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The weakest classical system is sometimes referred to as E and is non-normal. Both algebraic and neighborhood semantics characterize familiar classical modal systems that are weaker than the weakest normal modal logic K. Every regular modal logic is classical, and every normal modal logic is regular and hence classical.
When a Sahlqvist formula is used as an axiom in a normal modal logic, the logic is guaranteed to be complete with respect to the basic elementary class of frames the axiom defines. This result comes from the Sahlqvist completeness theorem [Modal Logic, Blackburn et al., Theorem 4.42]. But there is also a converse theorem, namely a theorem that ...
A non-normal modal logic is a variant of modal logic that deviates from the basic principles of normal modal logics. Normal modal logics adhere to the distributivity axiom ( ( p → q ) → ( p → q ) {\displaystyle \Box (p\to q)\to (\Box p\to \Box q)} ) and the necessitation principle which states that "a tautology must be necessarily true ...
In theoretical computer science, the modal μ-calculus (Lμ, L μ, sometimes just μ-calculus, although this can have a more general meaning) is an extension of propositional modal logic (with many modalities) by adding the least fixed point operator μ and the greatest fixed point operator ν, thus a fixed-point logic.
Modal logic is a kind of logic used to represent statements about necessity and possibility.It plays a major role in philosophy and related fields as a tool for understanding concepts such as knowledge, obligation, and causation.
Informally, the number of transitions in the 'longest chain' of transitions in the first-order formula is the modal depth of the formula. The modal depth of the formula used in the example above is two. The first-order formula indicates that the transitions from to and from to are needed to verify the validity of the formula. This is also the ...
In modal logic, a regular modal logic is a modal logic containing (as axiom or theorem) the duality of the modal operators:
The modal depth of a formula indicates 'how far' one needs to look in a Kripke model when checking the validity of the formula. For each modal operator, one needs to transition from a world in the model to a world that is accessible through the accessibility relation. The modal depth indicates the longest 'chain' of transitions from a world to ...