@article{AA_2003_15_4_a1,
     author = {M. Sh. Birman},
     title = {Average procedure for periodic operators near an edge of internal gap},
     journal = {Algebra i analiz},
     pages = {61--71},
     publisher = {mathdoc},
     volume = {15},
     number = {4},
     year = {2003},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AA_2003_15_4_a1/}
}