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COMPLETE ADDITIVITY AND MODAL INCOMPLETENESS

Published online by Cambridge University Press:  04 July 2019

WESLEY H. HOLLIDAY*
Affiliation:
Department of Philosophy and Group in Logic and the Methodology of Science, University of California, Berkeley
TADEUSZ LITAK*
Affiliation:
Chair for Theoretical Computer Science (Informatik 8), FAU Erlangen-Nuremberg
*
*DEPARTMENT OF PHILOSOPHY AND GROUP IN LOGIC AND THE METHODOLOGY OF SCIENCE UNIVERSITY OF CALIFORNIA, BERKELEY BERKELEY, CA 94720, USA E-mail: wesholliday@berkeley.edu
CHAIR FOR THEORETICAL COMPUTER SCIENCE (INFORMATIK 8) FAU ERLANGEN-NUREMBERG 91058 ERLANGEN, GERMANY E-mail: tadeusz.litak@fau.de

Abstract

In this article, we tell a story about incompleteness in modal logic. The story weaves together an article of van Benthem (1979), “Syntactic aspects of modal incompleteness theorems,” and a longstanding open question: whether every normal modal logic can be characterized by a class of completely additive modal algebras, or as we call them, ${\cal V}$-baos. Using a first-order reformulation of the property of complete additivity, we prove that the modal logic that starred in van Benthem’s article resolves the open question in the negative. In addition, for the case of bimodal logic, we show that there is a naturally occurring logic that is incomplete with respect to ${\cal V}$-baos, namely the provability logic $GLB$ (Japaridze, 1988; Boolos, 1993). We also show that even logics that are unsound with respect to such algebras do not have to be more complex than the classical propositional calculus. On the other hand, we observe that it is undecidable whether a syntactically defined logic is ${\cal V}$-complete. After these results, we generalize the Blok Dichotomy (Blok, 1978) to degrees of ${\cal V}$-incompleteness. In the end, we return to van Benthem’s theme of syntactic aspects of modal incompleteness.

Information

Type
Research Article
Copyright
Copyright © Association for Symbolic Logic 2019 

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