Collage grammars generate picture languages in a context-free way. The generating process is based on the replacement of atomic nonterminal items and can be seen as an adaptation of the notion of hyperedge replacement known from the area of context-free graph generation. While a pumping lemma holds for hyperedge replacement graph grammars and is quite useful to show that certain graph languages...

The close relationship between Petri nets and language theory has been extensively studied for a long time [1, 2]. Results from the theory of Petri nets have been applied successfully to provide elegant solutions to complicated problems from language theory [4, 7]. A context-free grammar can be associated with a context-free (communica-tion-free) Petri net, whose places and transitions, corresp...

This paper presents some new results concerning the descriptional complexity of partially parallel grammars. Specifically, it proves that every recursively enumerable language is generated (i) by a four-nonterminal scattered context grammar with no more than four non-context-free productions, (ii) by a two-nonterminal multisequential grammar with no more than two selectors, or (iii) by a three-...

We present a new variant of the SyntaxAugmented Machine Translation (SAMT) formalism with a category-coarsening algorithm originally developed for tree-to-tree grammars. We induce bilingual labels into the SAMT grammar, use them for category coarsening, then project back to monolingual labeling as in standard SAMT. The result is a “collapsed” grammar with the same expressive power and format as...

We investigate the complexity of a variety of normal-form transformations for extended context-free grammars, where by extended we mean that the set of right-hand sides for each nonterminal in such a grammar is a regular set. The study is motivated by the implementation project GraMa which will provide a C++ toolkit for the symbolic manipulation of context-free objects just as Grail does for re...

It is shown that the shortest-grammar problem remains NP-complete if the alphabet is fixed and has a size of at least 24 (which settles an open question). On the other hand, this problem can be solved in polynomial-time, if the number of nonterminals is bounded, which is shown by encoding the problem as a problem on graphs with interval structure. Furthermore, we present an O(3) exact exponenti...

Selecting a set of nonterminals for the synchronous CFGs underlying the hierarchical phrase-based models is usually done on the basis of a monolingual resource (like a syntactic parser). However, a standard bilingual resource like word alignments is itself rich with reordering patterns that, if clustered somehow, might provide labels of different (possibly complementary) nature to monolingual l...

We consider the problem of designing a preemptive protocol that is both fair and efficient when one is only concerned with the sojourn time of the job and not intermediate results. Our Fair Sojourn Protocol (FSP) is both efficient, in a strong sense (similar to the shortest remaining processing time protocol – SRPT), and fair, in the sense of guaranteeing that it outperforms processor sharing (...

When people make judgments about the effects of a perturbation on populations of species in a food web, their judgments exhibit the dissipation effect: a tendency to judge that effects of the perturbation weaken or dissipate as they spread out through the food web from the locus of the perturbation. In the present research evidence for two more phenomena is reported. Terminal locations are poin...

This paper describes a factored approach to incorporating soft source syntactic constraints into a hierarchical phrase-based translation system. In contrast to traditional approaches that directly introduce syntactic constraints to translation rules by explicitly decorating them with syntactic annotations, which often exacerbate the data sparsity problem and cause other problems, our approach k...