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DISJUNCTION AND EXISTENCE PROPERTIES IN MODAL ARITHMETIC

Published online by Cambridge University Press:  23 December 2022

TAISHI KURAHASHI
Affiliation:
GRADUATE SCHOOL OF SYSTEM INFORMATICS KOBE UNIVERSITY 1-1 ROKKODAI, NADA, KOBE 657-8501, JAPAN E-mail: kurahashi@people.kobe-u.ac.jp
MOTOKI OKUDA
Affiliation:
INDEPENDENT SCHOLAR

Abstract

We systematically study several versions of the disjunction and the existence properties in modal arithmetic. First, we newly introduce three classes $\mathrm {B}$, $\Delta (\mathrm {B})$, and $\Sigma (\mathrm {B})$ of formulas of modal arithmetic and study basic properties of them. Then, we prove several implications between the properties. In particular, among other things, we prove that for any consistent recursively enumerable extension T of $\mathbf {PA}(\mathbf {K})$ with $T \nvdash \Box \bot $, the $\Sigma (\mathrm {B})$-disjunction property, the $\Sigma (\mathrm {B})$-existence property, and the $\mathrm {B}$-existence property are pairwise equivalent. Moreover, we introduce the notion of the $\Sigma (\mathrm {B})$-soundness of theories and prove that for any consistent recursively enumerable extension of $\mathbf {PA}(\mathbf {K4})$, the modal disjunction property is equivalent to the $\Sigma (\mathrm {B})$-soundness.

Type
Research Article
Copyright
© The Author(s), 2022. Published by Cambridge University Press on behalf of The Association for Symbolic Logic

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