@article{MAN_1990__287_1_164705,
     author = {W. Hansen and Z. Ma},
     title = {Perturbation by differences of unbounded potentials.},
     journal = {Mathematische Annalen},
     pages = {553--570},
     publisher = {mathdoc},
     volume = {287},
     number = {1},
     year = {1990},
     zbl = {0685.31005},
     url = {http://geodesic.mathdoc.fr/item/MAN_1990__287_1_164705/}
}