@article{ZNSL_1968_8_a25,
     author = {G. S. Tseitin},
     title = {On the complexity of proof in prepositional calculus},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {234--259},
     publisher = {mathdoc},
     volume = {8},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1968_8_a25/}
}