3,928 research outputs found
Fast Ewald summation for electrostatic potentials with arbitrary periodicity
A unified treatment for fast and spectrally accurate evaluation of
electrostatic potentials subject to periodic boundary conditions in any or none
of the three space dimensions is presented. Ewald decomposition is used to
split the problem into a real space and a Fourier space part, and the FFT based
Spectral Ewald (SE) method is used to accelerate the computation of the latter.
A key component in the unified treatment is an FFT based solution technique for
the free-space Poisson problem in three, two or one dimensions, depending on
the number of non-periodic directions. The cost of calculations is furthermore
reduced by employing an adaptive FFT for the doubly and singly periodic cases,
allowing for different local upsampling rates. The SE method will always be
most efficient for the triply periodic case as the cost for computing FFTs will
be the smallest, whereas the computational cost for the rest of the algorithm
is essentially independent of the periodicity. We show that the cost of
removing periodic boundary conditions from one or two directions out of three
will only marginally increase the total run time. Our comparisons also show
that the computational cost of the SE method for the free-space case is
typically about four times more expensive as compared to the triply periodic
case. The Gaussian window function previously used in the SE method, is here
compared to an approximation of the Kaiser-Bessel window function, recently
introduced. With a carefully tuned shape parameter that is selected based on an
error estimate for this new window function, runtimes for the SE method can be
further reduced. Keywords: Fast Ewald summation, Fast Fourier transform,
Arbitrary periodicity, Coulomb potentials, Adaptive FFT, Fourier integral,
Spectral accuracy.Comment: 21 pages, 11 figure
Estimating Functions of Probability Distributions from a Finite Set of Samples, Part 1: Bayes Estimators and the Shannon Entropy
We present estimators for entropy and other functions of a discrete
probability distribution when the data is a finite sample drawn from that
probability distribution. In particular, for the case when the probability
distribution is a joint distribution, we present finite sample estimators for
the mutual information, covariance, and chi-squared functions of that
probability distribution.Comment: uuencoded compressed tarfile, submitte
Approximation results for a general class of Kantorovich type operators
We introduce and study a family of integral operators in the Kantorovich
sense for functions acting on locally compact topological groups. We obtain
convergence results for the above operators with respect to the pointwise and
uniform convergence and in the setting of Orlicz spaces with respect to the
modular convergence. Moreover, we show how our theory applies to several
classes of integral and discrete operators, as the sampling, convolution and
Mellin type operators in the Kantorovich sense, thus obtaining a simultaneous
approach for discrete and integral operators. Further, we derive our general
convergence results for particular cases of Orlicz spaces, as spaces,
interpolation spaces and exponential spaces. Finally we construct some concrete
example of our operators and we show some graphical representations.Comment: 23 pages, 5 figure
A parametric study of aliasing error for a narrow field of view scanning radiometer
Starting from the general measurement equation, it is shown that a NFOV scanner can be approximated by a spatially invariant system whose point spread function depends on the detector shape and angular characteristics and electrical filter transfer function for given patches at the top of the atmosphere. The radiometer is modeled by a detector, electrical filter, analog to digital converter followed by a reconstruction filter. The errors introduced by aliasing and blurring into a reconstruction of the input radiant exitance are modeled and analyzed for various detector shapes, sampling intervals, electrical filters and scan types. Quantitative results on the errors introduced are presented showing the various tradeoffs between design parameters. The results indicate that proper selection of detector shape coupled with electrical filter can reduce aliasing errors significantly
A literature survey of low-rank tensor approximation techniques
During the last years, low-rank tensor approximation has been established as
a new tool in scientific computing to address large-scale linear and
multilinear algebra problems, which would be intractable by classical
techniques. This survey attempts to give a literature overview of current
developments in this area, with an emphasis on function-related tensors
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