Chomsky hierarchy

Within the field of computer science, specifically in the area of formal languages, the Chomsky hierarchy (occasionally referred to as Chomsky–Schützenberger hierarchy) is a containment hierarchy of classes of formal grammars.

This hierarchy of grammars was described by Noam Chomsky in 1956.[1] It is also named after Marcel-Paul Schützenberger, who played a crucial role in the development of the theory of formal languages.

Formal grammars

A formal grammar of this type consists of:

• a finite set of terminal symbols
• a finite set of nonterminal symbols
• a finite set of production rules with a left and a right-hand side consisting of a sequence of these symbols
• a start symbol

A formal grammar defines (or generates) a formal language, which is a (usually infinite) set of finite-length sequences of symbols (i.e. strings) that may be constructed by applying production rules to another sequence of symbols which initially contains just the start symbol. A rule may be applied to a sequence of symbols by replacing an occurrence of the symbols on the left-hand side of the rule with those that appear on the right-hand side. A sequence of rule applications is called a derivation. Such a grammar defines the formal language: all words consisting solely of terminal symbols which can be reached by a derivation from the start symbol.

Nonterminals are usually represented by uppercase letters, terminals by lowercase letters, and the start symbol by S. For example, the grammar with terminals {a,b}, nonterminals {S,A,B}, production rules

S $\rightarrow \,$ ABS
S $\rightarrow \,$ ε (where ε is the empty string)
BA $\rightarrow \,$ AB
BS $\rightarrow \,$ b
Bb $\rightarrow \,$ bb
Ab $\rightarrow \,$ ab
Aa $\rightarrow \,$ aa

and start symbol S, defines the language of all words of the form anbn (i.e. n copies of a followed by n copies of b). The following is a simpler grammar that defines the same language: Terminals {a,b}, Nonterminals {S}, Start symbol S, Production rules

S $\rightarrow \,$ aSb
S $\rightarrow \,$ ε

The hierarchy

Set inclusions described by the Chomsky hierarchy

The Chomsky hierarchy consists of the following levels:

• Type-0 grammars (unrestricted grammars) include all formal grammars. They generate exactly all languages that can be recognized by a Turing machine. These languages are also known as the recursively enumerable languages. Note that this is different from the recursive languages which can be decided by an always-halting Turing machine.
• Type-1 grammars (context-sensitive grammars) generate the context-sensitive languages. These grammars have rules of the form $\alpha A\beta \rightarrow \alpha\gamma\beta$ with A a nonterminal and α, β and γ strings of terminals and nonterminals. The strings α and β may be empty, but γ must be nonempty. The rule $S \rightarrow \epsilon$ is allowed if S does not appear on the right side of any rule. The languages described by these grammars are exactly all languages that can be recognized by a linear bounded automaton (a nondeterministic Turing machine whose tape is bounded by a constant times the length of the input.)
• Type-2 grammars (context-free grammars) generate the context-free languages. These are defined by rules of the form $A \rightarrow \gamma$ with A a nonterminal and γ a string of terminals and nonterminals. These languages are exactly all languages that can be recognized by a non-deterministic pushdown automaton. Context-free languages are the theoretical basis for the syntax of most programming languages.
• Type-3 grammars (regular grammars) generate the regular languages. Such a grammar restricts its rules to a single nonterminal on the left-hand side and a right-hand side consisting of a single terminal, possibly followed (or preceded, but not both in the same grammar) by a single nonterminal. The rule $S \rightarrow \epsilon$ is also allowed here if S does not appear on the right side of any rule. These languages are exactly all languages that can be decided by a finite state automaton. Additionally, this family of formal languages can be obtained by regular expressions. Regular languages are commonly used to define search patterns and the lexical structure of programming languages.

Note that the set of grammars corresponding to recursive languages is not a member of this hierarchy.

Every regular language is context-free, every context-free language, not containing the empty string, is context-sensitive and every context-sensitive language is recursive and every recursive language is recursively enumerable. These are all proper inclusions, meaning that there exist recursively enumerable languages which are not context-sensitive, context-sensitive languages which are not context-free and context-free languages which are not regular.

