33,574 research outputs found
Taming computational complexity: efficient and parallel SimRank optimizations on undirected graphs
SimRank has been considered as one of the promising link-based ranking algorithms to evaluate similarities of web documents in many modern search engines. In this paper, we investigate the optimization problem of SimRank similarity computation on undirected web graphs. We first present a novel algorithm to estimate the SimRank between vertices in O(n3+ Kn2) time, where n is the number of vertices, and K is the number of iterations. In comparison, the most efficient implementation of SimRank algorithm in [1] takes O(K n3 ) time in the worst case. To efficiently handle large-scale computations, we also propose a parallel implementation of the SimRank algorithm on multiple processors. The experimental evaluations on both synthetic and real-life data sets demonstrate the better computational time and parallel efficiency of our proposed techniques
Microscopic origin of light emission in Al_yGa_{1-y}N/GaN superlattice: Band profile and active site
We present first-principles calculations of AlGaN/GaN superlattice,
clarifying the microscopic origin of the light emission and revealing the
effect of local polarization within the quantum well. Profile of energy band
and distributions of electrons and holes demonstrate the existence of a main
active site in the well responsible for the main band-edge light emission. This
site appears at the position where the local polarization becomes zero. With
charge injection, the calculated optical spectra show that the broadening of
the band gap at the active site leads to the blueshift of emission wavelength
Bounce and cyclic cosmology in extended nonlinear massive gravity
We investigate non-singular bounce and cyclic cosmological evolutions in a
universe governed by the extended nonlinear massive gravity, in which the
graviton mass is promoted to a scalar-field potential. The extra freedom of the
theory can lead to certain energy conditions violations and drive cyclicity
with two different mechanisms: either with a suitably chosen scalar-field
potential under a given Stuckelberg-scalar function, or with a suitably chosen
Stuckelberg-scalar function under a given scalar-field potential. Our analysis
shows that extended nonlinear massive gravity can alter significantly the
evolution of the universe at both early and late times.Comment: 20 pages, 5 figures, version published at JCA
Probing and modelling the localized self-mixing in a GaN/AlGaN field-effect terahertz detector
In a GaN/AlGaN field-effect terahertz detector, the directional photocurrent
is mapped in the two-dimensional space of the gate voltage and the drain/source
bias. It is found that not only the magnitude, but also the polarity, of the
photocurrent can be tuned. A quasistatic self-mixing model taking into account
the localized terahertz field provides a quantitative description of the
detector characteristics. Strongly localized self-mixing is confirmed. It is
therefore important to engineer the spatial distribution of the terahertz field
and its coupling to the field-effect channel on the sub-micron scale.Comment: 12 pages, 4 figures, submitted to AP
Local Measurement of Current Density by Magneto-Optical Current Reconstruction in Normally and Overpressure Processed Bi-2223 Tapes
Magneto-optical current reconstruction has been used for detailed analysis of
the local critical current density (Jc) variation in monocore Bi-2223 tapes. We
find, even in high quality tapes with bulk transport Jc ~ 40 kA/cm^2 (77K, 0T),
that there exist local regions which possess current densities of more than 200
kA/cm^2. Overpressure processing at 148 bar significantly improved Jc to 48
kA/cm^2 by improving the connectivity. For the overpressure-processed sample we
find that the current distribution is more uniform and that the maximum local
current density at 77 K is increased almost to 300 kA/cm^2.Comment: Presented at Applied Superconductivity Conference, Houston, August
4th -9th, 200
FPTAS for Weighted Fibonacci Gates and Its Applications
Fibonacci gate problems have severed as computation primitives to solve other
problems by holographic algorithm and play an important role in the dichotomy
of exact counting for Holant and CSP frameworks. We generalize them to weighted
cases and allow each vertex function to have different parameters, which is a
much boarder family and #P-hard for exactly counting. We design a fully
polynomial-time approximation scheme (FPTAS) for this generalization by
correlation decay technique. This is the first deterministic FPTAS for
approximate counting in the general Holant framework without a degree bound. We
also formally introduce holographic reduction in the study of approximate
counting and these weighted Fibonacci gate problems serve as computation
primitives for approximate counting. Under holographic reduction, we obtain
FPTAS for other Holant problems and spin problems. One important application is
developing an FPTAS for a large range of ferromagnetic two-state spin systems.
This is the first deterministic FPTAS in the ferromagnetic range for two-state
spin systems without a degree bound. Besides these algorithms, we also develop
several new tools and techniques to establish the correlation decay property,
which are applicable in other problems
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