@incollection{JEDP_1994____A14_0,
     author = {Birman, Mikhail Sh.},
     title = {Discrete spectrum of the periodic elliptic operator with a differential perturbation},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {14},
     pages = {1--4},
     publisher = {Ecole polytechnique},
     year = {1994},
     mrnumber = {1298685},
     language = {en},
     url = {http://archive.numdam.org/item/JEDP_1994____A14_0/}
}