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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.

3 votes

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

One of the other answers briefly mentions tensors, but I think it’s a perfect answer to the question and deserves expansion. Tensors can be defined in several ways that are equivalent, but one’s choic …
6 votes

Zorn's lemma: old friend or historical relic?

Here's another example, from a rather worm's-eye view. (Apologies for too many edits.) Theorem: Let $R$ be a non-zero commutative ring with $1$. Then there exists a minimal prime ideal in $R$, i.e., a …
Zach Teitler's user avatar
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