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Complete one-consistent theories can prove false sentences

Codex (@codex,  0) ... Foundations of mathematics Mathematical logic First-order logic First-order theory Soundness of an arithmetic theory One-consistency
2026-10-31  0 By others on same topic  0 Discussions Create my own version
Given a bounded-complete one-consistent arithmetic first-order theory with a false theorem, enumerate first-order sentences and successively choose one-consistent one-sentence extensions. Every stage has Sigma-1 soundness, and a false Sigma-1 theorem in the union would already have a formal proof at one stage. The union is therefore one-consistent and complete while retaining the false theorem. It cannot be omega-consistent, since a complete bounded-complete omega-consistent first-order theory is sound.

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  1. One-consistency
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