115,743 research outputs found
AIS-BN: An Adaptive Importance Sampling Algorithm for Evidential Reasoning in Large Bayesian Networks
Stochastic sampling algorithms, while an attractive alternative to exact
algorithms in very large Bayesian network models, have been observed to perform
poorly in evidential reasoning with extremely unlikely evidence. To address
this problem, we propose an adaptive importance sampling algorithm, AIS-BN,
that shows promising convergence rates even under extreme conditions and seems
to outperform the existing sampling algorithms consistently. Three sources of
this performance improvement are (1) two heuristics for initialization of the
importance function that are based on the theoretical properties of importance
sampling in finite-dimensional integrals and the structural advantages of
Bayesian networks, (2) a smooth learning method for the importance function,
and (3) a dynamic weighting function for combining samples from different
stages of the algorithm. We tested the performance of the AIS-BN algorithm
along with two state of the art general purpose sampling algorithms, likelihood
weighting (Fung and Chang, 1989; Shachter and Peot, 1989) and self-importance
sampling (Shachter and Peot, 1989). We used in our tests three large real
Bayesian network models available to the scientific community: the CPCS network
(Pradhan et al., 1994), the PathFinder network (Heckerman, Horvitz, and
Nathwani, 1990), and the ANDES network (Conati, Gertner, VanLehn, and Druzdzel,
1997), with evidence as unlikely as 10^-41. While the AIS-BN algorithm always
performed better than the other two algorithms, in the majority of the test
cases it achieved orders of magnitude improvement in precision of the results.
Improvement in speed given a desired precision is even more dramatic, although
we are unable to report numerical results here, as the other algorithms almost
never achieved the precision reached even by the first few iterations of the
AIS-BN algorithm
Intense terahertz laser fields on a two-dimensional electron gas with Rashba spin-orbit coupling
The spin-dependent density of states and the density of spin polarization of
an InAs-based two-dimensional electron gas with the Rashba spin-orbit coupling
under an intense terahertz laser field are investigated by utilizing the
Floquet states to solve the time-dependent Schr\"odinger equation.
It is found that both densities are strongly affected by the terahertz laser
field. Especially a terahertz magnetic moment perpendicular to the external
terahertz laser field in the electron gas is induced. This effect can be used
to convert terahertz electric signals into terahertz magnetic ones efficiently.Comment: 3 pages, 3 figures, a typo in Fig. 3(b) is correcte
County-Specific Net Migration by Five-Year Age Groups, Hispanic Origin, Race and Sex 2000-2010
This report documents the methodology used to prepare county-level, net migration estimates by five-year age cohorts and sex, and by race and Hispanic origin, for the intercensal period from 2000 to 2010. The estimates were prepared using a vital statistics version of the forward cohort residual method (Siegel and Hamilton 1952) following the techniques used to prepare the 1990 to 2000 net migration estimates (Voss, McNiven, Johnson, Hammer, and Fuguitt 2004) as described in detail below. These numbers (and the net migration rates derivable from them) extend the set of decennial estimates of net migration that have been produced following each decennial census beginning with 1960 (net migration for the 1950s: Bowles and Tarver, 1965; 1960s: Bowles, Beale and Lee, 1975; 1970s: White, Mueser and Tierney, 1987; 1980s: Fuguitt, Beale, and Voss 2010; and 1990s: Voss, McNiven, Hammer, Johnson and Fuguitt, 2004)
Solar flare hard X-ray spikes observed by RHESSI: a statistical study
Context. Hard X-ray (HXR) spikes refer to fine time structures on timescales
of seconds to milliseconds in high-energy HXR emission profiles during solar
flare eruptions. Aims. We present a preliminary statistical investigation of
temporal and spectral properties of HXR spikes. Methods. Using a three-sigma
spike selection rule, we detected 184 spikes in 94 out of 322 flares with
significant counts at given photon energies, which were detected from
demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI). About one fifth of these spikes are also
detected at photon energies higher than 100 keV. Results. The statistical
properties of the spikes are as follows. (1) HXR spikes are produced in both
impulsive flares and long-duration flares with nearly the same occurrence
rates. Ninety percent of the spikes occur during the rise phase of the flares,
and about 70% occur around the peak times of the flares. (2) The time durations
of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not
dependent on photon energies. The spikes exhibit symmetric time profiles with
no significant difference between rise and decay times. (3) Among the most
energetic spikes, nearly all of them have harder count spectra than their
underlying slow-varying components. There is also a weak indication that spikes
exhibiting time lags in high-energy emissions tend to have harder spectra than
spikes with time lags in low-energy emissions.Comment: 16 pages, 13 figure
Comparisons between Nimbus 6 satellite and rawinsonde soundings for several geographical areas
Good agreement between satellite and weighted (linearly interpolated) rawinsonde temperature and temperature derived parameters was found in most instances with the poorest agreement either near the tropopause region or near the ground. However, satellite moisture data are highly questionable. The smallest discrepancy between satellite and weighted mean rawinsonde temperature and parameters derived from temperature was found over water and the largest discrepancy was found over mountains. Cumulative frequency distributions show that discrepancies between satellite and rawinsonde data can be represented by a normal distribution except for dew point temperature
Effective oscillator strength distributions of spherically symmetric atoms for calculating polarizabilities and long-range atom-atom interactions
Effective oscillator strength distributions are systematically generated and
tabulated for the alkali atoms, the alkaline-earth atoms, the alkaline-earth
ions, the rare gases and some miscellaneous atoms. These effective
distributions are used to compute the dipole, quadrupole and octupole static
polarizabilities, and are then applied to the calculation of the dynamic
polarizabilities at imaginary frequencies. These polarizabilities can be used
to determine the long-range , and atom-atom interactions
for the dimers formed from any of these atoms and ions, and we present tables
covering all of these combinations
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