621,755 research outputs found
A fixed point formula for the index of multi-centered N=2 black holes
We propose a formula for computing the (moduli-dependent) contribution of
multi-centered solutions to the total BPS index in terms of the
(moduli-independent) indices associated to single-centered solutions. The main
tool in our analysis is the computation of the refined index Tr(-y)^{2J_3} of
configurational degrees of freedom of multi-centered BPS black hole solutions
in N=2 supergravity by localization methods. When the charges carried by the
centers do not allow for scaling solutions (i.e. solutions where a subset of
the centers can come arbitrarily close to each other), the phase space of
classical BPS solutions is compact and the refined index localizes to a finite
set of isolated fixed points under rotations, corresponding to collinear
solutions. When the charges allow for scaling solutions, the phase space is
non-compact but appears to admit a compactification with finite volume and
additional non-isolated fixed points. We give a prescription for determining
the contributions of these fixed submanifolds by means of a `minimal
modification hypothesis', which we prove in the special case of dipole halo
configurations.Comment: 61 pages, 3 figure
Athena: A New Code for Astrophysical MHD
A new code for astrophysical magnetohydrodynamics (MHD) is described. The
code has been designed to be easily extensible for use with static and adaptive
mesh refinement. It combines higher-order Godunov methods with the constrained
transport (CT) technique to enforce the divergence-free constraint on the
magnetic field. Discretization is based on cell-centered volume-averages for
mass, momentum, and energy, and face-centered area-averages for the magnetic
field. Novel features of the algorithm include (1) a consistent framework for
computing the time- and edge-averaged electric fields used by CT to evolve the
magnetic field from the time- and area-averaged Godunov fluxes, (2) the
extension to MHD of spatial reconstruction schemes that involve a
dimensionally-split time advance, and (3) the extension to MHD of two different
dimensionally-unsplit integration methods. Implementation of the algorithm in
both C and Fortran95 is detailed, including strategies for parallelization
using domain decomposition. Results from a test suite which includes problems
in one-, two-, and three-dimensions for both hydrodynamics and MHD are given,
not only to demonstrate the fidelity of the algorithms, but also to enable
comparisons to other methods. The source code is freely available for download
on the web.Comment: 61 pages, 36 figures. accepted by ApJ
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Centralized vs. decentralized computing : organizational considerations and management options
The long-standing debate over whether to centralize or decentralize computing is examined in terms of the fundamental organizational and economic factors at stake. The traditional debate is examined and found to focus predominantly on issues of efficiency vs. effectiveness, with solutions based on a rationalistic strategy of optimizing in this tradeoff. A more behavioralistic assessment suggests that the driving issues in the debate are the politics of organization and resources, centering on the issue of control. The economics of computing deployment decisions is presented as an important issue, but one that often serves as a field of argument that is based on more political concerns. The current situation facing managers of computing, given the advent of small and comparatively inexpensive computers, is examined in detail, and a set of management options for dealing with this persistent issue is presented
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