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Journal of Logic and Computation, Volume 9, Issue 5, pp. 601-621: Abstract.

Points, lines and diamonds: a two-sorted modal logic for projective planes

Y Venema

Institute of Logic, Language and Information, University of Amsterdam, Plantage Muidergracht 24, 1018 TV Amsterdam, The Netherlands, E-mail: yde@wins.uva.nl

We introduce a modal language for talking about projective planes. This language is two-sorted, containing formulas to be evaluated at points and at lines, respectively. The language has two diamonds whose intended accessibility relations are the two directions of the incidence relation between points and lines. We provide a sound and complete axiomatization for the formulas that are valid in the class of projective planes. We also show that it is decidable whether a given formula is satisfiable in a projective plane, and we characterize the computational complexity of this satisfaction problem.

Keywords: Modal logic, logic of space, projective geometry, completeness, decidability, complexity

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