@article{AA_2000_12_4_a2, author = {M. Sh. Birman and A. Laptev and T. A. Suslina}, title = {The discrete spectrum of a~two-dimensional second-order periodic elliptic operator perturbed by a~decreasing {potential.~I.} {A~semi-infinite} gap}, journal = {Algebra i analiz}, pages = {36--78}, publisher = {mathdoc}, volume = {12}, number = {4}, year = {2000}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/AA_2000_12_4_a2/} }