@article{RM_1966_21_5_a0,
     author = {N. V. Efimov},
     title = {Surfaces with a~slowly changing negative curvature},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {1--55},
     publisher = {mathdoc},
     volume = {21},
     number = {5},
     year = {1966},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1966_21_5_a0/}
}