18,118 research outputs found
Analyses of silicon dioxide, magnesium oxide, lead fluoride, bismuth as low-pass velocity filters for neutrons
Transmission measurement of neutrons by filter materials for low energy neutrons is important for the study of structure and dynamics of condensed matter. Since only thermal neutrons are useful for such experiments, filter materials that transmit thermal neutrons while attenuating fast neutrons and gamma rays are of considerable interest
Perfect Simulation of Queues
In this paper we describe a perfect simulation algorithm for the stable
queue. Sigman (2011: Exact Simulation of the Stationary Distribution of
the FIFO M/G/c Queue. Journal of Applied Probability, 48A, 209--213) showed how
to build a dominated CFTP algorithm for perfect simulation of the super-stable
queue operating under First Come First Served discipline, with
dominating process provided by the corresponding queue (using Wolff's
sample path monotonicity, which applies when service durations are coupled in
order of initiation of service), and exploiting the fact that the workload
process for the queue remains the same under different queueing
disciplines, in particular under the Processor Sharing discipline, for which a
dynamic reversibility property holds. We generalize Sigman's construction to
the stable case by comparing the queue to a copy run under Random
Assignment. This allows us to produce a naive perfect simulation algorithm
based on running the dominating process back to the time it first empties. We
also construct a more efficient algorithm that uses sandwiching by lower and
upper processes constructed as coupled queues started respectively from
the empty state and the state of the queue under Random Assignment. A
careful analysis shows that appropriate ordering relationships can still be
maintained, so long as service durations continue to be coupled in order of
initiation of service. We summarize statistical checks of simulation output,
and demonstrate that the mean run-time is finite so long as the second moment
of the service duration distribution is finite.Comment: 28 pages, 5 figure
Entanglement consumption of instantaneous nonlocal quantum measurements
Relativistic causality has dramatic consequences on the measurability of
nonlocal variables and poses the fundamental question of whether it is
physically meaningful to speak about the value of nonlocal variables at a
particular time. Recent work has shown that by weakening the role of the
measurement in preparing eigenstates of the variable it is in fact possible to
measure all nonlocal observables instantaneously by exploiting entanglement.
However, for these measurement schemes to succeed with certainty an infinite
amount of entanglement must be distributed initially and all this entanglement
is necessarily consumed. In this work we sharpen the characterisation of
instantaneous nonlocal measurements by explicitly devising schemes in which
only a finite amount of the initially distributed entanglement is ever
utilised. This enables us to determine an upper bound to the average
consumption for the most general cases of nonlocal measurements. This includes
the tasks of state verification, where the measurement verifies if the system
is in a given state, and verification measurements of a general set of
eigenstates of an observable. Despite its finiteness the growth of entanglement
consumption is found to display an extremely unfavourable exponential of an
exponential scaling with either the number of qubits needed to contain the
Schmidt rank of the target state or total number of qubits in the system for an
operator measurement. This scaling is seen to be a consequence of the
combination of the generic exponential scaling of unitary decompositions
combined with the highly recursive structure of our scheme required to overcome
the no-signalling constraint of relativistic causality.Comment: 32 pages and 14 figures. Updated to published versio
A RE-EXAMINATION OF EVENT STUDIES APPLIED TO CHALLENGED HORIZONTAL MERGERS
A growing body of empirical studies have been interpreted as support for a laissez-faire policy towards mergers. These "event studies" examine the reaction of stock market prices of firms that announce an agreement to merge. The type ~f reaction reveals whether a merger is motivated by a desire for market power or purely to improve market efficiency. In this paper, a version of the capital asset pricing model (CAPM) is applied to determine if abnormal returns are earned by rivals of 22 pairs of firms whose attempted horizontal mergers were challenged by the federal antitrust agencies. At most eight, and possibly only five, of the cases were found to be motivated by efficiency in seeking merger, and at most six, and possibly only one, were motivated by market power; the rest were inconclusive. The event-study technique is highly flawed for the study of business-regulation effects. Numerous unrealistic assumptions, inappropriate data constraints, and questionable interpretations hamper .the application of this technique to policy analysis.Industrial Organization,
Comparative evaluation of twenty pilot workload assessment measure using a psychomotor task in a moving base aircraft simulator
A comparison of the sensitivity and intrusion of twenty pilot workload assessment techniques was conducted using a psychomotor loading task in a three degree of freedom moving base aircraft simulator. The twenty techniques included opinion measures, spare mental capacity measures, physiological measures, eye behavior measures, and primary task performance measures. The primary task was an instrument landing system (ILS) approach and landing. All measures were recorded between the outer marker and the middle marker on the approach. Three levels (low, medium, and high) of psychomotor load were obtained by the combined manipulation of windgust disturbance level and simulated aircraft pitch stability. Six instrument rated pilots participated in four seasons lasting approximately three hours each
Local growth of icosahedral quasicrystalline tilings
Icosahedral quasicrystals (IQCs) with extremely high degrees of translational
order have been produced in the laboratory and found in naturally occurring
minerals, yet questions remain about how IQCs form. In particular, the
fundamental question of how locally determined additions to a growing cluster
can lead to the intricate long-range correlations in IQCs remains open. In
answer to this question, we have developed an algorithm that is capable of
producing a perfectly ordered IQC, yet relies exclusively on local rules for
sequential, face-to-face addition of tiles to a cluster. When the algorithm is
seeded with a special type of cluster containing a defect, we find that growth
is forced to infinity with high probability and that the resultant IQC has a
vanishing density of defects. The geometric features underlying this algorithm
can inform analyses of experimental systems and numerical models that generate
highly ordered quasicrystals.Comment: 13 pages, 15 figures, 1 tabl
The Web as an Adaptive Network: Coevolution of Web Behavior and Web Structure
Much is known about the complex network structure of the Web, and about behavioral dynamics on the Web. A number of studies address how behaviors on the Web are affected by different network topologies, whilst others address how the behavior of users on the Web alters network topology. These represent complementary directions of influence, but they are generally not combined within any one study. In network science, the study of the coupled interaction between topology and behavior, or state-topology coevolution, is known as 'adaptive networks', and is a rapidly developing area of research. In this paper, we review the case for considering the Web as an adaptive network and several examples of state-topology coevolution on the Web. We also review some abstract results from recent literature in adaptive networks and discuss their implications for Web Science. We conclude that adaptive networks provide a formal framework for characterizing processes acting 'on' and 'of' the Web, and offers potential for identifying general organizing principles that seem otherwise illusive in Web Scienc
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