Abstract
With any structural inference rule A/B, we associate the rule \({(A \lor p)/(B \lor p)}\) , providing that formulas A and B do not contain the variable p. We call the latter rule a join-extension (\({\lor}\) -extension, for short) of the former. Obviously, for any intermediate logic with disjunction property, a \({\lor}\) -extension of any admissible rule is also admissible in this logic. We investigate intermediate logics, in which the \({\lor}\) -extension of each admissible rule is admissible. We prove that any structural finitary consequence operator (for intermediate logic) can be defined by a set of \({\lor}\) -extended rules if and only if it can be defined through a set of well-connected Heyting algebras of a corresponding quasivariety. As we exemplify, the latter condition is satisfied for a broad class of algebraizable logics.
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Citkin, A. A note on admissible rules and the disjunction property in intermediate logics. Arch. Math. Logic 51, 1–14 (2012). https://doi.org/10.1007/s00153-011-0250-y
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DOI: https://doi.org/10.1007/s00153-011-0250-y