Bar-Ilan University, Ramat-Gan, Israel
Beginning in 1960, and continuing for about a decade and a half, Shepherdson's self-referential formulae dominated the applications of diagonalization in metamathematics. In 1976, it was doubly toppled from its position of supremacy by two demonstrably more powerful such sentences introduced by D. Guaspari. The goal of the present note is to understand and explain the first (and, for the time being, more important) of Guaspari's self-referential sentences, which we dub “Guaspari sentences of the first kind”, or, less poetically, “Guaspari fixed points”.
Both Shepherdson's and Guaspari's fixed points are generalizations of the Rosser sentence. But, where Shepherdson merely tacks on side-formulae, Guaspari takes a more revolutionary step: He views the basic components, Pr T (⌈¬φ⌉) and Pr T (⌈φ⌉), of the Rosser sentence as attempts to refute something and replaces them by refutations of something else. Ignoring their Shepherdsonesque side-formulae (tacked on for the sake of more esoteric applications), our analysis of Guaspari fixed points can be viewed as merely the isolation of those properties whose refutations can be used in this context.
In §1 we offer the outcome of this analysis—i.e. a delineation of properties whose refutations can so be used. Some simple examples are cited and the Guaspari fixed points are defined. The main theorem—a master Fixed Point Calculation—is proven in §2.
(Received May 01 1979)