We study the following problem: given two words w and x, with jwj; jxj n, what is the size of the smallest context-free grammar G which generates exactly one of fw; xg? If jwj 6 = jxj, then we prove there exists a G separating w from x of size O(log log n), and this bound is best possible. If jwj = jxj, then we get an upper bound on the size of G of O(log n), and a lower bound of (logn loglog n).

Article history: Available online xxxx

This paper answers three open questions concerning the generative power of some simple variants of context-free grammars regulated by context conditions. Specifically, it discusses the generative power of so-called contextfree semi-conditional grammars (which are random context grammars where permitting and forbidding sets are replaced with permitting and forbidding strings) where permitting an...

We show that, given a string s of length n, with constant memory and logarithmic passes over a constant number of streams we can build a context-free grammar that generates s and only s and whose size is within an O (

The ability to detect ambiguities in context-free grammars is vital for their use in several fields, but the problem is undecidable in the general case. We present a safe, conservative approach, where approximations cannot result in overlooked ambiguous cases. We analyze the complexity of its use, and compare it with other ambiguity detection methods.

We generalize Uno and Yagiura’s algorithm for finding all common intervals of two permutations to the setting of two sequences with many-to-many alignment links across the two sides. We show how to maximally decompose a word-aligned sentence pair in linear time, which can be used to generate all possible phrase pairs or a Synchronous Context-Free Grammar (SCFG) with the simplest rules possible....

In this paper, we propose a new method for training translation rules for a Synchronous Context-free Grammar. A bilingual chart parser is used to generate the parse forest, and EM algorithm to estimate expected counts for each rule of the ruleset. Additional rules are constructed as combinations of reliable rules occurring in the parse forest. The new method of proposing additional translation ...

We introduce a large family of k-context-free sequences for wich we give a polynomial upper bound of their complexity functions.