@article{SM_1993_76_2_a4,
     author = {D. V. Tunitsky},
     title = {Regular isometric immersion in the~large of two-dimensional metrics of nonpositive curvature},
     journal = {Sbornik. Mathematics},
     pages = {317--329},
     year = {1993},
     volume = {76},
     number = {2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1993_76_2_a4/}
}