On Approximating a Scheduling Problem

Given a set of communication tasks (best described in terms of a weighted bipartite graph where one set of nodes denotes the senders, the other set the receivers, edges are communication tasks, and the weight of an edge is the time required for transmission), we wish to minimize the total time required for the completion of all communication tasks assuming that tasks can be preempted (that is, each edge can be subdivided into many edges with weights adding up to the edge's original weight) and that preemption comes with a cost. In this paper, we first prove that one cannot approximate this problem within a factor smaller than unless P=NP. It is known that a simple approximation algorithm achieves within a ratio of two (H. Choi and S.L. Hakimi, Algorithmica, vol. 3, pp. 223–245, 1988). However, our experimental results show that its performance is worse than the originally proposed heuristic algorithm (I.S. Gopal and C.K. Wong, IEEE Transactions on Communications, vol. 33, pp. 497–501, 1985). We devise a more sophisticated algorithm, called the potential function algorithm which, on the one hand, achieves a provable approximation ratio of two, and on the other hand, shows very good experimental performance. Moreover, the way in which our more sophisticated algorithm derives from the simple one, suggests a hierarchy of algorithms, all of which have a worst-case performance at most two, but which we suspect to have increasingly better performance, both in worst case and with actual instances.     

Becoming en‐wheeled: the situated accomplishment of re‐embodiment as a wheelchair user after spinal cord injury

This paper discusses the creative process of re‐embodiment experienced by physically disabled adults who become wheelchair users. Interviews and observational data of adults (rehabilitation patients and persons living in the community) who use wheelchairs show how they redefine, re‐examine or modify past experiences, abilities, lifestyles and habits in their efforts towards re‐embodiment. The aim of this paper is to document the process of learning to use a wheelchair and making it a part of one's embodied existence. The paper shows that this process involves the negotiation of past and new habits, abilities and ways of doing. It argues against conceptualizing disability as an all encompassing state of being. Rather, the competence and abilities required to achieve wheelchair embodiment are analyzed as a situated accomplishment with social and political consequences.     

Preliminary Evidence on Wage Setting in Greek Manufacturing

Abstract. The negotiating power of the unions and their role in wage determination depends on the level of centralization of the wage bargaining system as well as on other institutional arrangements. In this study, we estimate a set of wage equations in order to study the employment effects of different unions’ stances in wage determination as well as to investigate other characteristics of employment in Greek manufacturing. We discriminate between workers and employees (blue‐ and white‐collar workers) and between sectors in order to capture differences in the relative supply as well as differences in the negotiating power among the staff of the same industry. Empirical findings reveal that the persistence of unemployment arises as a result of the great bargaining power of the unions in negotiations at a sectoral level. White‐collar workers exhibit a stronger negotiating power compared with blue‐collar workers because of differences in the institutional framework.     

Estimating the earthquake occurrence rates in Corinth Gulf (Greece) through Markovian arrival process modeling

The Markovian Arrival Process (MAP) is applied as a candidate model to describe the time-varying earthquake activity in Corinth Gulf, Greece. To the best of our knowledge, this is the first attempt to study the earthquake temporal evolution with the specific class of MAPs. A complete catalogue is used for the earthquake temporal distribution investigation, along with data sets of different magnitude cutoffs. The study area is divided into its western and eastern subareas, and possible variations in the earthquake occurrence times were sought. Hidden states of MAPs correspond to different levels of seismicity, and hence various numbers of states are examined. Akaike and Bayes information criteria are implemented for identifying the best model, and comparison to the most known and broadly accepted theoretical interevent time distributions is provided. In all cases, the fitted MAPs with phase type distributed intearrival times outperform the models with other distributions. Important indicators of the underlying Markov process are computed, and the earthquake frequency is approximated by the counting process. The analysis demonstrates high index of burstiness for the earthquake generation in the eastern part, i.e. long quiescent periods alternate with short ones of intense seismic activity.