Additional Key Words and Phrases: Relational Data File, retrieval system, question-answering system, predicate calculus, interpretation, definite formulas, subformula, proper formula, recursively unsolvable decision problem, recursively enumerable, prenex form, Skolem form, solvable cases of the decision problem
- Robert A. Di Paola. The recursive unsolvability of the decision problem for the class of definite formulas. Journal of the ACM, 16(2):324-327, April 1969.