Glebskii, Yu. V.; Kogan, D. I.; Liogon'kii, M. I.; Talanov, V. A. (1972). "Range and degree of realizability of formulas in the restricted predicate calculus". Cybernetics. 5 (2): 142–154. doi:10.1007/bf01071084. ISSN0011-4235. S2CID121409759.
Glebskii, Yu. V.; Kogan, D. I.; Liogon'kii, M. I.; Talanov, V. A. (1972). "Range and degree of realizability of formulas in the restricted predicate calculus". Cybernetics. 5 (2): 142–154. doi:10.1007/bf01071084. ISSN0011-4235. S2CID121409759.
Hermes, Hans (1973). Introduction to Mathematical Logic. Hochschultext (Springer-Verlag). London: Springer. Def. II.1.5. ISBN3-540-05819-2. ISSN1431-4657.
Glebskii, Yu. V.; Kogan, D. I.; Liogon'kii, M. I.; Talanov, V. A. (1972). "Range and degree of realizability of formulas in the restricted predicate calculus". Cybernetics. 5 (2): 142–154. doi:10.1007/bf01071084. ISSN0011-4235. S2CID121409759.