@article{CMH_1982__57_139912,
     author = {John N. Mather},
     title = {Concavity of the {Lagrangian} for quasi-periodic orbits.},
     journal = {Commentarii mathematici Helvetici},
     pages = {356--376},
     publisher = {mathdoc},
     volume = {57},
     year = {1982},
     zbl = {0508.58037},
     url = {http://geodesic.mathdoc.fr/item/CMH_1982__57_139912/}
}