@article{CMH_1982__57_139898,
     author = {E. Trubowitz and B.E.J. Dahlberg},
     title = {A remark on two dimensional periodic potentials.},
     journal = {Commentarii mathematici Helvetici},
     pages = {130--134},
     publisher = {mathdoc},
     volume = {57},
     year = {1982},
     zbl = {0539.35059},
     url = {http://geodesic.mathdoc.fr/item/CMH_1982__57_139898/}
}