Alternating context-free and conjunctive grammars are two formalisms that extend context-free grammars with an operator for universal choice. The classes of languages derived by such grammars lie in between CFL and CSL. Here we show that not only the two classes coincide but that the language of a grammar is invariant under these two semantics. Furthermore, we suggest a third equivalent semantics which allows words to have exponentially shorter derivations and which is therefore much more suited for parsing in these grammars.

We introduce a generic extension of the popular branching-time 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.

We show interreducibility under linear-time (Turing) reductions between three families of problems parametrised by classes of formal languages: the problem of reachability in a directed graph constrained by a formal language, the model checking problem for Propositional Dynamic Logic over some class of formal languages, and the problem of deciding whether or not the intersection of a language of some class with a regular language is empty. This allows several decidability and complexity results to be transferred, mainly from the area of formal languages to the areas of graph algorithms and modal logics.

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 NP and thus solvable by a reduction to the
satisfiability problem for propositional logic. We present such encodings -- fully utilizing
the power of incrementat SAT solvers -- prove correctness and validate this approach with some
benchmarks.

We present a model checking algorithm for HFL1, the first-order fragment
of Higher-Order Fixpoint Logic. This logic is capable of expressing many interesting properties
which are not regular and, hence, not expressible in the modal
μ-calculus.

The algorithm avoids best-case exponential behaviour by localising
the computation of functions and can be implemented symbolically using BDDs.

We show how insight into the behaviour of this procedure, when run on a fixed formula,
can be used to obtain specialised algorithms for particular problems. This yields, for example,
the competitive antichain algorithm for NFA universality but also a new algorithm for
a string matching problem.

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 its fragments HFL^{k,m} which are formed using types of bounded order k
and arity m only. We establish k-ExpTime-completeness for model checking each HFL^{k,m}
fragment. For the upper bound we reduce the problem to the problem of solving rather large
parity games of small index. As a consequence of this we obtain an ExpTime upper bound on the
expression complexity of each HFL^{k,m}.

The lower bound is established by a reduction from the word problem for alternating
(k-1)-fold exponential space bounded Turing Machines. As a corollary we obtain
k-ExpTime-completeness for the data complexity of HFL^{k,m} already when m ≥ 4.