1,731 research outputs found
Semidefinite code bounds based on quadruple distances
Let be the maximum number of words of length , any two
having Hamming distance at least . We prove , which implies
that the quadruply shortened Golay code is optimal. Moreover, we show
, , , ,
, , , ,
, , , ,
, , and .
The method is based on the positive semidefiniteness of matrices derived from
quadruples of words. This can be put as constraint in a semidefinite program,
whose optimum value is an upper bound for . The order of the matrices
involved is huge. However, the semidefinite program is highly symmetric, by
which its feasible region can be restricted to the algebra of matrices
invariant under this symmetry. By block diagonalizing this algebra, the order
of the matrices will be reduced so as to make the program solvable with
semidefinite programming software in the above range of values of and .Comment: 15 page
The Lutz-Kelker bias in trigonometric parallaxes
The theoretical prediction that trigonometric parallaxes suffer from a
statistical effect, has become topical again now that the results of the
Hipparcos satellite have become available. This statistical effect, the
so-called Lutz-Kelker bias, causes measured parallaxes to be too large. This
has the implication that inferred distances, and hence inferred luminosities
are too small. Published analytic calculations of the Lutz-Kelker bias indicate
that the inferred luminosity of an object is, on average, 30% too small when
the error in the parallax is only 17.5%. Yet, this bias has never been
determined empirically. In this paper we investigate whether there is such a
bias by comparing the best Hipparcos parallaxes which ground-based
measurements. We find that there is indeed a large bias affecting parallaxes,
with an average and scatter comparable to predictions. We propose a simple
method to correct for the LK bias, and apply it successfully to a sub-sample of
our stars. We then analyze the sample of 26 `best' Cepheids used by Feast &
Catchpole (1997) to derive the zero-point of the fundamental mode pulsators and
leads to a distance modulus to the Large Magellanic Cloud - based on Cepheid
parallaxes- of 18.56 +/- 0.08, consistent with previous estimates.Comment: MNRAS Letters in press; 6 pages LaTeX, 6 ps figure
Reconstructing the Local Twist of Coronal Magnetic Fields and the Three-Dimensional Shape of the Field Lines from Coronal Loops in EUV and X-Ray Images
Non-linear force-free fields are the most general case of force-free fields,
but the hardest to model as well. There are numerous methods of computing such
fields by extrapolating vector magnetograms from the photosphere, but very few
attempts have so far made quantitative use of coronal morphology. We present a
method to make such quantitative use of X-Ray and EUV images of coronal loops.
Each individual loop is fit to a field line of a linear force-free field,
allowing the estimation of the field line's twist, three-dimensional geometry
and the field strength along it.
We assess the validity of such a reconstruction since the actual corona is
probably not a linear force-free field and that the superposition of linear
force-free fields is generally not itself a force-free field. To do so, we
perform a series of tests on non-linear force-free fields, described in Low &
Lou (1990). For model loops we project field lines onto the photosphere. We
compare several results of the method with the original field, in particular
the three-dimensional loop shapes, local twist (coronal alpha), distribution of
twist in the model photosphere and strength of the magnetic field. We find
that, (i) for these trial fields, the method reconstructs twist with mean
absolute deviation of at most 15% of the range of photospheric twist, (ii) that
heights of the loops are reconstructed with mean absolute deviation of at most
5% of the range of trial heights and (iii) that the magnitude of non-potential
contribution to photospheric field is reconstructed with mean absolute
deviation of at most 10% of the maximal value.Comment: submitted to Ap
Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars
The objective of this paper is to investigate whether a convective dynamo can
account quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical simulations
of a turbulent dynamo driven by convection to characterize the dynamic
behavior, magnetic field strengths, and filling factors in a non-rotating
stratified medium, and to predict these magnetic properties at the surface of
cool stars. We use simple applications of stellar structure theory for the
convective envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0--M0 spectral range, which allows us to estimate
the unsigned magnetic flux on the surface of non-rotating reference stars. With
these estimates we use the recent results of \citet{Pevtsov03} to predict the
level of X-ray emission from such a turbulent dynamo, and find that our results
compare well with observed lower limits of surface X-ray flux. If we scale our
predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with
the observed lower limit of chromospheric emission in K dwarfs. This suggests
that dynamo action from a convecting, non-rotating plasma is a viable
alternative to acoustic heating models as an explanation for the basal emission
level seen in chromospheric, transition region, and coronal diagnostics from
late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure
Mg II h + k emission lines as stellar activity indicators of main sequence F-K stars
The main purpose of this study is to use the IUE spectra in the analysis of
magnetic activity of main sequence F-K stars. Combining IUE observations of
MgII and optical spectroscopy of Ca II, the registry of ctivity of stars can be
extended in time. We retrieved all the high-resolution spectra of F, G, and K
main sequence stars observed by IUE (i.e. 1623 spectra of 259 F to K dwarf
stars). We obtained the continuum surface flux near the Mg II h+k lines near
2800 \AA and the MgII line-core surface flux from the IUE spectra. We obtained
a relation between the mean continuum flux near the MgII lines with the colour
of the star. For a set of 117 nearly simultaneous observations of Mg II
and Ca II fluxes of 21 F5 to K3 main sequence stars, we obtained a colour
dependent relation between the Mount Wilson CaII S-index and the MgII emission
line-core flux. As an application of this calibration, we computed the Mount
Wilson index for all the dF to dK stars which have high resolution IUE spectra.
