8,237 research outputs found
Optimizing at the Ergodic Edge
Using a simple, annealed model, some of the key features of the recently
introduced extremal optimization heuristic are demonstrated. In particular, it
is shown that the dynamics of local search possesses a generic critical point
under the variation of its sole parameter, separating phases of too greedy
(non-ergodic, jammed) and too random (ergodic) exploration. Comparison of
various local search methods within this model suggests that the existence of
the critical point is essential for the optimal performance of the heuristic.Comment: RevTex4, 17 pages, 3 ps-figures incl., for related information, see
http://www.physics.emory.edu/faculty/boettcher/publications.htm
Extremal Optimization for Sherrington-Kirkpatrick Spin Glasses
Extremal Optimization (EO), a new local search heuristic, is used to
approximate ground states of the mean-field spin glass model introduced by
Sherrington and Kirkpatrick. The implementation extends the applicability of EO
to systems with highly connected variables. Approximate ground states of
sufficient accuracy and with statistical significance are obtained for systems
with more than N=1000 variables using bonds. The data reproduces the
well-known Parisi solution for the average ground state energy of the model to
about 0.01%, providing a high degree of confidence in the heuristic. The
results support to less than 1% accuracy rational values of for
the finite-size correction exponent, and of for the fluctuation
exponent of the ground state energies, neither one of which has been obtained
analytically yet. The probability density function for ground state energies is
highly skewed and identical within numerical error to the one found for
Gaussian bonds. But comparison with infinite-range models of finite
connectivity shows that the skewness is connectivity-dependent.Comment: Substantially revised, several new results, 5 pages, 6 eps figures
included, (see http://www.physics.emory.edu/faculty/boettcher/ for related
information
Numerical Results for Ground States of Spin Glasses on Bethe Lattices
The average ground state energy and entropy for +/- J spin glasses on Bethe
lattices of connectivities k+1=3...,26 at T=0 are approximated numerically. To
obtain sufficient accuracy for large system sizes (up to n=2048), the Extremal
Optimization heuristic is employed which provides high-quality results not only
for the ground state energies per spin e_{k+1} but also for their entropies
s_{k+1}. The results show considerable quantitative differences between
lattices of even and odd connectivities. The results for the ground state
energies compare very well with recent one-step replica symmetry breaking
calculations. These energies can be scaled for all even connectivities k+1 to
within a fraction of a percent onto a simple functional form, e_{k+1} = E_{SK}
sqrt(k+1) - {2E_{SK}+sqrt(2)} / sqrt(k+1), where E_{SK} = -0.7633 is the ground
state energy for the broken replica symmetry in the Sherrington-Kirkpatrick
model. But this form is in conflict with perturbative calculations at large
k+1, which do not distinguish between even and odd connectivities. We find
non-zero entropies s_{k+1} at small connectivities. While s_{k+1} seems to
vanish asymptotically with 1/(k+1) for even connectivities, it is
indistinguishable from zero already for odd k+1 >= 9.Comment: 11 pages, RevTex4, 28 ps-figures included, related papers available
at http://www.physics.emory.edu/faculty/boettcher
Coronal cooling and its signatures in the rapid aperiodic variability of Galactic black-hole candidates
The most popular models for the complex phase and time lags in the rapid
aperiodic variability of Galactic X-ray binaries are based Comptonization of
soft seed photons in a hot corona, where small-scale flares are induced by
flares of the soft seed photon input (presumably from a cold accretion disc).
However, in their original version, these models have neglected the additional
cooling of the coronal plasma due to the increased soft seed photon input, and
assumed a static coronal temperature structure. In this paper, our
Monte-Carlo/Fokker-Planck code for time-dependent radiation transfer and
electron energetics is used to simulate the self-consistent coronal response to
the various flaring scenarios that have been suggested to explain phase and
time lags observed in some Galactic X-ray binaries. It is found that the
predictions of models involving slab-coronal geometries are drastically
different from those deduced under the assumption of a static corona. However,
with the inclusion of coronal cooling they may even be more successful than in
their original version in explaining some of the observed phase and time lag
features. The predictions of the model of inward-drifting density perturbations
in an ADAF-like, two-temperature flow also differ from the static-corona case
previously investigated, but may be consistent with the alternating phase lags
seen in GRS 1915+105 and XTE J1550-564. Models based on flares of a cool disc
around a hot, inner two-temperature flow may be ruled out for most objects
where significant Fourier-frequency-dependent phase and time lags have been
observed.Comment: 23 pages, including 8 figures and 2 tables; accepted for publication
in ApJ; extended discussion w.r.t. original versio
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