@article{DAN_1987_294_2_a0,
     author = {A. A. Borisenko},
     title = {On extremal properties of compact surfaces on nonpositive extrinsic curvature},
     journal = {Doklady Akademii Nauk},
     pages = {265--268},
     publisher = {mathdoc},
     volume = {294},
     number = {2},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1987_294_2_a0/}
}