@article{VMUMM_2009_5_a4,
     author = {A. B. Ugol'nikov},
     title = {Equivalent transformations of formulas in $P_2$},
     journal = {Vestnik Moskovskogo universiteta. Matematika, mehanika},
     pages = {25--32},
     publisher = {mathdoc},
     number = {5},
     year = {2009},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/VMUMM_2009_5_a4/}
}