@article{SIGMA_2023_19_a59,
     author = {Robert L. Bryant},
     title = {On the {Convex} {Pfaff{\textendash}Darboux} {Theorem} of {Ekeland} and {Nirenberg}},
     journal = {Symmetry, integrability and geometry: methods and applications},
     year = {2023},
     volume = {19},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SIGMA_2023_19_a59/}
}