@article{MS2_1968__22_166029,
     author = {Erik Balslev},
     title = {Discreteness of the {Spectrum} of a {Second} {Order} {Elliptic} {Differential} {Operator} in {L2} (rn).},
     journal = {Mathematica Scandinavica},
     pages = {42--50},
     publisher = {mathdoc},
     volume = {22},
     year = {1968},
     zbl = {0176.39901},
     url = {http://geodesic.mathdoc.fr/item/MS2_1968__22_166029/}
}