The following table summarizes each of Chomsky's four types of grammars, the class of language it generates, the type of automaton that recognizes it, and the form its rules must have.

Grammar Languages Automaton Production rules (constraints)
Type-0 Recursively enumerable Turing machine $\alpha \rightarrow \beta$ (no restrictions)
Type-1 Context-sensitive Linear-bounded non-deterministic Turing machine $\alpha A\beta \rightarrow \alpha\gamma\beta$
Type-2 Context-free Non-deterministic pushdown automaton $A \rightarrow \gamma$
Type-3 Regular Finite state automaton $A \rightarrow a$
and
$A \rightarrow aB$

However, there are further categories of formal languages, some of which are given in the following table:

References

1. ^ Chomsky, Noam (1956). "Three models for the description of language". IRE Transactions on Information Theory (2): 113–124.
• Chomsky, Noam (1959). "On certain formal properties of grammars". Information and Control 2 (2): 137–167. doi:10.1016/S0019-9958(59)90362-6.
• Chomsky, Noam; Schützenberger, Marcel P. (1963). "The algebraic theory of context free languages". In Braffort, P.; Hirschberg, D.. Computer Programming and Formal Languages. Amsterdam: North Holland. pp. 118–161.
• Davis, Martin E.; Sigal, Ron; Weyuker, Elaine J. (1994). Computability, complexity, and languages: Fundamentals of theoretical computer science. Boston: Academic Press, Harcourt, Brace. pp. 327. ISBN 0122063821.

Wikimedia Foundation. 2010.

Look at other dictionaries:

• Chomsky hierarchy — A series of increasingly complex and comprehensive classes of formal languages. The simplest are finite state languages, followed by context free, context sensitive, recursive, and recursively enumerable languages. The descriptions refer to the… …   Philosophy dictionary

• Chomsky (disambiguation) — Chomsky may refer to: Noam Chomsky (born 1928), American linguist, philosopher, cognitive scientist, political activist, author, lecturer, professor emeritus at MIT, known for early work in transformational grammar and A.I. Chomsky (surname),… …   Wikipedia

• Hierarchy (mathematics) — In mathematics, a hierarchy is a preorder, i.e. an ordered set. The term is used to stress a natural hierarchical relation among the elements. In particular, it is the preferred terminology for posets whose elements are classes of objects of… …   Wikipedia

• Noam Chomsky — Chomsky redirects here. For other topics with the same name, see Chomsky (disambiguation). Noam Chomsky Noam Chomsky visiting Vancouver, Canada in 2004 …   Wikipedia

• Hiérarchie de Chomsky — En informatique théorique, en théorie des langages, et en calculabilité, la hiérarchie de Chomsky est une classification des langages formels et des grammaires formelles, décrite par Noam Chomsky en 1956[1]. Cette section ne cite pas suffisamment …   Wikipédia en Français

• Noam Chomsky's political views — Noam Chomsky at an antiwar rally in Vancouver, 2004 Noam Chomsky is a widely known intellectual, political activist, and …   Wikipedia

• Noam Chomsky bibliography — Bibliography of Noam Chomsky. Contents 1 Books by Chomsky 1.1 Linguistics 1.2 Computer science 1.3 Politics …   Wikipedia

• Manufacturing Consent: Noam Chomsky and the Media — This article is about a documentary film. For the similarly named book, see Manufacturing Consent: The Political Economy of the Mass Media. Manufacturing Consent: Noam Chomsky and the Media Film poster …   Wikipedia

• Containment hierarchy — A containment hierarchy is a hierarchical collection of strictly nested sets. Each entry in the hierarchy designates a set such that the previous entry is a strict superset, and the next entry is a strict subset. For example, all rectangles are… …   Wikipedia

• Social hierarchy — is a multi tiered pyramid like social or functional structure having an apex as the centralization of power. The term can also be applied to animal societies, but the term dominance hierarchy is preferred most times. Typically, institutions such… …   Wikipedia