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# Indexed language

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 Title: Indexed language Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

### Indexed language

Indexed languages are a class of formal languages discovered by Alfred Aho;[1] they are described by indexed grammars and can be recognized by nested stack automata.[2]

Indexed languages are a proper subset of context-sensitive languages.[1] They qualify as an abstract family of languages (furthermore a full AFL) and hence satisfy many closure properties. However, they are not closed under intersection or complement.[1]

The class of indexed languages has practical importance in natural language processing as a computationally affordable generalization of context-free languages, since indexed grammars can describe many of the nonlocal constraints occurring in natural languages.

Gerald Gazdar (1988)[3] and Vijay-Shanker (1987)[4] introduced a mildly context-sensitive language class now known as linear indexed grammars (LIG).[5] Linear indexed grammars have additional restrictions relative to IG. LIGs are weakly equivalent (generate the same language class) as tree adjoining grammars.[6]

## Examples

The following languages are indexed, but are not context-free:

\{a^n b^n c^n d^n| n \geq 1 \} [3]
\{a^n b^m c^n d^m | m,n \geq 0 \} [2]

These two languages are also indexed, but are not even mildly context sensitive under Gazdar's characterization:

\{a^{2^{n}} | n \geq 0 \} [2]
\{www | w \in \{a,b\}^+ \} [3]

On the other hand, the following language is not indexed:[7]

\{(a b^n)^n | n \geq 0 \}

## Properties

Hopcroft and Ullman tend to consider indexed languages as a "natural" class, since they are generated by several formalisms, such as:[9]

Hayashi[14] generalized the pumping lemma to indexed grammars. Conversely, Gilman[7][15] gives a "shrinking lemma" for indexed languages.

## References

1. ^ a b c d
2. ^ a b c
3. ^ a b c Gazdar, Gerald (1988). "Applicability of Indexed Grammars to Natural Languages". In U. Reyle and C. Rohrer. Natural Language Parsing and Linguistic Theories. pp. 69–94.
4. ^ http://search.proquest.com/docview/303610666
5. ^ Laura Kallmeyer (2010). Parsing Beyond Context-Free Grammars. Springer Science & Business Media. p. 31.
6. ^ Laura Kallmeyer (16 August 2010). Parsing Beyond Context-Free Grammars. Springer Science & Business Media. p. 32.
7. ^ a b Gilman, Robert H. (1996). "A Shrinking Lemma for Indexed Languages". Theoretical Computer Science 163 (1–2): 277–281.
8. ^
9. ^ Introduction to automata theory, languages, and computation,[8]Bibliographic notes, p.394-395
10. ^ Alfred Aho (1969). "Nested Stack Automata". Journal of the ACM 16 (3): 383–406.
11. ^ Michael J. Fischer (1968). "Grammars with Macro-Like Productions". Proc. 9th Ann. IEEE Symp. on Switching and Automata Theory (SWAT). pp. 131–142.
12. ^ Sheila A. Greibach (1970). "Full AFL's and Nested Iterated Substitution". Information and Control 16 (1): 7–35.
13. ^ T.S.E. Maibaum (1974). "A Generalized Approach to Formal Languages". J. Computer and System Sciences 8 (3): 409–439.
14. ^ T. Hayashi (1973). Theorem"uvxyz"On Derivation Trees of Indexed Grammars - An Extension of the . Publication of the Research Institute for Mathematical Sciences (Research Institute for Mathematical Sciences) 9 (1): 61–92.
15. ^ Robert H. Gilman (Sep 1995). "A Shrinking Lemma for Indexed Languages".