Probabilistic alternatives for competitive analysis

In the last 20 years competitive analysis has become the main tool for analyzing the quality of online algorithms. Despite of this, competitive analysis has also been criticized: it sometimes cannot discriminate between algorithms that exhibit significantly different empirical behavior or it even favors an algorithm that is worse from an empirical point of view. Therefore, there have been several approaches to circumvent these drawbacks. In this survey, we discuss probabilistic alternatives for competitive analysis.operations research and management science;

Probabilistic analysis of Online Bin Coloring algorithms via Stochastic Comparison

This paper proposes a new method for probabilistic analysis of online algorithms that is based on the notion of stochastic dominance. We develop the method for the Online Bin Coloring problem introduced by Krumke et al. Using methods for the stochastic comparison of Markov chains we establish the strong result that the performance of the online algorithm GreedyFit is stochastically dominated by the performance of the algorithm OneBin for any number of items processed. This result gives a more realistic picture than competitive analysis and explains the behavior observed in simulations.mathematical applications;

Simple optimality proofs for Least Recently Used in the presence of locality of reference

It is well known that competitive analysis yields results that do not reflect the observed performance of online paging algorithms. Many deterministic paging algorithms achieve the same competitive ratio, ranging from inefficient strategies as flush-when-full to the well-performing least-recently-used (LRU). In this paper, we study this fundamental online problem from the viewpoint of stochastic dominance. We give simple proofs that whensequences are drawn from distributions modelling locality of reference, LRU stochastically dominates any other online paging algorithm. As a byproduct, we obtain simple proofs of some earlier results.operations research and management science;

Constructing Internet access: The emergence and use of ICTs in new social spaces

This thesis examines the introduction of new Information and Communication Technologies (ICTs) into new social spaces. These new social spaces have been called the 'middle ground' (MG). They are found not in the workplace nor in the home but are located elsewhere in community centres, libraries and Internet cafes. The MG is used for education, communication and information retrieval. This thesis draws upon two case studies to explore the institutional, sociological and cultural development and use of such MG spaces. Theoretically, this thesis draws upon the Sociology of Science & Technology, specifically Actor Network Theory (ANT). According to ANT, to understand just how a technology becomes (or fails to become) a success we must follow and observe various innovators as they attempt to enrol others into their 'networks'. The utility of ANT is twofold. Firstly, it enables me to study 'innovation' and secondly to move beyond overarching general notions (such as technological determinism) to understand the complex ways new ICTs are being utilised in the 'middle ground'. The first case study examines the development of a 'virtual university' project 'Televersity'. Here I examine the relationship that develops between the key actors involved with the project, the strategic co-operation between the actors and the use of representations of the Local Learning Centres (LLCs) and identity of the users. The second case study examines the establishment and use of an Internet cafe. I explore the representational resources that the founders utilise in their attempts to stabilise the identity of the Internet cafe and its users. I then proceed to compare these with accounts of use, derived through a series of interviews with (and observations of) the users. The thesis thus aims to illuminate the heterogeneous and complex nature of the MG and the cooperation and flexibility needed at a policy level for such places to be sustained. Furthermore, by using an approach like ANT, we can begin to understand the micro-level shaping of the MG and the relation between these local processes and the wider dynamics within which they are embedded

Improving the perfomance of elevator systems using exact reoptimization algorithms

The task of an elevator control is to schedule the elevators of a group such that small average and maximal waiting and travel times for the passengers are obtained. We present a novel exact reoptimization algorithm for this problem. A reoptimization algorithm computes a new optimal schedule for the elevator group each time a new passenger arrives. Our algorithm uses column generation techniques and is, to the best of our knowledge, the first exact reoptimization algorithm for a group of elevators. We use our algorithm to compare the potential performance that can be achieved for conventional (ie up/down buttons) and two variants of destination call systems, where a passenger enters his destination floor when calling an elevator. This research is part of an ongoing project with our industry partner Kollmorgen Steuerungstechnik

