Journal of Symbolic Logic

A Quantitative Analysis of Modal Logic

Ronald Fagin

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We do a quantitative analysis of modal logic. For example, for each Kripke structure $M$, we study the least ordinal $\mu$ such that for each state of $M$, the beliefs of up to level $\mu$ characterize the agents' beliefs (that is, there is only one way to extend these beliefs to higher levels). As another example, we show the equivalence of three conditions, that on the face of it look quite different, for what it means to say that the agents' beliefs have a countable description, or putting it another way, have a "countable amount of information". The first condition says that the beliefs of the agents are those at a state of a countable Kripke structure. The second condition says that the beliefs of the agents can be described in an infinitary language, where conjunctions of arbitrary countable sets of formulas are allowed. The third condition says that countably many levels of belief are sufficient to capture all of the uncertainty of the agents (along with a technical condition). The fact that all of these conditions are equivalent shows the robustness of the concept of the agents' beliefs having a "countable description".

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J. Symbolic Logic, Volume 59, Issue 1 (1994), 209-252.

First available in Project Euclid: 6 July 2007

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Fagin, Ronald. A Quantitative Analysis of Modal Logic. J. Symbolic Logic 59 (1994), no. 1, 209--252.

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