@incollection{JEDP_1989____A14_0,
     author = {O. A. Olejnik},
     title = {On the spectrum of singularly perturbed operators},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {14},
     pages = {1--10},
     publisher = {\'Ecole polytechnique},
     year = {1989},
     zbl = {0675.47010},
     language = {en},
     url = {https://proceedings.centre-mersenne.org/item/JEDP_1989____A14_0/}
}