In 1984, Danecki proved that satisfiability in IPDL, i.e., Propositional Dynamic Logic (PDL) extended with an intersection operator on programs, is decidable in deterministic double exponential time. Since then, the exact complexity of IPDL has remained an open problem: the best known lower bound was the ExpTime one stemming from plain PDL until, in 2004, the first author established ExpSpace-hardness. In this paper, we finally close the gap and prove that IPDL is hard for 2-ExpTime, thus 2-ExpTime-complete. We then sharpen (...) our lower bound, showing that it even applies to IPDL without the test operator interpreted on tree structures. (shrink)

The modal -calculus extends basic modal logic with second-order quantification in terms of arbitrarily nested fixpoint operators. Its satisfiability problem is EXPTIME-complete. Decision procedures for the modal -calculus are not easy to obtain though since the arbitrary nesting of fixpoint constructs requires some combinatorial arguments for showing the well-foundedness of least fixpoint unfoldings. The tableau-based decision procedures so far also make assumptions on the unfoldings of fixpoint formulas, e.g., explicitly require formulas to be in guarded normal form. In this paper (...) we present a tableau calculus for deciding satisfiability of arbitrary, i.e., not necessarily guarded -calculus formulas. It is based on a new unfolding rule for greatest fixpoint formulas which allows unguarded formulas to be handled without an explicit transformation into guarded form, thus avoiding a exponential blow-up in formula size. We prove soundness and completeness of the calculus, and compare it empirically to using guarded transformation instead. The new unfolding rule can also be used to handle nested star operators in PDL formulas correctly. (shrink)

The paper defines focus games for satisfiability of linear time temporal logic with past operators. The games are defined in such a way that a complete axiomatisation can easily be obtained from the game rules.

We consider the model checking problem for Hybrid Logic. Known algorithms so far are global in the sense that they compute, inductively, in every step the set of all worlds of a Kripke structure that satisfy a subformula of the input. Hence, they always exploit the entire structure. Local model checking tries to avoid this by only traversing necessary parts of the input in order to establish or refute the satisfaction relation between a given world and a formula. We present (...) a framework for local model checking of Hybrid Logic based on games. We show that these games are simple reachability games for ordinary Hybrid Logic and weak Büchi games for Hybrid Logic with operators interpreted over the transitive closure of the accessibility relation of the underlying Kripke frame, and show how to solve these games thus solving the local model checking problem. Since the first-order part of Hybrid Logic is inherently hard to localise in model checking, we give examples, in the end, of how global model checkers can be optimised in certain special cases using well-established techniques like fixpoint approximations and divide-and-conquer algorithms. (shrink)

Epigenetic modifications, such as DNA methylation and post‐translation modifications of histones, have been shown to play an important role in chromatin structure, promoter activity, and cellular reprogramming. Large protein complexes, such as Polycomb and trithorax, often harbor multiple activities which affect histone tail modification. Nevertheless, the mechanisms underlying the deposition of these marks, their propagation during cell replication, and the alteration on their distribution during transformation still require further study. Here we review recent data on those processes in both normal (...) and cancer cells, and we propose that the unscheduled expression of oncogenic transcription factors causes reprogramming of normal cells into cancer stem cells. (shrink)

Head-driven phrase structure grammar (HPSG) is one of the most prominent theories employed in deep parsing of natural language. Many linguistic theories are arguably best formalized in extensions of modal or dynamic logic (Keller, Feature logics, infinitary descriptions and grammar, 1993; Kracht, Linguistics Philos 18:401–458, 1995; Moss and Tiede, In: Blackburn, van Benthem, and Wolther (eds.) Handbook of modal logic, 2006), and HPSG seems to be no exception. Adequate extensions of dynamic logic have not been studied in detail, however; the (...) most important aspect is the reference to sets of substructures. In this paper, an adequate extension is identified, and some important results are established: Satisfiability is highly undecidable, and model checking is shown to be in EXPTIME and PSPACE-hard. A fragment with polynomial time model checking procedures is identified; it is shown to cover considerable fragments of HPSG. (shrink)