Workspace Theorems for Regular-Controlled Grammars

Regular-controlled context-free grammars, workspace theorems, removal of erasing rules

This paper establishes a workspace theorem in terms of regular-controlled (context-free) grammars. It proves that, if, for a regular-controlled grammar H, there is a positive integer k such that H generates every sentence y in L(H) by a derivation in which every sentential form x contains at most (k-1)|x|/k occurrences of nonterminals that are erased throughout the rest of the derivation, where |x| denotes the length of x, then the language of H is generated by a propagating regular-controlled grammar. An analogical workspace theorem is demonstrated for regular-controlled grammars with appearance checking. The paper provides an algorithm that removes all erasing rules from any regular-controlled grammar (possibly with appearance checking) that satisfies the workspace condition above without affecting the generated language. In its conclusion, the paper points out a relationship of the workspace theorems to other areas of formal language theory.