Download PDFOpen PDF in browserStone duality for firstorder logic: a nominal approach to logic and topology32 pages•Published: February 12, 2014AbstractWhat are variables, and what is universal quantification over a variable?Nominal sets are a notion of `sets with names', and using equational axioms in nominal algebra these names can be given substitution and quantification actions. So we can axiomatise firstorder logic as a nominal logical theory. We can then seek a nominal sets representation theorem in which predicates are interpreted as sets; logical conjunction is interpreted as sets intersection; negation as complement. Now what about substitution; what is it for substitution to act on a predicateinterpretedasaset, in which case universal quantification becomes an infinite sets intersection? Given answers to these questions, we can seek notions of topology. What is the general notion of topological space of which our sets representation of predicates makes predicates into `open sets'; and what specific class of topological spaces corresponds to the image of nominal algebras for firstorder logic? The classic Stone duality answers these questions for Boolean algebras, representing them as Stone spaces. Nominal algebra lets us extend Boolean algebras to `FOLalgebras', and nominal sets let us correspondingly extend Stone spaces to `∀Stone spaces'. These extensions reveal a wealth of structure, and we obtain an attractive and selfcontained account of logic and topology in which variables directly populate the denotation, and open predicates are interpreted as sets rather than functions from valuations to sets. In: Andrei Voronkov and Margarita Korovina (editors). HOWARD60. A Festschrift on the Occasion of Howard Barringer's 60th Birthday, vol 42, pages 178209
