Relaxed Spanners for Directed Disk Graphs

Let (V,delta) be a finite metric space, where V is a set of n points and delta is a distance function defined for these points. Assume that (V,delta) has a doubling dimension d and assume that each point paV has a disk of radius r(p) around it. The disk graph that corresponds to V and r(a <...) is a directed graph I(V,E,r), whose vertices are the points of V and whose edge set includes a directed edge from p to q if delta(p,q)a parts per thousand currency signr(p). In Peleg and Roditty (Proc. 7th Int. Conf. on Ad-Hoc Networks and Wireless (AdHoc-NOW), pp. 622-633, 2008) we presented an algorithm for constructing a (1+I mu)-spanner of size O(nI mu (-d) logM), where M is the maximal radius r(p). The current paper presents two results. The first shows that the spanner of Peleg and Roditty (in Proc. 7th Int. Conf. on Ad-Hoc Networks and Wireless (AdHoc-NOW), pp. 622-633, 2008) is essentially optimal, i.e., for metrics of constant doubling dimension it is not possible to guarantee a spanner whose size is independent of M. The second result shows that by slightly relaxing the requirements and allowing a small augmentation of the radius assignment, considerably better spanners can be constructed. In particular, we show that if it is allowed to use edges of the disk graph I(V,E,r (1+I mu) ), where r (1+I mu) (p)=(1+I mu)a <...r(p) for every paV, then it is possible to get a (1+I mu)-spanner of size O(n/I mu (d) ) for I(V,E,r). Our algorithm is simple and can be implemented efficiently.