@article{SJIM_2001_4_2_a4,
     author = {O. Bertrand and B. Philippe},
     title = {Counting the eigenvalues surrounded by a~closed curve},
     journal = {Sibirskij \v{z}urnal industrialʹnoj matematiki},
     pages = {73--94},
     publisher = {mathdoc},
     volume = {4},
     number = {2},
     year = {2001},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SJIM_2001_4_2_a4/}
}