@article{DA_1985_26_a2,
     author = {A. N. Burdukovskii},
     title = {Conditions for the optimality of singular controls in the {Goursat{\textendash}Darboux} problem},
     journal = {Diskretnyj analiz i issledovanie operacij},
     pages = {31--39},
     year = {1985},
     number = {26},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DA_1985_26_a2/}
}