Linear indexed languages

AbstractIn this paper one characterization of linear indexed languages based on controlling linear context-free grammars with context-free languages and one based on homomorphic images of context-free languages are given. By constructing a generator for the family of linear indexed languages, it is shown that this family is a full principal semi-AFL. Furthermore a Parikh theorem for linear indexed languages is stated which implies that there are indexed languages which are not linear

The equivalence of four extensions of context-free grammars

There is currently considerable interest among computational linguists in grammatical formalisms with highly restricted generative power. This paper concerns the relationship between the class of string languages generated by several such formalisms, namely, combinatory categorial grammars, head grammars, linear indexed grammars, and tree adjoining grammars. Each of these formalisms is known to generate a larger class of languages than context-free grammars. The four formalisms under consideration were developed independently and appear superficially to be quite different from one another. The result presented in this paper is that all four of the formalisms under consideration generate exactly the same class of string languages