@article{SM_1977_31_4_a4,
     author = {A. A. Agrachev},
     title = {A~second-order necessary condition for optimality in the general nonlinear case},
     journal = {Sbornik. Mathematics},
     pages = {493--506},
     year = {1977},
     volume = {31},
     number = {4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1977_31_4_a4/}
}