dankane comments on Robust Cooperation in the Prisoner's Dilemma - Less Wrong

69 Post author: orthonormal 07 June 2013 08:30AM

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Comment author: dankane 08 June 2013 07:34:03PM 1 point [-]

Ah, never mind I see it. You just need to modify the proof of Lob's Theorem. Instead of using L = (there exists a proof of L) -> S you instead use L = (there exists a proof of L of length at most Ackermann(10)) -> S and the rest of the proof of Lob's Theorem still works.

Comment author: cousin_it 09 June 2013 01:39:30PM *  3 points [-]

Yeah, that's the main idea. Note that to state the bounded version of Lob's theorem, you actually have to use two separate limits: "if having a bounded-size proof of statement S would imply the truth of S, and that fact itself has a short enough proof, then S is provable". The proof is similar to the regular Lob's theorem, but each step has to keep track of length limits, and these bookkeeping details eventually determine how high the limits have to be in the first place for the theorem to be true.