Barwise compactness theorem

In mathematical logic, the Barwise compactness theorem, named after Jon Barwise, is a generalization of the usual compactness theorem for first-order logic to a certain class of infinitary languages. It was stated and proved by Barwise in 1967.

Statement of the theorem

Let A be a countable admissible set. Let L be an A-finite relational language. Suppose \Gamma is a set of L_A-sentences, where \Gamma is a \Sigma_1 set with parameters from A, and every A-finite subset of \Gamma is satisfiable. Then \Gamma is satisfiable.

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