On Minimal Grammar Problems for Finite Languages

We investigate the grammatical complexity of finite languages w.r.t. context-free grammars and variants thereof. For fixed alphabets, it is shown that both the minimal number of productions and the minimal size of a context-free grammar generating a finite language cannot be approximated within a factor of o(p(1/6)) and o(s(1/7)), respectively, unless P = NP. Here, p is the number of productions and s the size of the given grammar. Similar inapproximability results also hold for linear context-free and right-linear (or regular) grammars. As a byproduct, we show that the language of all cubes of a given length requires an exponential number of context-free productions and we also investigate upper and lower bounds on the complexity of the operations union and concatenation for finite languages.