@article{CMH_1979__54_139800,
     author = {Yves Colin de Verdi\`ere},
     title = {Sur le spectre des op\'erateurs elliptiques \`a bicaract\'eristiques toutes les p\'eriodiques.},
     journal = {Commentarii mathematici Helvetici},
     pages = {508--522},
     publisher = {mathdoc},
     volume = {54},
     year = {1979},
     zbl = {0459.58014},
     url = {http://geodesic.mathdoc.fr/item/CMH_1979__54_139800/}
}