@article{DE_1977_13_3_a4,
     author = {F. Clarke},
     title = {Extremal arcs and extended {Hamiltonian} systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {427--442},
     publisher = {mathdoc},
     volume = {13},
     number = {3},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1977_13_3_a4/}
}