# Ruitenburg's Theorem via Duality and Bounded Bisimulations

For a given intuitionistic propositional formula A and a propositional variable x occurring in it, define the infinite sequence of formulae A _i | i>1 by letting A_1 be A and A_i+1 be A(A_i/x). Ruitenburg's Theorem [8] says that the sequence A _i (modulo logical equivalence) is ultimately periodic with period 2, i.e. there is N > 0 such that A N+2 A N is provable in intuitionistic propositional calculus. We give a semantic proof of this theorem, using duality techniques and bounded bisimulations ranks.

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