@article{IM2_1990_35_2_a0,
     author = {L. D. Beklemishev},
     title = {On the classification of propositional provability logics},
     journal = {Izvestiya. Mathematics },
     pages = {247--275},
     publisher = {mathdoc},
     volume = {35},
     number = {2},
     year = {1990},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1990_35_2_a0/}
}