@article{ZNSL_1968_8_a22,
     author = {V. P. Orevkov},
     title = {Two unsolvable classes of formulas of classical predicate calculus},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {202--210},
     publisher = {mathdoc},
     volume = {8},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1968_8_a22/}
}