@article{DAN_1968_183_1_a18,
     author = {Yu. A. Kravtsov},
     title = {{\textquotedblleft}Quasi-isotropic{\textquotedblright} approximation of geometrical optics},
     journal = {Doklady Akademii Nauk},
     pages = {74--76},
     year = {1968},
     volume = {183},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1968_183_1_a18/}
}