@article{VMUMM_1993_3_a2,
     author = {P. G. Naumov},
     title = {Undecidability of a second-order provability logic with witness comparison},
     journal = {Vestnik Moskovskogo universiteta. Matematika, mehanika},
     pages = {14--17},
     publisher = {mathdoc},
     number = {3},
     year = {1993},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/VMUMM_1993_3_a2/}
}