We introduce and investigate the notion of uniform Lyndon interpolation property (ULIP) which is a strengthening of both uniform interpolation property and Lyndon interpolation property. We prove several propositional modal logics including K, KB, GL and Grz enjoy ULIP. Our proofs are modifications of Visser's proofs of uniform interpolation property using bounded bisimulations [33]. Also we give a new upper bound on the complexity of uniform interpolants for GL and Grz. * kurahashi@n.kisarazu.ac.jp recently settled affirmatively for GL by Shamkanov [29] and for Grz by Maksimova [24]. Recently, Kuznets [18] proved LIP for a wider class of propositional modal logics including the logics in the so-called modal cube of [12]. Maksimova [21] showed that there exist normal extensions of S5 having CIP but do not have LIP (see also [11]).Pitts [26] proved that intuitionistic propositional logic has the uniform interpolation property. A logic L is said to have the uniform interpolation property (UIP) if for any formula ϕ and any finite set P of propositional variables, there exists a formula θ such that θ does not contain propositional variables in P and it uniformly interpolates all L-provable implications ϕ → ψ in L where ψ does not contain propositional variables in P . Shavrukov [30] proved that the propositional modal logic GL has UIP. UIP for K, Grz, and KT were proved by Ghilardi [13] and Visser [33], Visser [33], and Bílková [2], respectively. See also [3,17]. However, it was proved by Ghilardi and Zawadowski [14] that the modal logic S4 does not enjoy UIP, and Bílková [2] also showed the same result for K4.So far, it has been studied separately that each logic has UIP and that logic has LIP. In this paper, we give a framework which can simultaneously derive that a logic enjoys both UIP and LIP. Namely, we introduce the notion of uniform Lyndon interpolation property (ULIP), and investigate this newly introduced notion.In Section 2, we show that ULIP is actually stronger than both UIP and LIP. Also we prove several basic behaviors of ULIP. Then we show that ULIP for the propositional modal logics K5, KD5, K45, KD45, KB5 and S5 easily follows from LIP for each of them. In Section 3, we introduce the notion of bounded (P, Q)-bisimulation between Kripke models which is a main tool of our proofs. ULIP for the propositional modal logics K, KD, KT, KB, KDB and KTB is proved in Section 4. Consequently, we obtain both UIP and LIP for these logics. UIP for KB, KDB and KTB are probably new. At last, we prove ULIP for GL and Grz in Section 5. Our proofs of ULIP are modifications of Visser's proofs [33] of UIP using bounded bisimulations. Especially for GL and Grz, we give a new upper bound on the complexity of uniform interpolants.
Interpolation properties in propositional modal logicsIn this section, we introduce some variations of interpolation property. In particular, we newly introduce the notion of uniform Lyndon interpolation property, and we investigate several basic behaviors of uniform Lyndon interpolation propert...