5,767 research outputs found

    Submodular Welfare Maximization

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    An overview of different variants of the submodular welfare maximization problem in combinatorial auctions. In particular, I studied the existing algorithmic and game theoretic results for submodular welfare maximization problem and its applications in other areas such as social networks

    Excitation of stellar p-modes by turbulent convection: 1. Theoretical formulation

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    Stochatic excitation of stellar oscillations by turbulent convection is investigated and an expression for the power injected into the oscillations by the turbulent convection of the outer layers is derived which takes into account excitation through turbulent Reynolds stresses and turbulent entropy fluctuations. This formulation generalizes results from previous works and is built so as to enable investigations of various possible spatial and temporal spectra of stellar turbulent convection. For the Reynolds stress contribution and assuming the Kolmogorov spectrum we obtain a similar formulation than those derived by previous authors. The entropy contribution to excitation is found to originate from the advection of the Eulerian entropy fluctuations by the turbulent velocity field. Numerical computations in the solar case in a companion paper indicate that the entropy source term is dominant over Reynold stress contribution to mode excitation, except at high frequencies.Comment: 14 pages, accepted for publication in A&

    The Effect of Large Scale Magnetic Field on Outflow in ADAFs: an Odd Symmetry Configuration

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    We construct self-similar inflow-outflow solutions for a hot viscous-resistive accretion flow with large scale magnetic fields that have odd symmetry with respect to the equatorial plane in BθB_\theta, and even symmetry in BrB_r and BϕB_\phi. Following previous authors, we also assume that the polar velocity vθv_\theta is nonzero. We focus on four parameters: βr0\beta_{r0}, βϕ0\beta_{\phi0} (the plasma beta parameters for associated with magnetic field components at the equatorial plane), the magnetic resistivity η0\eta_0, and the density index n=dlnρ/dlnrn=-d\ln\rho/d\ln r. The resulting flow solutions are divided into two parts consisting of an inflow region with a negative radial velocity (vr0v_r0. Our results show that stronger outflows emerge for smaller βr0\beta_{r0} (102\le10^{-2} for n>1n>1) and larger values of βϕ0\beta_{\phi0}, η0\eta_0 and nn.Comment: 10 pages, 9 figures, Accepted for publication MNRA

    Excitation of non-radial stellar oscillations by gravitational waves: a first model

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    The excitation of solar and solar-like g modes in non-relativistic stars by arbitrary external gravitational wave fields is studied starting from the full field equations of general relativity. We develop a formalism that yields the mean-square amplitudes and surface velocities of global normal modes excited in such a way. The isotropic elastic sphere model of a star is adopted to demonstrate this formalism and for calculative simplicity. It is shown that gravitational waves solely couple to quadrupolar spheroidal eigenmodes and that normal modes are only sensitive to the spherical component of the gravitational waves having the same azimuthal order. The mean-square amplitudes in case of stationary external gravitational waves are given by a simple expression, a product of a factor depending on the resonant properties of the star and the power spectral density of the gravitational waves' spherical accelerations. Both mean-square amplitudes and surface velocities show a characteristic R^8-dependence (effective R^2-dependence) on the radius of the star. This finding increases the relevance of this excitation mechanism in case of stars larger than the Sun.Comment: 8 pages, to be published in MNRAS (in press); corrected typo
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