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.
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.
You said arbitrarily large finite models, however. First-order arithmetic has no finite models. : )
Oh, yeah, that’s a typo. Fixed, thanks.