Chomsky hierarchy

About: Chomsky hierarchy is a research topic. Over the lifetime, 601 publications have been published within this topic receiving 31067 citations. The topic is also known as: Chomsky–Schützenberger hierarchy.

Closure of language classes under bounded duplication

Abstract: Duplication is an operation generating a language from a single word by iterated application of rewriting rules u →uu on factors. We extend this operation to entire languages and investigate, whether the classes of the Chomsky hierarchy are closed under duplication. Here we treat mainly bounded duplication, where the factors duplicated cannot exceed a given length. While over two letters the regular languages are closed under bounded duplication, over three or more letters they are not, if the length bound is 4 or greater. For 2 they are closed under duplication, the case of 3 remains open. Finally, the class of context-free languages is closed under duplication over alphabets of any size.

Clearing Restarting Automata

Abstract: Restarting automata were introduced as a model for analysis by reduction, which is a linguistically motivated method for checking correctness of a sentence. We propose a new restricted version of restarting automata called clearing restarting automata with a very simple definition but simultaneously with interesting properties with respect to their possible applications. The new model can be learned very efficiently from positive examples and its stronger version can be used to learn effectively a large class of languages. We relate the class of languages recognized by clearing restarting automata to the Chomsky hierarchy.

A Finite-State Approach to Shallow Parsing and Grammatical Functions Annotation of German

Abstract: Chapter 2 gives an introduction to the nature and architecture of our parser. Section 2.1 presents finite-state automata (FSAs) as a parsing model and illustrates the motivation for using this very model for the parser at hand. Section 2.2 explains the motivation for and the main features of the parsing approach used in the dissertation at hand, namely the annotation of constituent structures and grammatical functions with hand-crafted rules. Section 2.3 introduces and illustrates principles spanning the annotation of all linguistic phenomena (i.e. robustness, modularity and underspecification) and, thus, all components of the parser. Section 2.4 shows how the FSAs are implemented with the help of the tool fsgmatch by giving illustrative examples of typical linguistic phenomena. 2.1 FSAs as a Parsing Model A parser is the implementation of an abstract automaton. In the thesis at hand, we use finite-state automata (FSAs) as the abstract automaton which serves as the basis of our parser because FSAs are very effective formalisms for parsing. The reason for this is that FSAs can be described by regular grammars, which are the most restricted and, thus, least powerful type of grammar in the Chomsky Hierarchy (Chomsky, 1956). Since the power of the formalism is restricted, for example it cannot deal with recursive structures, it is considerably faster than other approaches as Abney (1996c) has shown.