@article{AL_1987_26_4_a1,
     author = {Yu. M. Vazhenin},
     title = {Algorithmic problems and hierarchies of first-order languages},
     journal = {Algebra i logika},
     pages = {419--434},
     year = {1987},
     volume = {26},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1987_26_4_a1/}
}