume = {48},
     number = {2},
     year = {1991},
     mrnumber = {1127119},
     zbl = {0773.47031},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_1991__48_2_115748/}
}