We introduce the synchronization of a pushdown automaton by a sequential transducer associating an integer to each input word. The visibly pushdown automata are the automata synchronized by an one state transducer whose output labels are −1, 0, 1. For each transducer, we can decide whether a pushdown automaton is synchronized. The pushdown automata synchronized by a given transducer accept lang...

We present a pumping lemma for each level of the collapsible pushdown graph hierarchy in analogy to the second author’s pumping lemma for higher-order pushdown graphs (without collapse). Using this lemma, we give the first known examples that separate the levels of the collapsible pushdown graph hierarchy and of the collapsible pushdown tree hierarchy, i.e., the hierarchy of trees generated by ...

Visibly pushdown automata are input-driven pushdown automata that recognize some non-regular context-free languages while preserving the nice closure and decidability properties of finite automata. Visibly pushdown automata with multiple stacks have been considered recently by La Torre, Madhusudan, and Parlato, who exploit the concept of visibility further to obtain a rich pushdown-automata cla...

Higher-order pushdown systems generalize pushdown systems by using higher-order stacks, which are nested stacks of stacks. In this article, we consider parity games defined by higher-order pushdown systems and provide a k-Exptime algorithm to compute finite representations of positional winning strategies for both players for games defined by level-k higher-order pushdown automata. Our result i...

Visibly pushdown languages properly generalise regular languages and are properly contained in deterministic context-free languages. The complexity of their membership problem is equivalent to that of regular languages. However, the corresponding counting problem – computing the number of accepting paths in a visibly pushdown automaton – could be harder than counting paths in a non-deterministi...

We introduce and study input-driven deterministic and nondeterministic double-head pushdown automata. A double-head pushdown automaton is a slight generalization of an ordinary pushdown automaton working with two input heads that move in opposite directions on the common input tape. In every step one head is moved and the automaton decides on acceptance if the heads meet. Demanding the automato...

The solution of parity games over pushdown graphs (Walukiewicz ’96) was the first step towards an effective theory of infinite-state games. It was shown that winning strategies for pushdown games can be implemented again as pushdown automata. We continue this study and investigate the connection between game presentations and winning strategies in altogether six cases of game arenas, among them...

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...

Flip-pushdown automaton is pushdown automaton which has ability to flip its pushdown throughout the computation. This model was introduced in [3] by Sarkar. Here we solve in the affirmative the following open problem posed by Holzer and Kutrib in [1]: What is the power of ε-moves for nondeterministic flip-pushdown automata – can they be removed without affecting the computational capacity? (ε d...