@article{SM_1989_64_2_a9,
     author = {V. A. Baikov and R. K. Gazizov and N. Kh. Ibragimov},
     title = {Approximate symmetries},
     journal = {Sbornik. Mathematics},
     pages = {427--441},
     publisher = {mathdoc},
     volume = {64},
     number = {2},
     year = {1989},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1989_64_2_a9/}
}