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In Mendelson's Introduction to Mathematical Logic, the proof of Godel's Theorem for S (his axiomatic arithmetic) goes via proving that a sentence that can be interpreted as "This statement has no proof in S" cannot be proved either false or true in S, if S is consistent.

According to the completeness of the Predicate Calculus, any logically valid wf of a theory K is a theorem of K. The statement I interpret as "This statement has no proof in S" cannot be proved either false or true, so I presume that there are models of arithmetic (or rather of axiom system S) in which it is false, and models in which it is true. Is this correct?

A model of arithmetic in which it is true seems sane enough. Are there models of S in which a proof of what is interpreted as "This statement has no proof in S" turns up as some sort of non-standard number, or have I got completely confused? I have a vision in my mind of a non-standard number encoding "1 is not a proof of S. 2 is not a proof of S. 3 is not a proof of S...." or in some other way satisfying the equation that asserts that X is a proof of Y, if not the mathematician posing the equation :-)

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up vote 4 down vote accepted

A model of arithmetic in which the G"odel sentence "I am unprovable" is false is necessarily a non-standard model. It contains an infinite element which satisfies, in the model, the formula expressing the property of being a proof of the G"odel sentence --- a formula that is not satisfied by any standard natural number (not even in a non-standard model).

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You can type o with an umlaut as ö but not in a comment, apparently :( – Nate Eldredge Sep 27 '10 at 21:22
(ö)/ There's no reason to be diacritical. – Eric Tressler Sep 27 '10 at 23:28

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