@article{DAN_1976_228_5_a7,
     author = {S. V. Popov},
     title = {On the complexity of derivations in classical propositional calculus},
     journal = {Doklady Akademii Nauk},
     pages = {1041--1044},
     year = {1976},
     volume = {228},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1976_228_5_a7/}
}