1,125 research outputs found
Effective order strong stability preserving RungeâKutta methods
We apply the concept of effective order to strong stability preserving (SSP) explicit RungeâKutta methods. Relative to classical RungeâKutta methods, effective order methods are designed to satisfy a relaxed set of order conditions, but yield higher order accuracy when composed with special starting and stopping methods. The relaxed order conditions allow for greater freedom in the design of effective order methods. We show that this allows the construction of four-stage SSP methods with effective order four (such methods cannot have classical order four). However, we also prove that effective order five methodsâlike classical order five methodsârequire the use of non-positive weights and so cannot be SSP. By numerical optimization, we construct explicit SSP RungeâKutta methods up to effective order four and establish the optimality of many of them. Numerical experiments demonstrate the validity of these methods in practice
Rettigheder uden pligter, mÄl uden formÄl, velfÊrd uden omtanke - en kritisk vurdering af Socialdemokraternes og Regeringens forslag til velfÊrds- og kvalitetsreform
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Overvejelser om forbrugerpolitik fra markedslogik og egeninteresse til informeret og ansvarlig selvregulering.
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The otherworldly view of economics - and its consequences.
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The Ship of Fools - a society of selfish individuals
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Distortions of modern management theory - and an attempt to correct them
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An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization
calculations of broad emission line clouds in quasars, spanning seven orders of
magnitude in hydrogen ionizing flux and particle density. The equivalent widths
of 42 quasar emission lines are presented as contours in the particle density -
ionizing flux plane for a typical incident continuum shape, solar chemical
abundances, and cloud column density of . Results are
similarly given for a small subset of emission lines for two other column
densities ( and ), five other incident
continuum shapes, and a gas metallicity of 5 \Zsun. These graphs should prove
useful in the analysis of quasar emission line data and in the detailed
modeling of quasar broad emission line regions. The digital results of these
emission line grids and many more are available over the Internet.Comment: 16 pages, LaTeX (AASTeX aaspp4.sty); to appear in the 1997 ApJS: full
contents of the 9 photoionization grids presented in this paper may be found
at http://www.pa.uky.edu/~korista/grids/grids.htm
Photoexcited electron dynamics in Kondo insulators and heavy fermions
We have studied the photoexcited carrier relaxation dynamics in the Kondo
insulator SmB6 and the heavy fermion metal YbAgCu4 as a function of temperature
and excitation level. The dynamic response is found to be both strongly
temperature dependent and nonlinear. The data are analyzed with a
Rothwarf-Taylor bottleneck model, where the dynamics are governed by the
presence of a narrow gap in the density of states near the Fermi level. The
remarkable agreement with the model suggests that carrier relaxation in a broad
class of heavy electron systems (both metals and insulators) is governed by the
presence of a (weakly temperature dependent) hybridization gap.Comment: accepted for publication in Physical Review Letter
A softer look at MCG--6-30-15 with XMM-Newton
We present analysis and results from the Reflection Grating Spectrometer
during the 320 ks XMM observation of the Seyfert 1 galaxy MCG-6-30-15. The
spectrum is marked by a sharp drop in flux at 0.7 keV which has been
interpreted by Branduardi-Raymont et al. as the blue wing of a relativistic
OVIII emission line and by Lee at al. as a dusty warm absorber. We find that
the drop is well explained by the FeI L2,3 absorption edges and obtain
reasonable fits over the 0.32-1.7 keV band using a multizone, dusty warm
absorber model. Some residuals remain which could be due to emission from a
relativistic disc, but at a much weaker level than from any model relying on
relativistic emission lines alone. A model based on such emission lines can be
made to fit if sufficient (warm) absorption is added, although the line
strengths exceed those expected. The EPIC pn difference spectrum between the
highest and lowest flux states of the source indicates that this is a power-law
in the 3-10 keV band which, if extrapolated to lower energies, reveals the
absorption function acting on the intrinsic spectrum, provided that any
emission lines do not scale exactly with the continuum. We find that this
function matches our dusty warm absorber model well. The soft X-ray spectrum is
therefore dominated by absorption structures, with the equivalent width of any
individual emission lines in the residuals being below about 30 eV. (abridged)Comment: 8 pages, 10 figures, submitted to MNRA
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