Abstract
Those who enjoy programming enjoy programming competitions, either as contestants or as coaches. Often coaches are teachers, who, aiming at better results in the future, would like to have more and more students participating, from earlier stages. Learning all the basic algorithms takes some time, of course; on the other hand, competition environments can be introduced right from the beginning as a pedagogical tool. If handled correctly, this can be very effective in helping to reach the goals of the course and, as a side-effect, in bringing larger crowds of students into the programming competition arena. © 2008 Institute of Mathematics and Informatics, Vilnius.

Abstract
System L is a linear version of Gödel's System T, where the ?-calculus is replaced with a linear calculus; or alternatively a linear ?-calculus enriched with some constructs including an iterator. There is thus at the same time in this system a lot of freedom in reduction and a lot of information about resources, which makes it an ideal framework to start a fresh attempt at studying reduction strategies in ?-calculi. In particular, we show that call-by-need, the standard strategy of functional languages, can be defined directly and effectively in System L, and can be shown minimal among weak strategies.

Abstract
We consider the issue of confidentiality in multicast network coding, by assuming that the encoding matrices, based upon variants of random linear network coding, are given only to the source and sinks. Based on this assumption, we provide a characterization of the mutual information between the encoded data and the two elements that can lead to information disclosure: the matrices of random coefficients and, naturally, the original data itself. Our results, some of which hold even with finite block lengths, show that, predicated on optimal source-coding, information-theoretic security is achievable for any field size without loss in terms of decoding probability. It follows that protecting the encoding matrix is generally sufficient to ensure confidentiality of network coded data.

Abstract
Under the emerging network coding paradigm, intermediate nodes in the network are allowed not only to store and forward packets but also to process and mix different data flows. We propose a low-complexity cryptographic scheme that exploits the inherent security provided by random linear network coding and offers the advantage of reduced overhead in comparison to traditional end-to-end encryption of the entire data. Confidentiality is achieved by protecting (or "locking") the source coefficients required to decode the encoded data, without preventing intermediate nodes from running their standard network coding operations. Our scheme can be easily combined with existing techniques that counter active attacks.

Abstract
Discussion forums are presently one of the most important tools in assisting distance education. Web learning is accomplished by using these communication tools and, particularly, by the interactions that take place in these settings. Therefore, students' participations in a discussion forum, the frequency and the way they participate, the types of interactions that they create with their colleagues and with the professor, can and should be analyzed in order to fully understand the group and, consequently, allow a more efficient and student focused education. In this article, suggest graphical representations (the iGraph) of these interactions and by using these tools, we describe forum participation according to the centralization of information, the density and intensity of interactions, and yet the quality of the moderation.

Abstract
We present an architecture that allows the seamless configuration of computer labs to work as dedicated computing clusters during periods of user inactivity. The operation of the cluster is fully automated by making use of differentiated network booting and a job management system. We have prepared it to be plugged to a larger computational grid. We provide some preliminary performance results obtained.