Evangelia Pyrga

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We consider two approaches that model timetable information in public transportation systems as shortest-path problems in weighted graphs. In the <i>time-expanded</i> approach, every event at a station, e.g., the departure of a train, is modeled as a node in the graph, while in the <i>time-dependent</i> approach the graph contains only one node per station.(More)
We describe a new technique for proving the existence of small 2-nets for hypergraphs satisfying certain simple conditions. The technique is particularly useful for proving o( 1 2 log 1 2 ) upper bounds which is not possible using the standard VC dimension theory. We apply the technique to several geometric hypergraphs and obtain simple proofs for the(More)
We reconsider the well-studied Selfish Routing game with affine latency functions. The Price of Anarchy for this class of games takes maximum value 4/3; this maximum is attained already for a simple network of two parallel links, known as Pigou’s network. We improve upon the value 4/3 by means of Coordination Mechanisms. We increase the latency functions of(More)
We continue the study of the effects of selfish behavior in the network design problem. We provide new bounds for the price of stability for network design with fair cost allocation for undirected graphs. We consider the most general case, for which the best known upper bound is the Harmonic number Hn, where n is the number of agents, and the best(More)
Weconsider theRailwayTravelingSalesmanProblem (RTSP) in which a salesman using the railway network wishes to visit a certain number of cities to carry out his/her business, starting and ending at the same city, and having as goal to minimize the overall time of the journey. RTSP is an NP-hard problem. Although it is related to the Generalized Asymmetric(More)
We consider optimal itinerary problems in time-table information systems supporting a vast number of on-line queries. We exhibit two important extensions of the time-dependent approach to model realistic versions of the Earliest Arrival and Minimum Number of Transfer problems, as well as of a combination of them, that could not be modeled by the original(More)
We consider two approaches that model timetable information in public transportation systems as shortestpath problems in weighted graphs. In the time-expanded approach every event at a station, e.g., the departure of a train, is modeled as a node in the graph, while in the time-dependent approach the graph contains only one node per station. Both approaches(More)
Point samples of a surface in R3 are the dominant output of a multitude of 3D scanning devices. The usefulness of these devices rests on being able to extract properties of the surface from the sample. We show that, under certain sampling conditions, the minimum cycle basis of a nearest neighbor graph of the sample encodes topological information about the(More)
We describe a new technique for proving the existence of small &#956;-nets for hypergraphs satisfying certain simple conditions. The technique is particularly useful for proving <i>o</i>(1/&#956; log 1/&#956;) upper bounds which the standard VC-dimension theory does not allow. We apply the technique to several geometric hypergraphs and obtain simple proofs(More)
In many communications settings, such as wired and wireless local-area networks, when multiple users attempt to access a communication channel at the same time, a conflict results and none of the communications are successful. Contention resolution is the study of distributed transmission and retransmission protocols designed to maximize notions of utility(More)