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 }i≥1 (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.