Abstract. We consider bounded versions of undecidable problems about context-free languages which restrict the domain of words to some finite length: inclusion, intersection, universality, equivalence, and ambiguity. These are in (co)-NP and thus solvable by a reduction to the (un-)satisfiability problem for propositional logic. We present such encodings -fully utilizing the power of incrementat SAT solvers -prove correctness and validate this approach with benchmarks.

Abstract. Higher-Order Fixpoint Logic (HFL) is a hybrid of the simply typed λ-calculus and the modal µ-calculus. This makes it a highly expressive temporal logic that is capable of expressing various interesting correctness properties of programs that are not expressible in the modal µ-calculus.This paper provides complexity results for its model checking problem. In particular, we consider those fragments of HFL that are built by using only types of bounded order k and arity m. We establish k-fold exponential time completeness for model checking each such fragment. For the upper bound we use fixpoint elimination to obtain reachability games that are singly-exponential in the size of the formula and k-fold exponential in the size of the underlying transition system. These games can be solved in deterministic linear time. As a simple consequence, we obtain an exponential time upper bound on the expression complexity of each such fragment.The lower bound is established by a reduction from the word problem for alternating (k − 1)-fold exponential space bounded Turing Machines. Since there are fixed machines of that type whose word problems are already hard with respect to k-fold exponential time, we obtain, as a corollary, k-fold exponential time completeness for the data complexity of our fragments of HFL, provided m exceeds 3. This also yields a hierarchy result in expressive power.

Abstract. We introduce a generic extension of the popular branchingtime logic CTL which refines the temporal until and release operators with formal languages. For instance, a language may determine the moments along a path that an until property may be fulfilled. We consider several classes of languages leading to logics with different expressive power and complexity, whose importance is motivated by their use in model checking, synthesis, abstract interpretation, etc. We show that even with context-free languages on the until operator the logic still allows for polynomial time model-checking despite the significant increase in expressive power. This makes the logic a promising candidate for applications in verification. In addition, we analyse the complexity of satisfiability and compare the expressive power of these logics to CTL * and extensions of PDL.

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