@article{DAN_1982_267_4_a10,
     author = {G. G. Ponomarenko},
     title = {On $r$-indistinguishability of finite automata},
     journal = {Doklady Akademii Nauk},
     pages = {806--807},
     year = {1982},
     volume = {267},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1982_267_4_a10/}
}