Mildly Context sensitive Grammars

Question

Can Anyone clearly explain what is exactly mildly context sensitive grammars? Are these grammars can be used to model Natural Languages?

Moreover, are the grammars like indexed grammar,head grammar and tree grammar belong to mildly context sensitive grammars?

Answer 1

The term ‘mildly context-sensitive grammars’ was introduced by Joshi (1985). The intention was to characterize formal grammars that are adequate for the description of natural language. They should be more powerful than context-free grammar (which Huybregts [1984] and Shieber [1985] had shown to be inadequate for natural language) but less powerful than general context-sensitive grammars (which, among other drawbacks, cannot be parsed in polynomial time).

Joshi’s characterization of mildly context-sensitive grammars was biased toward his work on tree-adjoining grammar (TAG). However, together with his students Vijay Shanker and David Weir, Joshi soon discovered that TAGs are equivalent, in terms of the generated languages, to the independently introduced head grammar, linear indexed grammar, and combinatory categorial grammar. This showed that the notion of mildly context-sensitivity is a very general one and not tied to a specific formalism.

Today, the term mildly context-sensitive grammar formalism is used to refer to several grammar formalisms that have some or all of the characteristic properties put forth by Joshi. Many of them are being researched and applied in descriptive and, most prominently, computational linguistics.

References

Riny Huybregts. The Weak Inadequacy of Context-Free Phrase Structure Grammars. In Ger de Haan, Mieke Trommelen, and Wim Zonneveld, editors, Van periferie naar kern, pages 81–99. Foris, Dordrecht, The Netherlands, 1984.

Aravind K. Joshi. Tree Adjoining Grammars: How Much Context-Sensitivity Is Required to Provide Reasonable Structural Descriptions?. In David R. Dowty, Lauri Karttunen, and Arnold M. Zwicky, editors, Natural Language Parsing, pages 206–250. Cambridge University Press, 1985.

Stuart M. Shieber. Evidence Against the Context-Freeness of Natural Language. Linguistics and Philosophy, 8(3):333–343, 1985.