How to calculate symmetries of Petri nets

Symmetric net structure yields symmetric net behaviour. Thus, knowing the symmetries of a net, redundant calculations can be skipped. We present a framework for the calculation of symmetries for several net classes including place/transition nets, timed nets, stochastic nets, self–modifying nets, nets with inhibitor arcs, and many others. Our approach allows the specification of different symmetry groups. Additionally it provides facilities either to calculate symmetries on demand while running the actual analysis algorithm, or to calculate them in advance. For the latter case we define and calculate a ground set of symmetries. Such a set has polynomial size and is sufficient for an efficient implementation of the for all symmetries loop and the partition of net elements into equivalence classes. These two constructions are the usual way to integrate symmetries into an analysis algorithm. Received 7 July 1997 / 10 August 1999     

Advanced networking services for distributed multimedia streaming applications

Increased network speeds coupled with new services delivered via the Internet have increased the demand for intelligence and flexibility in network systems. This paper argues that both can be provided by new hardware platforms comprised of heterogeneous multi-core systems with specialized communication support. We present and evaluate an experimental network service platform that uses an emergent class of devices—network processors—as its communication support, coupled via a dedicated interconnect to a host processor acting as a computational core. A software infrastructure spanning both enables the dynamic creation of application-specific services on the network processor, mediated by middleware and controlled by kernel-level communication support. Experimental evaluations use a Pentium IV-based computational core coupled with an IXP 2400 network processor. The sample application services run on both include an image manipulation application and application-level multicasting.

Dynamic Programming for Minimum Steiner Trees

We present a new dynamic programming algorithm that solves the minimum Steiner tree problem on graphs with k terminals in time O^*(c^k) for any c > 2. This improves the running time of the previously fastest parameterized algorithm by Dreyfus-Wagner of order O^*(3^k) and the so-called "full set dynamic programming" algorithm solving rectilinear instances in time O^*(2.38^k).     

Sharp bends with low losses in dielectric optical waveguides

A new method is proposed for making sharp bends with low radiation losses in dielectric optical waveguides. By modifying the transverse refractive-index profile at curved sections both the pure bend and the transition losses can be minimized. The optimum gradient-index profile requires an inhomogeneous medium. But in practice this can be replaced by a layered medium. By using four homogeneous layers the permitted radius of curvature of a slab waveguide can be reduced, e.g., from 6400 to only 100 wavelengths.     

Spherical sample holders to improve the susceptibility measurement of superparamagnetic materials

The design of two custom sample holders with a spherical cavity for commercial vibrating sample magnetometer systems is described. For such cavities, the magnetization M[over ->] and the internal magnetic field H(i)[over ->] of a sample are both homogeneous. Consequently, the material parameter M(H(i)) of a sample can be determined even for liquids and powders with a high magnetic susceptibility.     

On the Hitting Times of Quantum Versus Random Walks

The hitting time of a classical random walk (Markov chain) is the time required to detect the presence of—or equivalently, to find—a marked state. The hitting time of a quantum walk is subtler to define; in particular, it is unknown whether the detection and finding problems have the same time complexity. In this paper we define new Monte Carlo type classical and quantum hitting times, and we prove several relationships among these and the already existing Las Vegas type definitions. In particular, we show that for some marked state the two types of hitting time are of the same order in both the classical and the quantum case. Then, we present new quantum algorithms for the detection and finding problems. The complexities of both algorithms are related to the new, potentially smaller, quantum hitting times. The detection algorithm is based on phase estimation and is particularly simple. The finding algorithm combines a similar phase estimation based procedure with ideas of Tulsi from his recent theorem (Tulsi A.: Phys. Rev. A 78:012310 2008) for the 2D grid. Extending his result, we show that we can find a unique marked element with constant probability and with the same complexity as detection for a large class of quantum walks—the quantum analogue of state-transitive reversible ergodic Markov chains. Further, we prove that for any reversible ergodic Markov chain P, the quantum hitting time of the quantum analogue of P has the same order as the square root of the classical hitting time of P. We also investigate the (im)possibility of achieving a gap greater than quadratic using an alternative quantum walk. In doing so, we define a notion of reversibility for a broad class of quantum walks and show how to derive from any such quantum walk a classical analogue. For the special case of quantum walks built on reflections, we show that the hitting time of the classical analogue is exactly the square of the quantum walk.     

Exploring Appropriation of Enterprise Wikis:

The purpose of this paper is to provide both application-oriented researchers and practitioners with detailed insights into conception, implementation, and utilization of intra-organizational wikis to support knowledge management and group work. Firstly, we report on three case studies and describe how wikis have been appropriated in the context of a concrete practice. Our study reveals that the wikis have been used as Knowledge Base, Encyclopedia and Support Base, respectively. We present the identified practices as a result of the wiki appropriation process and argue that due to their open and flexible nature these wikis have been appropriated according to the users’ needs. Our contribution helps to understand how platforms support working practices that have not been supported by groupware before, or at least not in the same way. Secondly, three detailed implementation reports uncover many aspects of wiki projects, e.g., different viewpoints of managers and users, an investigation of other sources containing business-relevant information, and perceived obstacles to wiki projects. In this context, our study generates a series of lessons learned for people who intend to implement wikis in their own organizations, including the awareness of usage potential, the need for additional managerial support, and clear communication strategies to promote wiki usage.     

Novel 3D manufacturing method combining microelectrial discharge machining and electrochemical polishing

This paper reports on the potential of microelectrical discharge machining (μEDM) as an innovative method for the fabrication of 3D microdevices. To demonstrate the wide capabilities of μEDM two different microsystems—a microfluidic device for the dispersion of nanoparticles and a star probe for microcoordinate metrology—are presented. To gain optimized process conditions as well as a high surface quality an adequate adaption of the single erosion parameters such as energy, pulse frequency and spark gap has to be carried out and is discussed below. Thus, a surface roughness of R_a?=?80?nm is achieved at the channel bottom. The fabricated stylus elements for the star probe have sphere diameters of 40–200?μm. For further surface quality enhancement a subsequent electrochemical polishing step is investigated. In case of the dispersion micromodule a combined process chain of μEDM-milling and electropolishing has reached a surface improvement above 70%.