@article{IM2_1991_36_2_a7,
     author = {V. V. Rybakov},
     title = {Admissibility of rules of inference, and logical equations, in modal logics axiomatizing provability},
     journal = {Izvestiya. Mathematics },
     pages = {369--390},
     publisher = {mathdoc},
     volume = {36},
     number = {2},
     year = {1991},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1991_36_2_a7/}
}