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Definition (satisfiability): a set $\Gamma$ of formulas of a first order language is satisfiable if there exist a structure $S$, a Boolean algebra $B$ and an evaluation function $V$ such that $V(C)=1_B$ for every formula $C \in \Gamma$.

Theorem (Compactness): a set $\Gamma$ of formulas of a first order language is satisfiable if and only if every finite subset of $\Gamma$ is satisfiable.

I tried to prove the Compactness theorem in a constructive way but I didn't succeed, so now I wonder if the Compactness theorem constructively implies some other statement that cannot be proved in constructive mathematics (an intuitionistic taboo). Can someone help me?

effezeta
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  • Not quite duplicate, but I think answers there answer your question too? https://math.stackexchange.com/q/632394/659499 (compactness is equal to weak Konig's lemma, so can't be proven constructively) – mihaild Apr 17 '22 at 14:21
  • Interesting! How can be constructively proved that the compactness theorem implies the weak Konig's lemma? – effezeta Apr 18 '22 at 14:32

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