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

SzigetiWG4

Answered question

2022-11-22

Understanding compactness theorem on modeling a sentence
T is a theory and ϕ is a sentence with T ϕ. I read notes with a quote like this:
By Compactness Theorem, a finite subset T 0 T has T 0 ϕ.
I thought the Compactness Theorem was something like "a theory has a model iff every subset of the theory has a model". That is M T M T 0 . (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 ϕ being a sentence. If we replaced ϕ with an infinite theory T′ then we cannot claim T 0 T .

Answer & Explanation

Teagan Gamble

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 T ϕ, the theory T { ¬ ϕ } has no models. Thus, by compactness, there is a finite subset Δ T { ¬ ϕ } that has no models. Let T 0 T be the finitely many sentences from T that appear in Δ. Then T 0 { ¬ ϕ } has no models, so every model of T 0 is not a model of ¬ ϕ, and hence T 0 ϕ.

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