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  • On Correspondence of Standa...
    Mruczek-Nasieniewska, Krystyna; Nasieniewski, Marek

    Studia logica, 10/2020, Volume: 108, Issue: 5
    Journal Article

    In the context of modal logics one standardly considers two modal operators: possibility ( ◊ ) and necessity ( □ ) see for example Chellas (Modal logic. An introduction, Cambridge University Press, Cambridge, 1980). If the classical negation is present these operators can be treated as inter-definable. However, negative modalities ( ◊ ¬ ) and ( □ ¬ ) are also considered in the literature see for example Béziau (Log Log Philos 15:99–111, 2006. https://doi.org/10.12775/LLP.2006.006 ); Došen (Publ L’Inst Math, Nouv Sér 35(49):3–14, 1984); Gödel, in: Feferman (ed.), Collected works, vol 1, Publications 1929–1936, Oxford University Press, New York, 1986, p. 300; Lewis and Langford (Symbolic logic, Dover Publications Inc., New York, 1959, p. 497). Both of them can be treated as negations. In Béziau (Log Log Philos 15:99–111, 2006. https://doi.org/10.12775/LLP.2006.006 ) a logic Z has been defined on the basis of the modal logic S 5 . Z is proposed as a solution of so-called Jaśkowski’s problem see also Jaśkowski (Stud Soc Sci Torun 5:57–77, 1948). The only negation considered in the language of Z is ‘it is not necessary’. It appears that logic Z and S 5 inter-definable. This initial correspondence result between S 5 and Z has been generalised for the case of normal logics, in particular soundness-completeness results were obtained see Marcos (Log Anal 48(189–192):279–300, 2005); Mruczek-Nasieniewska and Nasieniewski (Bull Sect Log 34(4):229–248, 2005). In Mruczek-Nasieniewska and Nasieniewski (Log Univ 12:207–219, 2018. https://doi.org/10.1007/s11787-018-0184-9 ) it has been proved that there is a correspondence between Z -like logics and regular extensions of the smallest deontic logic. To obtain this result both negative modalities were used. This result has been strengthened in Mruczek-Nasieniewska and Nasieniewski (Bull Sect Log 46(3–4):261–280, 2017) since on the basis of classical positive logic it is enough to solely use □ ¬ to equivalently express both positive modalities and negation. Here we strengthen results given in Mruczek-Nasieniewska and Nasieniewski (Log Univ 12:207–219, 2018. https://doi.org/10.1007/s11787-018-0184-9 ) by showing correspondence for the smallest regular logic. In particular we give a syntactic formulation of a logic that corresponds to the smallest regular logic. As a result we characterise all logics that arise from regular logics. From this follows via respective translations a characterisation of a class of logics corresponding to some quasi-regular logics where S 2 0 is the smallest element. Moreover, if a given quasi-regular logic is characterised by some class of models, the same class can be used to semantically characterise the logic obtained by our translation.