Optimal bounds for a colorful Tverberg--Vrecica type problem

We prove the following optimal colorful Tverberg-Vrecica type transversal theorem: For prime r and for any k+1 colored collections of points C^l of size |C^l|=(r-1)(d-k+1)+1 in R^d, where each C^l is a union of subsets (color classes) C_i^l of size smaller than r, l=0,...,k, there are partition of the collections C^l into colorful sets F_1^l,...,F_r^l such that there is a k-plane that meets all the convex hulls conv(F_j^l), under the assumption that r(d-k) is even or k=0. Along the proof we obtain three results of independent interest: We present two alternative proofs for the special case k=0 (our optimal colored Tverberg theorem (2009)), calculate the cohomological index for joins of chessboard complexes, and establish a new Borsuk-Ulam type theorem for (Z_p)^m-equivariant bundles that generalizes results of Volovikov (1996) and Zivaljevic (1999).Comment: Substantially revised version: new notation, improved results, additional references; 12 pages, 2 figure

Predictive Trajectory Control with Online MTPA Calculation and Minimization of the Inner Torque Ripple for Permanent-Magnet Synchronous Machines

This paper presents an extended predictive trajectory control scheme combined with an inner torque ripple minimization considering the current-, flux-linkage-, and voltage-planes of permanent magnet synchronous machines. The extension of a fundamental machine model with flux-linkage harmonics allows the calculation of the inner torque ripple and enables its minimization. For this, the control is divided in two cases: (1) The dynamic operation or large signal behavior which uses the maximal torque gradient for the trajectory strategy during each control period for fastest dynamic operation, and (2) The stationary operation or small signal behavior, utilizing a real time capable polynomial approximation of the rotor position dependent torque hyperbolas (iso-torque curves) of permanent magnet synchronous machines for the ideal torque to current reference values. Since dynamic and steady-state operation is covered, torque to current look-up tables, such as maximum torque per ampere (MTPA)/maximum torque per volt/voltage (MTPV) look-up tables, are not required anymore. The introduced, new control approach is implemented in Matlab/Simulink based on finite element analysis and measured data. Furthermore, test-bench implementations based on measurement data are presented to show the real-time capability and precision

Fast Mapping of Terahertz Bursting Thresholds and Characteristics at Synchrotron Light Sources

Dedicated optics with extremely short electron bunches enable synchrotron light sources to generate intense coherent THz radiation. The high degree of spatial compression in this so-called low-alpha optics entails a complex longitudinal dynamics of the electron bunches, which can be probed studying the fluctuations in the emitted terahertz radiation caused by the micro-bunching instability ("bursting"). This article presents a "quasi-instantaneous" method for measuring the bursting characteristics by simultaneously collecting and evaluating the information from all bunches in a multi-bunch fill, reducing the measurement time from hours to seconds. This speed-up allows systematic studies of the bursting characteristics for various accelerator settings within a single fill of the machine, enabling a comprehensive comparison of the measured bursting thresholds with theoretical predictions by the bunched-beam theory. This paper introduces the method and presents first results obtained at the ANKA synchrotron radiation facility.Comment: 7 pages, 7 figures, to be published in Physical Review Accelerators and Beam

Simultaneous Detection of Longitudinal and Transverse Bunch Signals at a Storage Ring

To understand and control the dynamics in the longitudinal phase space, time-resolved measurements of different bunch parameters are required. For a reconstruction of this phase space, the detector systems have to be synchronized. This reconstruction can be used e.g. for studies of the micro-bunching instability. It occurs if the interaction of the bunch with its own radiation leads to the formation of sub-structures on the longitudinal bunch profile. These sub-structures can grow rapidly -- leading to a sawtooth-like behaviour of the bunch. At KARA, we use a fast-gated intensified camera for energy spread studies, Schottky diodes for coherent synchrotron radiation studies as well as electro-optical spectral decoding for longitudinal bunch profile measurements. For a synchronization, a hardware synchronization scheme is used which compensates for eventual hardware delays. In this paper, the different experimental setups and their synchronization are discussed and first results of synchronous measurements are presented