Multimodal logic

A multimodal logic is a modal logic that has more than one primitive modal operator. They find substantial applications in theoretical computer science.

A modal logic with n primitive unary modal operators $\Box_i, i\in \{1,\ldots, n\}$ is called an n-modal logic. Given these operators and negation, one can always add $\Diamond_i$ modal operators defined as $\Diamond_i P$ if and only if $\lnot \Box_i \lnot P$ .

The first substantive example of a 2-modal logic is perhaps Arthur Prior's tense logic, with two modalities, F and P, corresponding to "sometime in the future" and "sometime in the past". A logic^[1] with infinitely many modalities is (propositional) dynamic logic, introduced in 1976 and having a separate modal operator for every regular expression. A version of temporal logic introduced in 1977 and intended for program verification has two modalities, corresponding to dynamic logic's [A] and [A*] modalities for a single program A, understood as the whole universe taking one step forwards in time. The term "multimodal logic" itself was not introduced until 1980. Another example of a multimodal logic is the Hennessy-Milner logic, itself a fragment of the more expressive modal μ-calculus, which additionally is also a fixed-point logic.

Multimodal logic can be used also to formalize a kind of knowledge representation: the motivation of epistemic logic is allowing several agents (they are regarded as subjects capable of forming beliefs, knowledge); and managing the belief or knowledge of each agent, so that epistemic assertions can be formed about them. The modal operator $\Box$ must be capable of bookkeeping the cognition of each agent, thus $\Box_i$ must be indexed on the set of the agents. The motivation is that $\Box_i \alpha$ should assert "The subject i has knowledge about $\alpha$ being true". But it can be used also for formalizing "the subject i believes $\alpha$ ". For formalization of meaning based on the possible world semantics approach, a multimodal generalization of Kripke semantics can be used: instead of a single "common" accessibility relation, there is a series of them indexed on the set of agents.^[2]

Notes

↑ Sergio Tessaris; Enrico Franconi; Thomas Eiter (2009). Reasoning Web. Semantic Technologies for Information Systems: 5th International Summer School 2009, Brixen-Bressanone, Italy, August 30 - September 4, 2009, Tutorial Lectures. Springer. p. 112. ISBN 978-3-642-03753-5.
↑ Ferenczi 2002: 257

References

Ferenczi, Miklós (2002). Matematikai logika (in Hungarian). Budapest: Műszaki könyvkiadó. ISBN 963-16-2870-1.

Dov M. Gabbay, A. Kurucz, F. Wolter, M. Zakharyaschev (2003). Many-dimensional modal logics: theory and applications. Elsevier. ISBN 978-0-444-50826-3.
Walter Carnielli; Claudio Pizzi (2008). Modalities and Multimodalities. Springer. ISBN 978-1-4020-8589-5.

External links

Stanford Encyclopedia of Philosophy: "Modal logic" – by James Garson.