Abstract
In this paper, we discuss the relationships between some characteristic formulas in normal modal logic with their frames. We find that the validities of the modal formulas are conditional even though some of them are intuitively valid. Finally, we prove the validities of two formulas of Position-Exchange-Principle (PEP) proposed by papers 1 to 3 by using of modal logic and Kripke’s semantics of possible worlds.Key wordsCharacteristic FormulasValidityPEPFrame
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