This article presents the basic theory of automata on infinite trees. We introduce the standard model of nondeterministic automata and the generalization to alternating automata. The central part of the theory considers the closure properties of tree languages definable by such automata, the equivalence of the different models, and their algorithmic properties. We show how these results are obt...

Accessible groups whose word problems are accepted by a deterministic nested stack automaton with limited erasing are virtually

Higher-order pushdown systems (PDSs) generalise pushdown systems through the use ofhigher-order stacks, that is, a nested “stack of stacks” structure. These systems may be usedto model higher-order programs and are closely related to the Caucal hierarchy of infinitegraphs and safe higher-order recursion schemes.We consider the backwards-reachability problem over higher-order Alt...

This work is a study of the expressive power of unambiguity in the case of infinite trees. An automaton is called unambiguous if it has at most one accepting run on every input, the language of such an automaton is called an unambiguous language. It is known that not every regular language of infinite trees is unambiguous. Except that, very little is known about which regular tree languages are...

Properties of probabilistic as well as \probabilistic plus nondeterministic" pushdown automata and auxiliary pushdown automata are studied. These models are analogous to their counterparts with nondeterministic and alternating states. Complete characterizations in terms of well-known complexity classes are given for the classes of languages recognized by polynomial time-bounded, logarithmic spa...

We show that the class of context free languages CFL is not equal to ⊕1−PDA (= ⊕CFL), the class of languages, which are recognized by a nondeterministic oneway push-down automaton equipped with parity acceptance. Furthermore we show that LOG(⊕CFL) = ⊕AuxPDApt contains all languages, which can be recognized by a uniform weak unambiguous AC1-circuit introduced in [LR90a]. Therefore, it contains a...

We consider past pushdown timed automata that are discrete pushdown timed automata with past formulas as enabling conditions. Using past formulas allows a past pushdown timed automaton to access the past values of the finite state variables in the automaton. We prove that the reachability (i.e., the set of reachable configurations from an initial configuration) of a past pushdown timed automato...

A well-known theorem in automata theory states that every context-free language is accepted by a pushdown automaton. We investigate this theorem in the setting of processes, using the rooted branching bisimulation and contrasimulation equivalences instead of language equivalence. In process theory, different from automata theory, interaction is explicit, so we realize a pushdown automaton as a ...

We consider past pushdown timed automata that are discrete pushdown timed automata [15] with past-formulas as enabling conditions. Using past formulas allows a past pushdown timed automaton to access the past values of the finite state variables in the automaton. We prove that the reachability (i.e., the set of reachable configurations from an initial configuration) of a past pushdown timed aut...

The concept of a synchronized extension system (SE-system, for short) has been introduced in by Ţiplea et al. [Acta Inform., to appear] as a 4-tuple G = (V ,L1,L2, S), where V is an alphabet and L1, L2 and S are languages over V . Such systems generate languages extending L1 by L2 to the left or to the right, and synchronizing on words in S. Ţiplea et al. showed that the language of type r− gen...