 LOGICS
Exercice 4.1. Prove, in a semantical way , that ? ? ?x¬¬(R(x) ? ¬R(x)). Exercice
4.2. 1 Find some Kripke structure K which is a countermodel for. (¬A ? B) ? (¬
B ? A). 2 Is the sequent ? ? (¬A ? B) ? (¬B ? A) derivable within LK? within
LJ? Exercice 4.3 LJ versus LK. Let us consider the following sequents, that were
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