WAOA 2006 Special Issue of TOCS

This special issue of Theory of Computing Systems is dedicated to selected papers from the Fourth Workshop on Approximation and Online Algorithms (WAOA 2006) that took place in Zurich, Switzerland in September 2006. Of 62 papers submitted to WAOA 2006, 26 were accepted for presentation at the workshop, and revised versions of 7 papers appear in this special issue. All the contributions presented here were invited to this special issue, but went through the standard refereeing process of Theory of Computing Systems. In their contribution Online Dynamic Programming Speedups, Bar-Noy, Golin and Zhang consider dynamic programming in an online context where the problem size is not known in advance and the coefficients of the dynamic programming matrix are revealed row by row. They show that if the matrix satisfies a certain stronger version of the Monge property, it is possible to achieve a linear speedup compared to the naive algorithm even in this online scenario. In Covering Many or Few Points with Unit Disks, de Berg, Cabello and Har-Peled consider, for a given set of weighted points in the plane, the problem of placing m unit disks so as to cover points of maximum total weight, and the problem of placing a single unit disk with center in a given region so as to cover points of minimum total weight. For both problems, they give near-linear-time approximation schemes. In Online k-Server Routing Problems, Bonifaci and Stougie study the problem of minimizing the makespan or the sum of completion times in the setting where k