Pushdown automaton

About: Pushdown automaton is a research topic. Over the lifetime, 1868 publications have been published within this topic receiving 35399 citations.

Limited Automata: Properties, Complexity and Variants

Abstract: Limited automata are single-tape Turing machines with severe rewriting restrictions. They have been introduced in 1967 by Thomas Hibbard, who proved that they have the same computational power as pushdown automata. Hence, they provide an alternative characterization of the class of context-free languages in terms of recognizing devices. After that paper, these models have been almost forgotten for many years. Only recently limited automata were reconsidered in a series of papers, where several properties of them and of their variants have been investigated. In this work we present an overview of the most important results obtained in these researches. We also discuss some related models and possible lines for future investigations.

Selected Ideas Used for Decidability and Undecidability of Bisimilarity

Abstract: The paper tries to highlight some crucial ideas appearing in the decidability and undecidability proofs for the bisimilarity problem on models originating in language theory, like context-free grammars and pushdown automata. In particular, it focuses on the method of finite bases of bisimulations in the case of decidability and the method of "Defender's forcing" in the case of undecidability. An intent was to write an easy-to-read article in a slightly informal way, which should nevertheless convey the basic ideas with sufficient precision.

A Grammatical Inference Approach to Language-Based Anomaly Detection in XML

Abstract: False-positives are a problem in anomaly-based intrusion detection systems. To counter this issue, we discuss anomaly detection for the extensible Markup Language (XML) in a language-theoretic view. We argue that many XML-based attacks target the syntactic level, i.e. the tree structure or element content, and syntax validation of XML documents reduces the attack surface. XML offers so-called schemas for validation, but in real world, schemas are often unavailable, ignored or too general. In this work-in-progress paper we describe a grammatical inference approach to learn an automaton from example XML documents for detecting documents with anomalous syntax. We discuss properties and expressiveness of XML to understand limits of learn ability. Our contributions are an XML Schema compatible lexical data type system to abstract content in XML and an algorithm to learn visibly pushdown automata (VPA) directly from a set of examples. The proposed algorithm does not require the tree representation of XML, so it can process large documents or streams. The resulting deterministic VPA then allows stream validation of documents to recognize deviations in the underlying tree structure or data types.

Kolmogorov Complexity and Deterministic Context-Free Languages

Abstract: We deal with a criterion for deterministic context-free languages that was originally formulated by Li and Vitanyi [SIAM J. Comput., 24 (1995), pp. 398--410]. Their result---called the KC-DCF lemma---relates Kolmogorov complexity to pushdown automata and works on a superset of examples compared to traditional iteration and pumping lemmas. Sadly, their KC-DCF lemma has a flaw. In this paper, we give a counterexample to the original KC-DCF lemma and also provide a corrected version.