Wyniki wyszukiwania - BazTech

1

Tinput-Driven Pushdown, Counter, and Stack Automata

Kutrib M., Malcher A., Wendlandt M.

Fundamenta Informaticae

|

2017

|

Vol. 155, nr 1/2

59--88

EN

In input-driven automata the input alphabet is divided into distinct classes and different actions on the storage medium are solely governed by the input symbols. For example, in inputdriven pushdown automata (IDPDA) there are three distinct classes of input symbols determining the action of pushing, popping, or doing nothing on the pushdown store. Here, input-driven automata are extended in such a way that the input is preprocessed by a deterministic sequential transducer. IDPDAs extended in this way are called tinput-driven pushdown automata (TDPDA) and it turns out that TDPDAs are more powerful than IDPDAs but still not as powerful as real-time deterministic pushdown automata. Nevertheless, even this stronger model has still good closure and decidability properties. In detail, it is shown that TDPDAs are closed under the Boolean operations union, intersection, and complementation. Furthermore, decidability procedures for the inclusion problem as well as for the questions of whether a given automaton is a TDPDA or an IDPDA are developed. Additionally, representation theorems for the context-free languages using IDPDAs and TDPDAs are established. Two other classes investigated are on the one hand TDPDAs restricted to tinput-driven counter automata and on the other hand TDPDAs generalized to tinput-driven stack automata. In both cases, it is possible to preserve the good closure and decidability properties of TDPDAs, namely, the closure under the Boolean operations as well as the decidability of the inclusion problem.

2

Reversible Limited Automata

Kutrib M., Wendlandt M.

Fundamenta Informaticae

|

2017

|

Vol. 155, nr 1/2

31--58

EN

A k-limited automaton is a linear bounded automaton that may rewrite each tape square only in the first k visits, where k≥ 0 is a fixed constant. It is known that these automata accept context-free languages only. We investigate deterministic k-limited automata towards their ability to perform reversible computations, that is, computations in which every configuration has at most one predecessor. A first result is that, for all k≥ 0, sweeping k-limited automata accept regular languages only. In contrast to reversible finite automata, all regular languages are accepted by sweeping 0-limited automata. Then we study the computational power gained in the number k of possible rewrite operations. It is shown that the reversible 2-limited automata accept regular languages only and, thus, are strictly weaker than general 2-limited automata. Furthermore, a proper inclusion between reversible 3-limited and 4-limited automata languages is obtained. The next levels of the hierarchy are separated between every k and k + 3 rewrite operations. We investigate closure properties of the family of languages accepted by reversible k-limited automata. It turns out that these families are not closed under intersection, but are closed under complementation. They are closed under intersection with regular languages, which leads to the non-closure under concatenation, iteration, and homomorphisms. Finally, it turns out that all k-limited automata accept Church-Rosser languages only, that is, the intersection between context-free and Church-Rosser languages contains an infinite hierarchy of language families beyond the deterministic context-free languages.

3

Reversible Queue Automata

Kutrib M., Malcher A., Wendlandt M.

Fundamenta Informaticae

|

2016

|

Vol. 148, nr 3/4

341--368

EN

Deterministic finite automata equipped with the storage medium of a queue are investigated towards their ability to perform reversible computations, that is, computations in which every occurring configuration has exactly one successor and exactly one predecessor. A first result is that any queue automaton can be simulated by a reversible one. So, reversible queue automata are as powerful as Turing machines. Therefore it is of natural interest to impose time restrictions to queue automata. Here we consider quasi realtime and realtime computations. It is shown that every reversible quasi realtime queue automaton can be sped up to realtime. On the other hand, under realtime conditions reversible queue automata are less powerful than general queue automata. Furthermore, we exhibit a lower bound of ...[formula] time steps for realtime queue automata witness languages to be accepted by any equivalent reversible queue automaton. We study the closure properties of reversible realtime queue automata and obtain similar results as for reversible deterministic pushdown automata. Finally, we investigate decidability questions and obtain that all commonly studied questions such as emptiness, finiteness, or equivalence are not semidecidable for reversible realtime queue automata. Furthermore, it is not semidecidable whether an arbitrary given realtime queue automaton is reversible.

4

On the Computational Complexity of Partial Word Automata Problems

Holzer M., Jakobi S., Wendlandt M.

Fundamenta Informaticae

|

2016

|

Vol. 148, nr 3/4

267--289

EN

We consider the computational complexity of problems related to partial word automata. Roughly speaking, a partial word is a word in which some positions are unspecified and a partial word automaton is a finite automaton that accepts a partial word language-here the unspecified positions in the word are represented by a "hole" symbol ◊ . A partial word language L' can be transformed into an ordinary language L by using a ◊-substitution. In particular, we investigate the complexity of the compression or minimization problem for partial word automata, which is known to be NP-hard. We improve on the previously known complexity on this problem, by showing PSPACE-completeness. In fact, it turns out that almost all problems related to partial word automata, such as, e.g., equivalence and universality, are already PSPACE- complete. Moreover, we also study these problems under the further restriction that the involved automata accept only finite languages. In this case, the complexities of the studied problems drop from PSPACE-completeness down to coNP-hardness and containment in ∑P2 depending on the problem investigated.

5

Deterministic One-Way Turing Machines with Sublinear Space

Kutrib M., Provillard J., Vaszil G., Wendlandt M.

Fundamenta Informaticae

|

2015

|

Vol. 136, nr 1/2

139--155

EN

Deterministic one-way Turing machines with sublinear space bounds are systematically studied. We distinguish among the notions of strong, weak, and restricted space bounds. The latter is motivated by the study of P automata. The space available on the work tape depends on the number of input symbols read so far, instead of the entire input. The class of functions space constructible by such machines is investigated, and it is shown that every function f that is space constructible by a deterministic two-way Turing machine, is space constructible by a strongly f space-bounded deterministic one-way Turing machine as well. We prove that the restricted mode coincides with the strong mode for space constructible functions. The known infinite, dense, and strict hierarchy of strong space complexity classes is derived also for the weak mode by Kolmogorov complexity arguments. Finally, closure properties under AFL operations, Boolean operations and reversal are shown.