NORMAL FORMS FOR PHRASE-STRUCTURE GRAMMARS

Some new normal forms for the phrase-structure grammars are presented. Each phrase-structure grammar can be replaced by an equivalent grammar with all of the context free rules being of the from S --> upsilon, where S is the initial nonterminal, what concerns non context free rules five different situations may occur: either two extra rules of the form AB --> epsilon, CD --> epsilon, or two extra rules AB --> epsilon, CC --> epsilon, or two extra rules AA --> epsilon, BBB --> epsilon, or even a single extra rule ABBBA --> epsilon, or a single extra rule ABC --> epsilon. In all cases, no additional nonterminal symbols are required.