@article{AL_1990_29_5_a6,
     author = {A. V. Chagrov},
     title = {Undecidable properties of extensions of provability {logic.~II}},
     journal = {Algebra i logika},
     pages = {613--623},
     publisher = {mathdoc},
     volume = {29},
     number = {5},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1990_29_5_a6/}
}