72,220 research outputs found

    L2L_2 boosting in kernel regression

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    In this paper, we investigate the theoretical and empirical properties of L2L_2 boosting with kernel regression estimates as weak learners. We show that each step of L2L_2 boosting reduces the bias of the estimate by two orders of magnitude, while it does not deteriorate the order of the variance. We illustrate the theoretical findings by some simulated examples. Also, we demonstrate that L2L_2 boosting is superior to the use of higher-order kernels, which is a well-known method of reducing the bias of the kernel estimate.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ160 the Bernoulli (http://isi.cbs.nl/bernoulli/) by the International Statistical Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm

    Relativistic r-modes in Slowly Rotating Neutron Stars: Numerical Analysis in the Cowling Approximation

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    We investigate the properties of relativistic rr-modes of slowly rotating neutron stars by using a relativistic version of the Cowling approximation. In our formalism, we take into account the influence of the Coriolis like force on the stellar oscillations, but ignore the effects of the centrifugal like force. For three neutron star models, we calculated the fundamental rr-modes with l′=m=2l'=m=2 and 3. We found that the oscillation frequency σˉ\bar\sigma of the fundamental rr-mode is in a good approximation given by σˉ≈κ0Ω\bar\sigma\approx \kappa_0 \Omega, where σˉ\bar\sigma is defined in the corotating frame at the spatial infinity, and Ω\Omega is the angular frequency of rotation of the star. The proportional coefficient κ0\kappa_0 is only weakly dependent on Ω\Omega, but it strongly depends on the relativistic parameter GM/c2RGM/c^2R, where MM and RR are the mass and the radius of the star. All the fundamental rr-modes with l′=ml'=m computed in this study are discrete modes with distinct regular eigenfunctions, and they all fall in the continuous part of the frequency spectrum associated with Kojima's equation (Kojima 1998). These relativistic rr-modes are obtained by including the effects of rotation higher than the first order of Ω\Omega so that the buoyant force plays a role, the situation of which is quite similar to that for the Newtonian rr-modes.Comment: 22 pages, 8 figures, accepted for publication in Ap

    R-Modes on Rapidly Rotating, Relativistic Stars: I. Do Type-I Bursts Excite Modes in the Neutron-Star Ocean?

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    During a Type-I burst, the turbulent deflagation front may excite waves in the neutron star ocean and upper atmosphere with frequencies, ω∼1\omega \sim 1 Hz. These waves may be observed as highly coherent flux oscillations during the burst. The frequencies of these waves changes as the upper layers of the neutron star cool which accounts for the small variation in the observed QPO frequencies. In principle several modes could be excited but the fundamental buoyant r−r-mode exhibits significantly larger variability for a given excitation than all of the other modes. An analysis of modes in the burning layers themselves and the underlying ocean shows that it is unlikely these modes can account for the observed burst oscillations. On the other hand, photospheric modes which reside in a cooler portion of the neutron star atmosphere may provide an excellent explanation for the observed oscillations.Comment: 18 pages, 1 figure, substantial changes and additions to reflect version to appear in Ap

    Effect of Plasma Irradiation on CdI2Cd I_2 films

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    The effect of plasma irradiation is studied systematically on a 4H polytype (002) oriented CdI2{\rm CdI_2} stoichiometric film having compressive residual stress. Plasma irradiation was found to change the orientation to (110) of the film at certain moderate irradiation distances. A linear decrease in grain size and residual stress was observed with decreasing irradiation distance (or increasing ion energy) consistent with both structural and morphological observations. The direct optical energy gap Eg{\rm E_g} was found to increase linearly at the rate 15μeV/atm{\rm 15\mu eV/atm} with the compressive stress. The combined data of present compressive stress and from earlier reported tensile stress show a consistent trend of Eg{\rm E_g} change with stress. The iodine-iodine distance in the unit cell could be responsible for the observed change in Eg{\rm E_g} with stress.Comment: 13 pages and 10 fi

    Vacuum as a less hostile environment to entanglement

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    We derive sufficient conditions for infinite-dimensional systems whose entanglement is not completely lost in a finite time during its decoherence by a passive interaction with local vacuum environments. The sufficient conditions allow us to clarify a class of bipartite entangled states which preserve their entanglement or, in other words, are tolerant against decoherence in a vacuum. We also discuss such a class for entangled qubits.Comment: Replaced by the published versio

    Quantum Phase Transitions of Hard-Core Bosons in Background Potentials

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    We study the zero temperature phase diagram of hard core bosons in two dimensions subjected to three types of background potentials: staggered, uniform, and random. In all three cases there is a quantum phase transition from a superfluid (at small potential) to a normal phase (at large potential), but with different universality classes. As expected, the staggered case belongs to the XY universality, while the uniform potential induces a mean field transition. The disorder driven transition is clearly different from both; in particular, we find z~1.4, \nu~1, and \beta~0.6.Comment: 4 pages (6 figures); published version-- 2 references added, minor clarification
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