@article{AL_1981_20_6_a3,
     author = {S. V. Popov},
     title = {An undecidable intermediate calculus},
     journal = {Algebra i logika},
     pages = {654--706},
     year = {1981},
     volume = {20},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1981_20_6_a3/}
}