13 March 2006, Colloquium on Mathematical Logic, Alexey Romanov

We may think of propositional formulas as schemas for formulas of a first-order language. The question which then naturally arises is, "What schemas are valid in a given theory?" De Jongh's theorem settles this question for HA. Since then, mostly extensions of HA have been considered in this context. This talk will deal with much weaker theories of equality instead.

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