@article{AL_1991_30_5_a4,
     author = {V. L. Selivanov},
     title = {Fine hierarchy of formulas},
     journal = {Algebra i logika},
     pages = {568--582},
     publisher = {mathdoc},
     volume = {30},
     number = {5},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1991_30_5_a4/}
}