Abstract
We present a construction of a satisfaction class in an arbitrary countable recursively saturated models of first-order arithmetic. Our construction is fully classical, namely, it employs nothing more than the classical techniques of formal proof theory.
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In their proof it is assumed that arithmetic is formulated in the relational language. See [1] for extending the result to the language with function symbols.
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This is the first difference between our proof and the original KKL’s construction.
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Proof systems with similar infinitary rules have already been studied in the literature in the context of cut elimination. See, for example, [9].
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This will happen if all the numerical instantiations of \(\varphi (x)\) are seen as false by the model, that is, if for all numerals a, the sentence \(\lnot \varphi (a)\) belongs to T.
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Without cut, (Tr-Lit) is the only rule that permits us to eliminate sentences in the proof and (Tr-Lit) can eliminate literals only.
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Thus, sequents which are initial in P have height 0 and the maximal height of a sequent in P is not larger than the height of P.
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Here the argument proceeds by cases, corresponding to various ways in which elements of \(Sent_{i}\) can be obtained from elements of \(Sent_{i+1}\).
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The case of the existential mix formula is closely analogous and I do not discuss it separately.
References
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Acknowledgements
The author was supported by a grant from the National Science Centre in Cracow (NCN), project number 2017/27/B/HS1/01830.
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Cieśliński, C. (2019). Satisfaction Classes via Cut Elimination. In: Khan, M., Manuel, A. (eds) Logic and Its Applications. ICLA 2019. Lecture Notes in Computer Science(), vol 11600. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-58771-3_12
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