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the mathematical discipline that applies mathematical methods to the study of mathematical theories themselves.

10 votes

Existential statement without witness

$\def\epn{\mathrm{EP}_\mathbb N}\def\dp{\mathrm{DP}}$When a theory $T$ (with a distinguished definable set of “natural numbers” satisfying a suitable minimal arithmetic) has the property in the questi …
Emil Jeřábek's user avatar
24 votes
Accepted

Are there examples of nonconstructive metaproofs?

In theory, David’s answer is correct. Nevertheless, in practice it is perfectly possible to prove the existence of a proof non-constructively (such as by manipulating models and then appealing to the …
Emil Jeřábek's user avatar
4 votes

Is every recursively axiomatizable and consistent theory interpretable in the true arithmeti...

The other answers are enough to answer the question as given. However, let me point out for the record that using the arithmetized completeness theorem over PA is quite an overkill. We can make do wit …
Emil Jeřábek's user avatar