@article{IM2_1990_34_1_a2,
     author = {Yu. E. Karpeshina},
     title = {Analytic perturbation theory for a~periodic potential},
     journal = {Izvestiya. Mathematics },
     pages = {43--64},
     publisher = {mathdoc},
     volume = {34},
     number = {1},
     year = {1990},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1990_34_1_a2/}
}