In mathematical logic, an abstract logic is a formal system consisting of a class of sentences and a satisfaction relation with specific properties related to occurrence, expansion, isomorphism, renaming and quantification.C. C. Chang and Jerome Keisler Model Theory, 1990 page 128
Based on Lindström's characterization, first-order logic is, up to equivalence, the only abstract logic that is countably compact and has Löwenheim number ω.C. C. Chang and Jerome Keisler Model Theory, 1990 page 132
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