@article{MZM_1991_50_2_a1,
     author = {V. I. Bernik and \'E. I. Kovalevskaya},
     title = {Two-dimensional extremal surfaces with jointly extremal partial derivatives},
     journal = {Matemati\v{c}eskie zametki},
     pages = {14--19},
     publisher = {mathdoc},
     volume = {50},
     number = {2},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1991_50_2_a1/}
}