The braid group Bn is the mapping class group of an n-punctured disk. An n-braid is reducible if it has an invariant essential curve system in the punctured disk. We show that for some class of reducible braids (including split braids), finding a reduction system is as easy as finding one element in the ultra summit set.

At any stage in a well-run conversation, a certain amount is presupposed. The parties to the conversation take it for granted; or at least they purport to, whether sincerely or just “for the sake of the argument”. Presuppositions can be created or destroyed in the course of a conversation. This change is rule-governed, at least up to a point. The presuppositions at time r’ depend, in a way abou...

Labyrinth weirs have not a straight crest in the perpendicular direction to the flow. The plan view of the weir consists of multiple and broken linear crests that cause the increase of its effective length. The advantages of this type of weirs are higher discharge capacity, easy aeration as well as low fluctuations of water surface at the weir upstream. This paper deals with the simulation of t...

Domain-independent planning is a hard combinatorial problem. Taking into account plan quality makes the task even more di cult. We introduce a new paradigm for e cient high-quality planning that exploits plan rewriting rules and e cient local search techniques to transform an easy-to-generate, but possibly suboptimal, initial plan into a low-cost plan. In addition to addressing the issues of e ...

The fundamental problem in discourse is "What structure is there in discourse above the level of the sentence?" This question can be asked in terms of text versus dialogue. Is there a kind of structure that is exhibited in text but not in dialogue, or in dialogue but not in text? What are the appropriate structural descriptions of text and of dialogue? The question can also be asked from the pe...

Adaptive navigation support normally attempts to make selecting a relevant hyperlink as easy as possible. However, in educational applications, this may have negative learning effects since selecting a link is sometimes an important educational problem for the student to solve. To provide appropriate scaffolding to students, it is necessary to understand how they navigate in hypermedia sites. B...

In a virtual university, advanced support for all aspects of handling assignments is needed. Homework assignments are particularly in need of help because communication between teachers and learners as well as between learners is not as easy as in presence universities. In this paper, we present an overview of the AT(x) approach (analyze-and-test) for automatically analyzing and testing program...

In this paper, we examine the evolution of the impact of non-elite journals. We attempt to answer two questions. First, what fraction of the top-cited articles are published in non-elite journals and how has this changed over time. Second, what fraction of the total citations are to non-elite journals and how has this changed over time. To answer these questions, we studied citations to article...