2,685 research outputs found
Irregular Input Data in Convergence Acceleration and Summation Processes: General Considerations and Some Special Gaussian Hypergeometric Series as Model Problems
Sequence transformations accomplish an acceleration of convergence or a
summation in the case of divergence by detecting and utilizing regularities of
the elements of the sequence to be transformed. For sufficiently large indices,
certain asymptotic regularities normally do exist, but the leading elements of
a sequence may behave quite irregularly. The Gaussian hypergeometric series 2F1
(a, b; c; z) is well suited to illuminate problems of that kind. Sequence
transformations perform quite well for most parameters and arguments. If,
however, the third parameter of a nonterminating hypergeometric series 2F1
is a negative real number, the terms initially grow in magnitude like the terms
of a mildly divergent series. The use of the leading terms of such a series as
input data leads to unreliable and even completely nonsensical results. In
contrast, sequence transformations produce good results if the leading
irregular terms are excluded from the transformation process. Similar problems
occur also in perturbation expansions. For example, summation results for the
infinite coupling limit k_3 of the sextic anharmonic oscillator can be improved
considerably by excluding the leading terms from the transformation process.
Finally, numerous new recurrence formulas for the 2F1 (a, b; c; z) are derived.Comment: 25 pages, 7 tables, REVTe
Scalar Levin-Type Sequence Transformations
Sequence transformations are important tools for the convergence acceleration
of slowly convergent scalar sequences or series and for the summation of
divergent series. Transformations that depend not only on the sequence elements
or partial sums but also on an auxiliary sequence of so-called remainder
estimates are of Levin-type if they are linear in the , and
nonlinear in the . Known Levin-type sequence transformations are
reviewed and put into a common theoretical framework. It is discussed how such
transformations may be constructed by either a model sequence approach or by
iteration of simple transformations. As illustration, two new sequence
transformations are derived. Common properties and results on convergence
acceleration and stability are given. For important special cases, extensions
of the general results are presented. Also, guidelines for the application of
Levin-type sequence transformations are discussed, and a few numerical examples
are given.Comment: 59 pages, LaTeX, invited review for J. Comput. Applied Math.,
abstract shortene
Tentative observation of a gamma-ray line at the Fermi LAT
Using 43 months of public gamma-ray data from the Fermi Large Area Telescope,
we find in regions close to the Galactic center at energies of 130 GeV a 4.6
sigma excess that is not inconsistent with a gamma-ray line from dark matter
annihilation. When taking into account the look-elsewhere effect, the
significance of the observed signature is 3.2 sigma. If interpreted in terms of
dark matter particles annihilating into a photon pair, the observations imply a
partial annihilation cross-section of about 10^-27 cm^3s^-1 and a dark matter
mass around 130 GeV. We review aspects of the statistical analysis and comment
on possible instrumental indications.Comment: 4 pages, 5 figures, Proceedings of the 5th International Symposium on
High-Energy Gamma-Ray Astronomy (Gamma2012
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