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computable sets and functions, Turing degrees, c.e. degrees, models of computability, primitive recursion, oracle computation, models of computability, decision problems, undecidability, Turing jump, halting problem, notions of computable randomness, computable model theory, computable equivalence relation theory, arithmetic and hyperarithmetic hierarchy, infinitary computability, $\alpha$-recursion, complexity theory.

13 votes

Which finitely presented groups can be distinguished by decidable properties?

The isomorphism relation for finitely presented groups is c.e., and in fact is Turing equivalent to the halting problem. Proof: To check whether two finitely presented groups $G$ and $H$ are i …
Bjorn Poonen's user avatar
  • 23.8k
23 votes

Is the theory of categories decidable?

This answer builds on those of F. G. Dorais and Joel David Hamkins to answer your "specific question", the question left open by them, namely whether the theory of abelian categories is decidable. Th …
Bjorn Poonen's user avatar
  • 23.8k