37,242 research outputs found
Truncating the loop series expansion for Belief Propagation
Recently, M. Chertkov and V.Y. Chernyak derived an exact expression for the
partition sum (normalization constant) corresponding to a graphical model,
which is an expansion around the Belief Propagation solution. By adding
correction terms to the BP free energy, one for each "generalized loop" in the
factor graph, the exact partition sum is obtained. However, the usually
enormous number of generalized loops generally prohibits summation over all
correction terms. In this article we introduce Truncated Loop Series BP
(TLSBP), a particular way of truncating the loop series of M. Chertkov and V.Y.
Chernyak by considering generalized loops as compositions of simple loops. We
analyze the performance of TLSBP in different scenarios, including the Ising
model, regular random graphs and on Promedas, a large probabilistic medical
diagnostic system. We show that TLSBP often improves upon the accuracy of the
BP solution, at the expense of increased computation time. We also show that
the performance of TLSBP strongly depends on the degree of interaction between
the variables. For weak interactions, truncating the series leads to
significant improvements, whereas for strong interactions it can be
ineffective, even if a high number of terms is considered.Comment: 31 pages, 12 figures, submitted to Journal of Machine Learning
Researc
Shape Memory Alloy Nanostructures With Coupled Dynamic Thermo-Mechanical Effects
Employing the Ginzburg-Landau phase-field theory, a new coupled dynamic
thermo-mechanical 3D model has been proposed for modeling the
cubic-to-tetragonal martensitic transformations in shape memory alloy (SMA)
nanostructures. The stress-induced phase transformations and thermo-mechanical
behavior of nanostructured SMAs have been investigated. The mechanical and
thermal hysteresis phenomena, local non-uniform phase transformations and
corresponding non-uniform temperature and deformations distributions are
captured successfully using the developed model. The predicted microstructure
evolution qualitatively matches with the experimental observations. The
developed coupled dynamic model has provided a better understanding of
underlying martensitic transformation mechanisms in SMAs, as well as their
effect on the thermo-mechanical behavior of nanostructures.Comment: 8 pages, 3 figure
The First Simultaneous 3.5 and 1.3mm Polarimetric Survey of Active Galactic Nuclei in the Northern Sky
Short millimeter observations of radio-loud AGN offer the opportunity to
study the physics of their inner relativistic jets, from where the bulk
millimeter emission is radiated. Millimeter jets are significantly less
affected by Faraday rotation and depolarization than in radio. Also, the
millimeter emission is dominated by the innermost jet regions, that are
invisible in radio owing to synchrotron opacity. We present the first dual
frequency simultaneous 86GHz and 229GHz polarimetric survey of all four Stokes
parameters of a large sample of 211 radio loud active galactic nuclei, designed
to be flux limited at 1Jy at 86GHz. The observations were most of them made in
mid August 2010 using the XPOL polarimeter on the IRAM 30 m millimeter radio
telescope. Linear polarization detections above 3 sigma median level of ~1.0%
are reported for 183 sources at 86GHz, and for 23 sources at 229GHz, where the
median 3 sigma level is ~6.0%. We show a clear excess of the linear
polarization degree detected at 229GHz with regard to that at 86GHz by a factor
of ~1.6, thus implying a progressively better ordered magnetic field for blazar
jet regions located progressively upstream in the jet. We show that the linear
polarization angle, both at 86 and 229GHz, and the jet structural position
angle for both quasars and BL Lacs do not show a clear preference to align in
either parallel or perpendicular directions. Our variability study with regard
to the 86GHz data from our previous survey points out a large degree variation
of total flux and linear polarization in time scales of years by median factors
of ~1.5 in total flux, and ~1.7 in linear polarization degree -maximum
variations by factors up to 6.3, and ~5, respectively-, with 86% of sources
showing linear polarization angles evenly distributed with regard to our
previous measurements.Comment: Submitted for Publication in Astronomy & Astrophysics. 14 pages
(including 2 tables and 18 figures
New Factorization Relations for Yang Mills Amplitudes
A double-cover extension of the scattering equation formalism of Cachazo, He
and Yuan (CHY) leads us to conjecture covariant factorization formulas of
n-particle scattering amplitudes in Yang-Mills theories. Evidence is given that
these factorization relations are related to Berends-Giele recursions through
repeated use of partial fraction identities involving linearized propagators.Comment: 7 pages, 3 figures, version to appear in PR
Recursive partitioned inversion of large (1500 x 1500) symmetric matrices
A recursive algorithm was designed to invert large, dense, symmetric, positive definite matrices using small amounts of computer core, i.e., a small fraction of the core needed to store the complete matrix. The described algorithm is a generalized Gaussian elimination technique. Other algorithms are also discussed for the Cholesky decomposition and step inversion techniques. The purpose of the inversion algorithm is to solve large linear systems of normal equations generated by working geodetic problems. The algorithm was incorporated into a computer program called SOLVE. In the past the SOLVE program has been used in obtaining solutions published as the Goddard earth models
The dust morphology of the elliptical Galaxy M86 with SPIRE
We present Herschel-SPIRE observations at 250–500 μm of the giant elliptical galaxy M 86 and examine the distribution of the resolved cold dust emission and its relation with other galactic tracers. The SPIRE images reveal three dust components: emission from the central region; a dust lane extending north-south; and a bright emission feature 10 kpc to the south-east. We estimate that ~10^6 M_☉ of dust is spatially coincident with atomic and ionized hydrogen, originating from stripped material from the nearby spiral NGC 4438 due to recent tidal interactions with M 86. The gas-to-dust ratio of the cold gas component ranges from ~20–80. We discuss the different heating mechanisms for the dust features
Social Networking: An astronomer's field guide
We present a brief introduction to the phenomenon of "social networking" and
its potentially powerful use as an astronomy outreach and educational tool. We
briefly discuss the development of applications for websites and facebook and
the use of web trackers e.g. Google Analytics to analyze your audience. Finally
we discuss how social bookmarking can be used to promote your work to
unexpected audiences.Comment: 10 pages, conferenc
Pair Plasma Dominance in the Parsec-Scale Relativistic Jet of 3C345
We investigate whether a pc-scale jet of 3C345 is dominated by a normal
plasma or an electron-positron plasma. We present a general condition that a
jet component becomes optically thick for synchrotron self-absorption, by
extending the method originally developed by Reynolds et al. The general
condition gives a lower limit of the electron number density, with the aid of
the surface brightness condition, which enables us to compute the magnetic
field density. Comparing the lower limit with another independent constraint
for the electron density that is deduced from the kinetic luminosity, we can
distinguish the matter content. We apply the procedure to the five components
of 3C345 (C2, C3, C4, C5, and C7) of which angular diameters and radio fluxes
at the peak frequencies were obtainable from literature. Evaluating the
representative values of Doppler beaming factors by their equipartition values,
we find that all the five components are likely dominated by an
electron-positron plasma. The conclusion does not depend on the lower cutoff
energy of the power-law distribution of radiating particles.Comment: 17 page
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