Essentially-undecidable theory

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An algorithmically-undecidable logical theory, all consistent extensions of which are also undecidable (see Undecidability). An elementary theory is an essentially-undecidable theory if and only if every model of it has an undecidable elementary theory. Every complete undecidable theory is an essentially-undecidable theory, as is formal arithmetic (cf. Arithmetic, formal); no theory with a finite model can be an essentially-undecidable theory.

The essential undecidability of a suitable finitely-axiomatizable elementary theory $S$ is often used in proving the undecidability of a given theory $T$ (see [1], [2]). In this proof, $S$ is interpreted in any model $M$ of $T$. The domain of interpretation and the values of the elements of the signature of $S$ are defined using values in the model $M$ of suitable formulas in the language of $T$. If the interpretation is a model of $S$, then $T$ is undecidable; moreover, this theory is hereditarily undecidable, i.e. all of its subtheories of the same signature as $T$ are undecidable. This method is used to prove the undecidability of elementary predicate logic, elementary group theory, elementary field theory, etc. Finitely-axiomatized formal arithmetic is often used as the essentially-undecidable theory $S$.


[1] A. Tarski, A. Mostowski, R.M. Robinson, "Undecidable theories" , North-Holland (1953)
[2] Yu.L. Ershov, I.A. Lavrov, A.D. Taimanov, M.A. Taitslin, "Elementary theories" Russian Math. Surveys , 20 : 4 (1965) pp. 35–105 Uspekhi Mat. Nauk , 20 : 4 (1965) pp. 37–108
[3] S.C. Kleene, "Introduction to metamathematics" , North-Holland (1951)
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Essentially-undecidable theory. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by A.L. Semenov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article