@article{AL_1991_30_2_a1,
     author = {Yu. M. Vazhenin},
     title = {Matrix hierarchies of first-order languages and their},
     journal = {Algebra i logika},
     pages = {154--167},
     year = {1991},
     volume = {30},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1991_30_2_a1/}
}