Modal Logic and the two - variable
نویسندگان
چکیده
We introduce a modal language L which is obtained from standard modal logic by adding the diierence operator and modal operators interpreted by boolean combinations and the converse of accessibility relations. It is proved that L has the same expressive power as the two-variable fragment F O 2 of rst-order logic but speaks less succinctly about relational structures: if the number of relations is bounded, then L-satissability is ExpTime-complete but F O 2 satissability is NEx-pTime-complete. We indicate that the relation between L and F O 2 provides a general framework for comparing modal and temporal languages with rst-order languages.
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