Yvonne Anne Oswald

Complexity in geometric SINR

In this paper we study the problem of scheduling wireless links in the geometric SINR model, which explicitly uses the fact that nodes are distributed in the Euclidean plane. We present the first NP-completeness proofs in such a model. In particular, we prove two problems to be NP-complete: Scheduling and One-Shot Scheduling. The first problem consists in finding a minimum-length schedule for a given set of links. The second problem receives a weighted set of links as input and consists in finding a maximum-weight subset of links to be scheduled simultaneously in one shot. In addition to the complexity proofs, we devise an approximation algorithm for each problem.

Word of Mouth: Rumor Dissemination in Social Networks

In this paper we examine the diffusion of competing rumors in social networks. Two players select a disjoint subset of nodes as initiators of the rumor propagation, seeking to maximize the number of persuaded nodes. We use concepts of game theory and location theory and model the selection of starting nodes for the rumors as a strategic game. We show that computing the optimal strategy for both the first and the second player is NP-complete, even in a most restricted model. Moreover we prove that determining an approximate solution for the first player is NP-complete as well. We analyze several heuristics and show that--counter-intuitively--being the first to decide is not always an advantage, namely there exist networks where the second player can convince more nodes than the first, regardless of the first players decision.

How Optimal are Wireless Scheduling Protocols

In wireless networks mutual interference impairs the quality of received signals and might even prevent the correct reception of messages. It is therefore of paramount importance to dispose of power control and scheduling algorithms, coordinating the transmission of communication requests. We propose a new measure disturbance in order to comprise the intrinsic difficulty of finding a short schedule for a problem instance. Previously known approaches suffer from extremely bad performance in certain network scenarios even if disturbance is low. To overcome this problem, we present a novel scheduling algorithm for which we give analytical worst-case guarantees on its performance. Compared to previously known solutions, the algorithm achieves a speed up, which can be exponential in the size of the network.

On the windfall of friendship: inoculation strategies on social networks

This paper studies a virus inoculation game on social networks. A framework is presented which allows the measuring of the windfall of friendship, i.e., how much players benefit if they care about the welfare of their direct neighbors in the social network graph compared to purely selfish environments. We analyze the corresponding equilibria and show that the computation of the worst and best Nash equilibrium is NP-hard. Intriguingly, even though the windfall of friendship can never be negative, the social welfare does not increase monotonically with the extent to which players care for each other. While these phenomena are known on an anecdotal level, our framework allows us to quantify these effects analytically.

What can be approximated locally?: case study: dominating sets in planar graphs

Whether local algorithms can compute constant approximations of NP-hard problems is of both practical and theoretical interest. So far, no algorithms achieving this goal are known, as either the approximation ratio or the running time exceed O(1), or the nodes are provided with non-trivial additional information. In this paper, we present the first distributed algorithm approximating a minimum dominating set on a planar graph within a constant factor in constant time. Moreover, the nodes do not need any additional information.

Tight bounds for delay-sensitive aggregation

This paper studies the fundamental trade-off between communication cost and delay cost arising in various contexts such as control message aggregation or organization theory. An optimization problem is considered where nodes are organized in a tree topology. The nodes seek to minimize the time until the root is informed about their states and to use as few transmissions as possible at the same time. We derive an upper bound on the competitive ratio of O(min(h,c)) where h is the trees height, and c is the transmission cost per edge. Moreover, we prove that this upper bound is tight in the sense that any oblivious algorithm has a ratio of at least Omega(min(h,c)). For chain networks, we prove a tight competitive ratio of Theta(min(sqrt{h},c)). Furthermore, the paper introduces a new model for online event aggregation where the importance of an event depends on its difference to previous events.

Optimization of the MOVA undeniable signature scheme

This article presents optimization results on the MOVA undeniable signature scheme presented last year by Monnerat and Vaudenay at PKC ’04 as well as its generalization proposed at Asiacrypt ’04 which is based on a secret group homomorphism. The original MOVA scheme uses characters on

Mechanism design by creditability

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Luby-Rackoff ciphers from weak round functions?

and some additional candidate homomorphisms were proposed with its generalization. We give an overview of the expected performance of the MOVA scheme depending on the group homomorphism. Our optimizations focus on the quartic residue symbol and a homomorphism based on the computation of a discrete logarithm in a hidden subgroup of

Manipulation in games

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