On differentiation functions, structure functions, and related languages of context-free grammars

¹ Otto-von-Guericke-Universität Magdeburg, Fakultät für Informatik, PSF 4120, 39016 Magdeburg, Germany; dassow@iws.cs.uni-magdeburg.de.; stiebe@iws.cs.uni-magdeburg.de.
² University of Bucharest, Institute of Mathematics, Str. Academiei 14, 70109 Bucuresti, Romania.
³ Rovira i Virgili University, Research Group in Mathematical Linguistics, Pça. Imperial Tarraco 1, 43005, Tarragona, Spain; vmi@fll.urv.es.
⁴ Institute of Mathematics of the Romanian Academy, PO Box 1–764, 70700 Bucuresti, Romania; George.Paun@imar.ro.

Received: 29 August 2001
Accepted: 10 April 2004

Abstract

We introduce the notion of a differentiation function of a context-free grammar which gives the number of terminal words that can be derived in a certain number of steps. A grammar is called narrow (or k-narrow) iff its differentiation function is bounded by a constant (by k). We present the basic properties of differentiation functions, especially we relate them to structure function of context-free languages and narrow grammars to slender languages. We discuss the decidability of the equivalence of grammars with respect to the differentiation function and structure function and prove the decidability of the k-narrowness of context-free grammars. Furthermore, we introduce languages representing the graph of the differentiation and structure function and relate these languages to those of the Chomsky hierarchy.