57,293 research outputs found
Discrepancy estimates for sequences: new results and open problems
In this paper we give an overview of recent results on (upper and lower)
discrepancy estimates for (concrete) sequences in the unit-cube. In particular
we also give an overview of discrepancy estimates for certain classes of hybrid
sequences. Here by a hybrid sequence we understand an -dimensional
sequence which is a combination of an -dimensional sequence of a certain
type (e.g. Kronecker-, Niederreiter-, Halton-,... type) and a -dimensional
sequence of another type. The analysis of the discrepancy of hybrid sequences
(and of their components) is a rather current and vivid branch of research. We
give a collection of some challenging open problems on this topic.Comment: 17 page
Lithium abundances of halo dwarfs based on excitation temperature. I. LTE
The discovery of the Spite plateau in the abundances of 7Li for metal-poor
stars led to the determination of an observationally deduced primordial lithium
abundance. However, after the success of the Wilkinson Microwave Anisotropy
Probe (WMAP) in determining the baryon density, OmegaBh^2, there was a
discrepancy between observationally determined and theoretically determined
abundances in the case of 7Li. One of the most important uncertain factors in
the calculation of the stellar 7Li abundance is the effective temperature,
Teff. We use sixteen metal-poor halo dwarfs to calculate new Teff values using
the excitation energy method. With this temperature scale we then calculate new
Li abundances for this group of stars in an attempt to resolve the 7Li
discrepancy. Using high signal-to-noise (S/N ~ 100) spectra of 16 metal-poor
halo dwarfs, obtained with the UCLES spectrograph on the AAT, measurements of
equivalent widths from a set of unblended FeI lines are made. These equivalent
widths are then used to calculate new Teff values with the use of the single
line radiative transfer program WIDTH6, where we have constrained the gravity
using either theoretical isochrones or the Hipparcos parallax, rather than the
ionization balance. The lithium abundances of the stars are calculated with
these temperatures. The physical parameters are derived for the 16 programme
stars, and two standards. These include Teff, log g, [Fe/H], microturbulence
and 7Li abundances. A comparison between the temperature scale of this work and
those adopted by others has been undertaken. We find good consistency with the
temperatures derived from the Halpha line by Asplund et al. (2006), but not
with the hotter scale of Melendez & Ramirez (2004).Comment: 13 pages, 9 figure
Discrepancy bounds for low-dimensional point sets
The class of -nets and -sequences, introduced in their most
general form by Niederreiter, are important examples of point sets and
sequences that are commonly used in quasi-Monte Carlo algorithms for
integration and approximation. Low-dimensional versions of -nets and
-sequences, such as Hammersley point sets and van der Corput sequences,
form important sub-classes, as they are interesting mathematical objects from a
theoretical point of view, and simultaneously serve as examples that make it
easier to understand the structural properties of -nets and
-sequences in arbitrary dimension. For these reasons, a considerable
number of papers have been written on the properties of low-dimensional nets
and sequences
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