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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.

4 votes

Asymmetric $A \iff B$ proofs

I asked about this before on MSE: https://math.stackexchange.com/q/1500691/101420 There are some arguments that Pythagoras theorem and its converse provide an example of what you're asking. (So given …
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16 votes

Taking a theorem as a definition and proving the original definition as a theorem

When I was teaching elementary geometry in high school in Holland 10 years ago, the concept of equivalent definitions was actually part of the curriculum. There the examples were more down to earth: y …