Damian Niwiński

Higher-Order Pushdown Trees Are Easy

We show that the monadic second-order theory of an infinite tree recognized by a higher-order pushdown automaton of any level is decidable. We also show that trees recognized by pushdown automata of level n coincide with trees generated by safe higher-order grammars of level n. Our decidability result extends the result of Courcelle on algebraic(pushdo wn of level 1) trees and our own result on trees of level 2.

Fixed points vs. infinite generation

The author characterizes Rabin definability (see M.O. Rabin, 1969) of properties of infinite trees of fixed-point definitions based on the basic operations of a standard powerset algebra of trees and involving the least and greatest fixed-point operators as well as the finite union operator and functional composition. A strict connection is established between a hierarchy resulting from alternating the least and greatest fixed-point operators and the hierarchy induced by Rabin indices of automata. The characterization result is actually proved on a more general level, namely, for arbitrary powerset algebra, where the concept of Rabin automaton is replaced by the more general concept of infinite grammar.<<ETX>>

Fixed point characterization of infinite behavior of finite-state systems

Abstract Infinite behavior of nondeterministic finite-state automata running over infinite trees or more generally over elements of an arbitrary algebraic structure is characterized by a calculus of fixed point terms interpreted in powerset algebras. These terms involve the least and greatest fixed point operators and disjunction as the only logical operation. A tight correspondence is established between a hierarchy of Rabin indices of automata and a hierarchy induced by alternation of the least and greatest fixed point operators. It is shown that, in the powerset algebra of trees constructed from a set of functional symbols, the fixed point hierarchy is infinite unless all the symbols are unary (i.e. trees are words). It is also shown that an interpretation of a closed fixed point term in any powerset algebra can be factorized through the interpretation of this term in the powerset algebra of trees, from which it is deduced that the question whether a term denotes always ∅ can be answered in polynomial time.

Unsafe grammars and panic automata

We show that the problem of checking if an infinite tree generated by a higher-order grammar of level 2 (hyperalgebraic) satisfies a given μ-calculus formula (or, equivalently, if it is accepted by an alternating parity automaton) is decidable, actually 2-Exptime-complete. Consequently, the monadic second-order theory of any hyperalgebraic tree is decidable, so that the safety restriction can be removed from our previous decidability result. The last result has been independently obtained by Aehlig, de Miranda and Ong. Our proof goes via a characterization of possibly unsafe second-order grammars by a new variant of higher-order pushdown automata, which we call panic automata. In addition to the standard pop1 and pop2 operations, these automata have an option of a destructive move called panic. The model-checking problem is then reduced to the problem of deciding the winner in a parity game over a suitable 2nd order pushdown system.

Games for the m-calculus

Abstract Given a formula of the propositional μ-calculus, we construct a tableau of the formula and define an infinite game of two players of which one wants to show that the formula is satisfiable, and the other seeks the opposite. The strategy for the first player can be further transformed into a model of the formula while the strategy for the second forms what we call a refutation of the formula. Using Martins Determinacy Theorem, we prove that any formula has either a model or a refutation. This completeness result is a starting point for the completeness theorem for the μ-calculus to be presented elsewhere. However, we argue that refutations have some advantages of their own. They are generated by a natural system of sound logical rules and can be presented as regular trees of the size exponential in the size of a refuted formula. This last aspect completes the small model theorem for the μ-calculus established by Emerson and Jutla (1988). Thus, on a more practical side, refutations can be used as small objects testifying incorrectness of a program specification expressed by a μ-formula, we illustrate this point by an example.

A gap property of deterministic tree languages

We show that a tree language recognized by a deterministic parity automaton is either hard for the co-Buchi level and therefore cannot be recognized by a weak alternating automaton, or is on a very low level in the hierarchy of weak alternating automata. A topological counterpart of this property is that a deterministic tree language is either Π11 complete (and hence nonBorel), or it is on the level Π30 of the Borel hierarchy. We also give a new simple proof of the strictness of the hierarchy of weak alternating automata.

Relating Hierarchies of Word and Tree Automata

For an ω-word language L, the derived tree language Path(L) is the language of trees having all their paths in L. We consider the hierarchies of deterministic automata on words and nondeterministic automata on trees with Rabin conditions in chain form. We show that L is on some level of the hierarchy of deterministic word automata iff Path(L) is on the same level of the hierarchy of nondeterministic tree automata.

Deciding monadic theories of hyperalgebraic trees

We show that the monadic second-order theory of any infinite tree generated by a higher-order grammar of level 2 subject to a certain syntactic restriction is decidable. By this we extend the result of Courcelle [6] that the MSO theory of a tree generated by a grammar of level 1 (algebraic) is decidable. To this end, we develop a technique of representing infinite trees by infinite λ-terms, in such a way that the MSO theory of a tree can be interpreted in the MSO theory of a λ-term.

Deciding Nondeterministic Hierarchy of Deterministic Tree Automata

We show an algorithm which, for a given deterministic parity automaton on infinite trees, computes the minimal Mostowski (or Rabin) index of a nondeterministic automaton recognizing the same language. This extends a previous result of Urbanski on deciding if a given deterministic Rabin automaton is equivalent to a nondeterministic Buchi automaton. The algorithm runs in the time of verifying the non-emptiness of nondeterministic parity automata.

A geometrical view of the determinization and minimization of finite-state automata

With every finite-state word or tree automaton, we associate a binary relation on words or trees. We then consider the “rectangular decompositions” of this relation, i.e., the various ways to express it as a finite union of Cartesian products of sets of words or trees, respectively. We show that the determinization and the minimization of these automata correspond to simple geometrical reorganizations of the rectangular decompositions of the associated relations.