@article{DAN_1968_181_5_a11,
     author = {I. I. Tugov},
     title = {Group properties (in the large) of second order partial differential equations},
     journal = {Doklady Akademii Nauk},
     pages = {1073--1076},
     publisher = {mathdoc},
     volume = {181},
     number = {5},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1968_181_5_a11/}
}