@article{10_4153_CMB_1988_063_x,
     author = {Krall, Allan M.},
     title = {Semibounded {Extensions} of {Singular} {Ordinary} {Differential} {Operators}},
     journal = {Canadian mathematical bulletin},
     pages = {432--438},
     year = {1988},
     volume = {31},
     number = {4},
     doi = {10.4153/CMB-1988-063-x},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1988-063-x/}
}