Bakkot comments on Godel's Completeness and Incompleteness Theorems - Less Wrong

34 Post author: Eliezer_Yudkowsky 25 December 2012 01:16AM

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Comment author: Bakkot 26 December 2012 09:26:05AM 4 points [-]

It is!? Does anyone know a proof of Compactness that doesn't use completeness as a lemma?

There's actually a direct one on ProofWiki. It's constructive, even, sort of. (Roughly: take the ultraproduct of all the models of the finite subsets with a suitable choice of ultrafilter.) If you've worked with ultraproducts at all, and maybe if you haven't, this proof is pretty intuitive.

As Qiaochu_Yuan points out, this is equivalent to the ultrafilter lemma, which is independent of ZF but strictly weaker than the Axiom of Choice. So, maybe it's not that intuitive.

Comment author: benelliott 27 December 2012 06:53:11PM 0 points [-]

That's really beautiful, thanks.