Exploring eternal stability with the simple harmonic universe

We construct nonsingular cyclic cosmologies that respect the null energy condition, have a large hierarchy between the minimum and maximum size of the universe, and are stable under linearized fluctuations. The models are supported by a combination of positive curvature, a negative cosmological constant, cosmic strings and matter that at the homogeneous level behaves as a perfect fluid with equation of state -1 < w < -1/3. We investigate analytically the stability of the perturbation equations and discuss the role of parametric resonances and nonlinear corrections. Finally, we argue that Casimir energy contributions associated to the compact spatial slices can become important at short scales and lift nonperturbative decays towards vanishing size. This class of models (particularly in the static limit) can then provide a useful framework for studying the question of the ultimate (meta)stability of an eternal universe.Comment: 22 pages, 2 figure

Enhanced quality factors and force sensitivity by attaching magnetic beads to cantilevers for atomic force microscopy in liquid

Dynamic-mode atomic force microscopy (AFM) in liquid remains complicated due to the strong viscous damping of the cantilever resonance. Here we show that a high-quality resonance (Q>20) can be achieved in aqueous solution by attaching a microgram-bead at the end of the nanogram-cantilever. The resulting increase in cantilever mass causes the resonance frequency to drop significantly. However, the force sensitivity --- as expressed via the minimum detectable force gradient --- is hardly affected, because of the enhanced quality factor. Via the enhancement of the quality factor, the attached bead also reduces the relative importance of noise in the deflection detector. It can thus yield an improved signal-to-noise ratio when this detector noise is significant. We describe and analyze these effects for a set-up which includes magnetic actuation of the cantilevers and which can be easily implemented in any AFM system that is compatible with an inverted optical microscope.Comment: The following article has been accepted by Journal of Applied Physics. After it is published, it will be found at http://jap.aip.org

Shift rostering using decomposition: assign weekend shifts first

This paper introduces a shift rostering problem that surprisingly has not been studied in literature: the weekend shift rostering problem. It is motivated by our experience that employees’ shift preferences predominantly focus on the weekends, since many social activities happen during weekends. The Weekend Rostering Problem (WRP) addresses the rostering of weekend shifts, for which we design a problem specific heuristic. We consider the WRP as the first phase of the shift rostering problem. To complete the shift roster, the second phase assigns the weekday shifts using an existing algorithm. We discuss effects of this two-phase approach both on the weekend shift roster and on the roster as a whole. We demonstrate that our first-phase heuristic is effective both on generated instances and real-life instances. For situations where the weekend shift roster is one of the key determinants of the quality of the complete roster, our two-phase approach shows to be effective when incorporated in a commercially implemented algorithm

A Review Of An Optimal Public Transport Service Network In The Randstad Area: Is The Randstad Area In Control?

The general belief in the Netherlands is that the public transport in the western urbanized part of the Netherlands (the Randstad area) with the main cities of Amsterdam, Rotterdam, The Hague and Utrecht can perform (much) better. The central research question of the article is: How does the optimal public transport service network in the Randstad area look like and how can actors use their responsibilities to contribute? The answer to the first part of the research question is ‘we don not know yet’. There is no clear picture of current infrastructure networks in the Randstad area and of the desired or actual quality levels of public transport in the area. The most important issues that are under control of the respective actors and can be improved in the short term are: coherence in spatial planning and investments in public transport infrastructure, public transport service coherence in the Randstad area, the quality of public transport services, and concessions. The most important issues that are not under control of the respective actors are: reliable relationships with private businesses in combined developments of public transport infrastructure and offices, and the national public rail transport services. The involved actors might focus on the respective issues they are able to (partly) control and see what their contribution could be for a PTSN for the Randstad area. For the longer term, an integrated picture of the current and 2050 public transport infrastructure network for the Randstad area, an integrated picture of the current and 2050 public transport services for the Randstad area and clarity about the degree of control of the different involved public actors and the role of the public transport authorities is needed. Overall, change the focus from problems to control issues and then start working on solutions for the short and longer term.Institute of Transport and Logistics Studies. Faculty of Economics and Business. The University of Sydne

Multi-unit Bilateral Trade

We characterise the set of dominant strategy incentive compatible (DSIC), strongly budget balanced (SBB), and ex-post individually rational (IR) mechanisms for the multi-unit bilateral trade setting. In such a setting there is a single buyer and a single seller who holds a finite number k of identical items. The mechanism has to decide how many units of the item are transferred from the seller to the buyer and how much money is transferred from the buyer to the seller. We consider two classes of valuation functions for the buyer and seller: Valuations that are increasing in the number of units in possession, and the more specific class of valuations that are increasing and submodular. Furthermore, we present some approximation results about the performance of certain such mechanisms, in terms of social welfare: For increasing submodular valuation functions, we show the existence of a deterministic 2-approximation mechanism and a randomised e/(1-e) approximation mechanism, matching the best known bounds for the single-item setting

The spin-torque transistor

A magnetoelectronic thin-film transistor is proposed that can display negative differential resistance and gain. The working principle is the modulation of the soure-drain current in a spin valve by the magnetization of a third electrode, which is rotated by the spin-torque created by a control spin-valve. The device can operate at room temperature, but in order to be useful, ferromagnetic materials with polarizations close to unity are required.Comment: to be published in Applied Physics Letter