@article{MS2_1980__47_166728,
     author = {Bent Birkeland},
     title = {A singular {Sturm-Liouville} problem treated by non-standard analysis.},
     journal = {Mathematica Scandinavica},
     pages = {275--294},
     publisher = {mathdoc},
     volume = {47},
     year = {1980},
     zbl = {0438.34019},
     url = {http://geodesic.mathdoc.fr/item/MS2_1980__47_166728/}
}