For some of the most frequently observed main sequence stars, we analysed the
Mount Wilson index S from the IUE spectra, together with the ones derived from
visible spectra. We confirm the cyclic chromospheric activity of epsilon Eri
(HD 22049) and beta Hydri (HD 2151), and we find a magnetic cycle in alpha Cen
B (HD 128621). Complete abstract in the paper.Comment: 10 pages, accepted for publication in Astronomy and Astrophysic
Single machine scheduling with controllable processing times by submodular optimization
In scheduling with controllable processing times the actual processing time of each job is to be chosen from the interval between the smallest (compressed or fully crashed) value and the largest (decompressed or uncrashed) value. In the problems under consideration, the jobs are processed on a single machine and the quality of a schedule is measured by two functions: the maximum cost (that depends on job completion times) and the total compression cost. Our main model is bicriteria and is related to determining an optimal trade-off between these two objectives. Additionally, we consider a pair of associated single criterion problems, in which one of the objective functions is bounded while the other one is to be minimized. We reduce the bicriteria problem to a series of parametric linear programs defined over the intersection of a submodular polyhedron with a box. We demonstrate that the feasible region is represented by a so-called base polyhedron and the corresponding problem can be solved by the greedy algorithm that runs two orders of magnitude faster than known previously. For each of the associated single criterion problems, we develop algorithms that deliver the optimum faster than it can be deduced from a solution to the bicriteria problem
Quasi-periodic Fast-mode Wave Trains Within a Global EUV Wave and Sequential Transverse Oscillations Detected by SDO/AIA
We present the first unambiguous detection of quasi-periodic wave trains
within the broad pulse of a global EUV wave (so-called "EIT wave") occurring on
the limb. These wave trains, running ahead of the lateral CME front of 2-4
times slower, coherently travel to distances along the solar
surface, with initial velocities up to 1400 km/s decelerating to ~650 km/s. The
rapid expansion of the CME initiated at an elevated height of 110 Mm produces a
strong downward and lateral compression, which may play an important role in
driving the primary EUV wave and shaping its front forwardly inclined toward
the solar surface. The waves have a dominant 2 min periodicity that matches the
X-ray flare pulsations, suggesting a causal connection. The arrival of the
leading EUV wave front at increasing distances produces an uninterrupted chain
sequence of deflections and/or transverse (likely fast kink mode) oscillations
of local structures, including a flux-rope coronal cavity and its embedded
filament with delayed onsets consistent with the wave travel time at an
elevated (by ~50%) velocity within it. This suggests that the EUV wave
penetrates through a topological separatrix surface into the cavity, unexpected
from CME caused magnetic reconfiguration. These observations, when taken
together, provide compelling evidence of the fast-mode MHD wave nature of the
{\it primary (outer) fast component} of a global EUV wave, running ahead of the
{\it secondary (inner) slow} component of CME-caused restructuring.Comment: 17 pages, 12 figures; accepted by ApJ, April 24, 201
New upper bounds for nonbinary codes based on the Terwilliger algebra and semidefinite programming
We give a new upper bound on the maximum size of a code of word length and minimum Hamming distance at least over the alphabet of letters. By block-diagonalizing the Terwilliger algebra of the nonbinary Hamming scheme, the bound can be calculated in time polynomial in using semidefinite programming. For this gives several improved upper bounds for concrete values of and . This work builds upon previous results of A. Schrijver [IEEE Trans. Inform. Theory 51 (2005), no. 8, 2859--2866] on the Terwilliger algebra of the binary Hamming schem
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