@inbook{CIF_1982-1983__18__A3_0,
     author = {Colin de Verdi\`ere, Yves},
     title = {Chapitre {III} {Calcul} des variations},
     booktitle = {G\'eom\'etrie symplectique},
     series = {Cours de l'institut Fourier},
     pages = {1--16},
     publisher = {Institut des Math\'ematiques Pures - Universit\'e Scientifique et M\'edicale de Grenoble},
     number = {18},
     year = {1982-1983},
     url = {http://archive.numdam.org/item/CIF_1982-1983__18__A3_0/}
}