CONJUNCTIVE GRAMMARS GENERATE NON-REGULAR UNARY LANGUAGES

Conjunctive grammars, introduced by Okhotin, extend context-free grammars by an additional operation of intersection in the body of any production of the grammar. Several theorems and algorithms for context-free grammars generalize to the conjunctive case. Okhotin posed nine open problems concerning those grammars. One of them was a question, whether a conjunctive grammars over a unary alphabet generate only regular languages. We give a negative answer, contrary to the conjectured positive one, by constructing a conjunctive grammar for the language {a4n : n ∈ ℕ}. We also generalize this result: for every set of natural numbers L we show that {an : n ∈ L} is a conjunctive unary language, whenever the set of representations in base-k system of elements of L is regular, for arbitrary k.

EFFICIENT AUTOMATON-BASED RECOGNITION FOR LINEAR CONJUNCTIVE LANGUAGES

Linear conjunctive grammars have recently been proved computationally equivalent to triangular trellis automata. The relation between these grammars and these automata resembles that between regular expressions and finite automata: while the former are better suited for human use, the latter are considerably easier to implement. This paper studies efficient algorithms for converting a linear conjunctive grammar to an equivalent triangular trellis automaton, and also proposes a number of techniques of reducing the size of these automata.