@article{DE_1990_26_6_a4,
     author = {K. B. Mansimov},
     title = {Conditions for second-order optimality in {Goursat--Darboux} systems in the presence of constraints},
     journal = {Differencialʹnye uravneni\^a},
     pages = {954--965},
     publisher = {mathdoc},
     volume = {26},
     number = {6},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1990_26_6_a4/}
}