Understanding compactness theorem on modeling a sentence. T is a theory and phi is a sentence with T⊨phi. I read notes with a quote like this: By Compactness Theorem, a finite subset T_0 subseteq T has T_0.⊨ phi
SzigetiWG4
Answered question
2022-11-22
Understanding compactness theorem on modeling a sentence T is a theory and is a sentence with . I read notes with a quote like this: By Compactness Theorem, a finite subset has . I thought the Compactness Theorem was something like "a theory has a model iff every subset of the theory has a model". That is . (I believe it follows from Completeness of FOL and proofs being finite). So how do we show the claim with compactness? I think it has something to do with being a sentence. If we replaced ϕ with an infinite theory T′ then we cannot claim .
Answer & Explanation
Teagan Gamble
Beginner2022-11-23Added 8 answers
Step 1 Your statement of compactness is not quite right (your statement is true, just not very strong, since T is a subset of itself!). A correct statement is: Let T be a first-order theory. Then T has a model if and only if every finite subset of T has a model. Step 2 Now to your actual question. Since , the theory has no models. Thus, by compactness, there is a finite subset that has no models. Let be the finitely many sentences from T that appear in . Then has no models, so every model of is not a model of , and hence .