Remarks on the nonexistence of some covering grammars

Abstract

The problem of covering context-free grammars by grammars in some normal forms is considered. It is shown that certain example grammars cannot be covered by grammars which are in ε-free form or in Greibach normal form or in Chomsky normal form. The results are generalized using a concept called the structural similarity of context-free grammars. Finally, a grammatical transformation method for constructing ε-free covers is given.