@article{ZNSL_1979_88_a11,
     author = {V. P. Orevkov},
     title = {Three ways of recognizing essential formulas in sequents},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {163--175},
     publisher = {mathdoc},
     volume = {88},
     year = {1979},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1979_88_a11/}
}