@article{AL_1991_30_6_a4,
     author = {A. I. Omarov},
     title = {Filtering formulas},
     journal = {Algebra i logika},
     pages = {693--704},
     year = {1991},
     volume = {30},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1991_30_6_a4/}
}