2,815 research outputs found

    Discovering predictive variables when evolving cognitive models

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    A non-dominated sorting genetic algorithm is used to evolve models of learning from different theories for multiple tasks. Correlation analysis is performed to identify parameters which affect performance on specific tasks; these are the predictive variables. Mutation is biased so that changes to parameter values tend to preserve values within the population's current range. Experimental results show that optimal models are evolved, and also that uncovering predictive variables is beneficial in improving the rate of convergence

    The scope for joint household/commercial waste collections: a case study

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    Although commercial and household wastes are compositionally similar, common UK practice is for separate collections. This paper uses vehicle routing and scheduling software to predict the benefits of allowing household and commercial wastes to be collected together by a common vehicle fleet. This was compared in a case study in which collections were made from over 25,000 households on an alternate weekly basis and from 577 commercial premises having one or more collections each week. Modelled joint collections reduced vehicle mileage by up to 9.8%, equating to an annual saving of around £36,800 and a carbon equivalent saving of 2688 kg per annum. The modelled benefits were greatest when a common starting time (6 a.m.)was adopted for the commercial and household collections. The modelled rounds were estimated to have sufficient time and vehicle capacity available to allow an additional 50% of commercial waste to be collected, equating to 35.8 tonnes per week

    Asymptotics of self-similar solutions to coagulation equations with product kernel

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    We consider mass-conserving self-similar solutions for Smoluchowski's coagulation equation with kernel K(ξ,η)=(ξη)λK(\xi,\eta)= (\xi \eta)^{\lambda} with λ(0,1/2)\lambda \in (0,1/2). It is known that such self-similar solutions g(x)g(x) satisfy that x1+2λg(x)x^{-1+2\lambda} g(x) is bounded above and below as x0x \to 0. In this paper we describe in detail via formal asymptotics the qualitative behavior of a suitably rescaled function h(x)=hλx1+2λg(x)h(x)=h_{\lambda} x^{-1+2\lambda} g(x) in the limit λ0\lambda \to 0. It turns out that h1+Cxλ/2cos(λlogx)h \sim 1+ C x^{\lambda/2} \cos(\sqrt{\lambda} \log x) as x0x \to 0. As xx becomes larger hh develops peaks of height 1/λ1/\lambda that are separated by large regions where hh is small. Finally, hh converges to zero exponentially fast as xx \to \infty. Our analysis is based on different approximations of a nonlocal operator, that reduces the original equation in certain regimes to a system of ODE

    Structural defects induced by Fe-ion implantation in TiO2

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    X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission spectroscopy (RXES) measurements of pellet and thin film forms of TiO2_2 with implanted Fe ions are presented and discussed. The findings indicate that Fe-implantation in a TiO2_2 pellet sample induces heterovalent cation substitution (Fe2+^{2+}\rightarrow Ti4+^{4+}) beneath the surface region. But in thin film samples, the clustering of Fe atoms is primarily detected. In addition to this, significant amounts of secondary phases of Fe3+^{3+} are detected on the surface of all doped samples due to oxygen exposure. These experimental findings are compared with density functional theory (DFT) calculations of formation energies for different configurations of structural defects in the implanted TiO2_2:Fe system. According to our calculations, the clustering of Fe-atoms in TiO2_2:Fe thin films can be attributed to the formation of combined substitutional and interstitial defects. Further, the differences due to Fe doping in pellet and thin film samples can ultimately be attributed to different surface to volume ratios.Comment: 7+ pages, 3 Figure, to appear in J. Appl. Phy

    Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material

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    Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. In such materials light propagation is unusual, leading to novel and often non-intuitive optical phenomena. Here we report infrared nano-imaging experiments demonstrating that crystals of hexagonal boron nitride (hBN), a natural mid-infrared hyperbolic material, can act as a "hyper-focusing lens" and as a multi-mode waveguide. The lensing is manifested by subdiffractional focusing of phonon-polaritons launched by metallic disks underneath the hBN crystal. The waveguiding is revealed through the modal analysis of the periodic patterns observed around such launchers and near the sample edges. Our work opens new opportunities for anisotropic layered insulators in infrared nanophotonics complementing and potentially surpassing concurrent artificial hyperbolic materials with lower losses and higher optical localization.Comment: 25 pages, 5 figure

    Wnt Signaling Through Nitric Oxide Synthase Promotes the Formation of Multi-Innervated Spines

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    Structural plasticity of synapses correlates with changes in synaptic strength. Dynamic modifications in dendritic spine number and size are crucial for long-term potentiation (LTP), the cellular correlate of learning and memory. Recent studies have suggested the generation of multi-innervated spines (MIS), in the form of several excitatory presynaptic inputs onto one spine, are crucial for hippocampal memory storage. However, little is known about the molecular mechanisms underlying MIS formation and their contribution to LTP. Using 3D enhanced resolution confocal images, we examined the contribution of Wnt synaptic modulators in MIS formation in the context of LTP. We show that blockage of endogenous Wnts with specific Wnt antagonists supresses the formation of MIS upon chemical LTP induction in cultured hippocampal neurons. Gain- and loss-of-function studies demonstrate that Wnt7a signaling promotes MIS formation through the postsynaptic Wnt scaffold protein Disheveled 1 (Dvl1) by stimulating neuronal nitric oxide (NO) synthase (nNOS). Subsequently, NO activates soluble guanylyl cyclase (sGC) to increase MIS formation. Consistently, we observed an enhanced frequency and amplitude of excitatory postsynaptic currents. Collectively, our findings identify a unique role for Wnt secreted proteins through nNOS/NO/sGC signaling to modulate MIS formation during LTP

    The extinction law in high redshift galaxies

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    We estimate the dust extinction laws in two intermediate redshift galaxies. The dust in the lens galaxy of LBQS1009-0252, which has an estimated lens redshift of zl~0.88, appears to be similar to that of the SMC with no significant feature at 2175 A. Only if the lens galaxy is at a redshift of zl~0.3, completely inconsistent with the galaxy colors, luminosity or location on the fundamental plane, can the data be fit with a normal Galactic extinction curve. The dust in the zl=0.68 lens galaxy for B0218+357, whose reddened image lies behind a molecular cloud, requires a very flat ultraviolet extinction curve with (formally) R(V)=12 +- 2. Both lens systems seem to have unusual extinction curves by Galactic standards.Comment: 15 pages, 3 figures. ApJ in pres

    Aberration-free ultra-thin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces

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    The concept of optical phase discontinuities is applied to the design and demonstration of aberration-free planar lenses and axicons, comprising a phased array of ultrathin subwavelength spaced optical antennas. The lenses and axicons consist of radial distributions of V-shaped nanoantennas that generate respectively spherical wavefronts and non-diffracting Bessel beams at telecom wavelengths. Simulations are also presented to show that our aberration-free designs are applicable to high numerical aperture lenses such as flat microscope objectives
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