@incollection{JEDP_1989____A14_0, author = {Olejnik, O. A.}, 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 = {http://www.numdam.org/item/JEDP_1989____A14_0/} }