So8res comments on Very Basic Model Theory - LessWrong

22 Post author: So8res 31 October 2013 07:06AM

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Comment author: So8res 01 November 2013 03:54:48PM 1 point [-]

3 and 4 are generalizations to sets of sentences. But you're right, the generalization is pretty simple.

Arbitrarily large models are allowed in the first-order theory of arithmetic, and no first-order theory of arithmetic can restrict models to only the integers. This is one of the surprising results of compactness.

Comment author: Will_Sawin 01 November 2013 06:39:28PM *  1 point [-]

You said arbitrarily large finite models, however. First-order arithmetic has no finite models. : )

Comment author: So8res 01 November 2013 06:43:38PM 1 point [-]

Oh, yeah, that's a typo. Fixed, thanks.