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@article{PORMA_1966__25_2_114978, author = {Stein, F. Max}, title = {A note on families of singular {Sturm-Liouville} systems}, journal = {Portugaliae mathematica}, pages = {91--98}, publisher = {mathdoc}, volume = {25}, number = {2}, year = {1966}, mrnumber = {0209550}, zbl = {0146.29703}, language = {en}, url = {http://geodesic.mathdoc.fr/item/PORMA_1966__25_2_114978/} }
Stein, F. Max. A note on families of singular Sturm-Liouville systems. Portugaliae mathematica, Tome 25 (1966) no. 2, pp. 91-98. http://geodesic.mathdoc.fr/item/PORMA_1966__25_2_114978/