@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/}
}