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first-order and higher-order logic, model theory, set theory, proof theory, computability theory, formal languages, definability, interplay of syntax and semantics, constructive logic, intuitionism, philosophical logic, modal logic, completeness, Gödel incompleteness, decidability, undecidability, theories of truth, truth revision, consistency.

5 votes
0 answers
64 views

Definable pseudo-standard predicates in Internal Set Theory

Consider the usual language $\mathcal{L}=(\in, \mathrm{st})$ of Nelson's Internal Set Theory, and a unary $\mathcal{L}$-predicate $P$. For an $\mathcal{L}$-formula $\varphi$, let $\varphi^P$ denote th …
Z. A. K.'s user avatar
  • 756
6 votes
1 answer
255 views

Existence property for second-order propositional logic

Consider the intuitionistic second-order propositional calculus (SOL) formulated in the full $\wedge,\vee,\rightarrow,\bot,\top,\forall,\exists$ language. Question: Assume that $\Gamma$ and $\Psi$ are …
Z. A. K.'s user avatar
  • 756
9 votes
2 answers
373 views

How big can function spaces get without extensionality?

In what follows we work in the usual formulation of Martin-Löf Type Theory including Axiom K [1]. Boldface numbers $\mathbf{n}$ denote the usual finite type with $n$ elements. Motivation Postulating f …
Z. A. K.'s user avatar
  • 756