4,039 research outputs found
Nullity and Loop Complementation for Delta-Matroids
We show that the symmetric difference distance measure for set systems, and
more specifically for delta-matroids, corresponds to the notion of nullity for
symmetric and skew-symmetric matrices. In particular, as graphs (i.e.,
symmetric matrices over GF(2)) may be seen as a special class of
delta-matroids, this distance measure generalizes the notion of nullity in this
case. We characterize delta-matroids in terms of equicardinality of minimal
sets with respect to inclusion (in addition we obtain similar characterizations
for matroids). In this way, we find that, e.g., the delta-matroids obtained
after loop complementation and after pivot on a single element together with
the original delta-matroid fulfill the property that two of them have equal
"null space" while the third has a larger dimension.Comment: Changes w.r.t. v4: different style, Section 8 is extended, and in
addition a few small changes are made in the rest of the paper. 15 pages, no
figure
Surface oscillations in channeled snow flows
An experimental device has been built to measure velocity profiles and
friction laws in channeled snow flows. The measurements show that the velocity
depends linearly on the vertical position in the flow and that the friction
coefficient is a first-order polynomial in velocity (u) and thickness (h) of
the flow. In all flows, oscillations on the surface of the flow were observed
throughout the channel and measured at the location of the probes. The
experimental results are confronted with a shallow water approach. Using a
Saint-Venant modeling, we show that the flow is effectively uniform in the
streamwise direction at the measurement location. We show that the surface
oscillations produced by the Archimedes's screw at the top of the channel
persist throughout the whole length of the channel and are the source of the
measured oscillations. This last result provides good validation of the
description of such channeled snow flows by a Saint-Venant modeling
Separation of foregrounds from cosmic microwave background observations with the MAP satellite
Simulated observations of a 10\dg \times 10\dg field by the Microwave
Anisotropy Probe (MAP) are analysed in order to separate cosmic microwave
background (CMB) emission from foreground contaminants and instrumental noise
and thereby determine how accurately the CMB emission can be recovered. The
simulations include emission from the CMB, the kinetic and thermal
Sunyaev-Zel'dovich (SZ) effects from galaxy clusters, as well as Galactic dust,
free-free and synchrotron. We find that, even in the presence of these
contaminating foregrounds, the CMB map is reconstructed with an rms accuracy of
about 20 K per 12.6 arcmin pixel, which represents a substantial
improvement as compared to the individual temperature sensitivities of the raw
data channels. We also find, for the single 10\dg \times 10\dg field, that
the CMB power spectrum is accurately recovered for \ell \la 600.Comment: 7 pages, 7 figures, MNRAS submitte
Spectral Orbits and Peak-to-Average Power Ratio of Boolean Functions with respect to the {I,H,N}^n Transform
We enumerate the inequivalent self-dual additive codes over GF(4) of
blocklength n, thereby extending the sequence A090899 in The On-Line
Encyclopedia of Integer Sequences from n = 9 to n = 12. These codes have a
well-known interpretation as quantum codes. They can also be represented by
graphs, where a simple graph operation generates the orbits of equivalent
codes. We highlight the regularity and structure of some graphs that correspond
to codes with high distance. The codes can also be interpreted as quadratic
Boolean functions, where inequivalence takes on a spectral meaning. In this
context we define PAR_IHN, peak-to-average power ratio with respect to the
{I,H,N}^n transform set. We prove that PAR_IHN of a Boolean function is
equivalent to the the size of the maximum independent set over the associated
orbit of graphs. Finally we propose a construction technique to generate
Boolean functions with low PAR_IHN and algebraic degree higher than 2.Comment: Presented at Sequences and Their Applications, SETA'04, Seoul, South
Korea, October 2004. 17 pages, 10 figure
Omega from the skewness of the cosmic velocity divergence
We propose a method for measuring the cosmological density parameter
from the statistics of the divergence field, , the
divergence of peculiar velocity, expressed in units of the Hubble constant, . The velocity field is spatially smoothed over to remove strongly nonlinear effects. Assuming weakly-nonlinear
gravitational evolution from Gaussian initial fluctuations, and using
second-order perturbative analysis, we show that \propto
-\Omega^{-0.6} ^2. The constant of proportionality depends on the
smoothing window. For a top-hat of radius R and volume-weighted smoothing, this
constant is , where . If the
power spectrum is a power law, , then . A Gaussian
window yields similar results. The resulting method for measuring is
independent of any assumed biasing relation between galaxies and mass.
The method has been successfully tested with numerical simulations. A
preliminary application to real data, provided by the POTENT recovery procedure
from observed velocities favors . However, because of an
uncertain sampling error, this result should be treated as an assessment of the
feasibility of our method rather than a definitive measurement of .Comment: 16 pages + 2 figures, uuencoded postscript file, also available by
anonymous ftp from ftp.cita.utoronto.ca in directory
/cita/francis/div_skewness, CITA 94-1
On the Minimum Degree up to Local Complementation: Bounds and Complexity
The local minimum degree of a graph is the minimum degree reached by means of
a series of local complementations. In this paper, we investigate on this
quantity which plays an important role in quantum computation and quantum error
correcting codes. First, we show that the local minimum degree of the Paley
graph of order p is greater than sqrt{p} - 3/2, which is, up to our knowledge,
the highest known bound on an explicit family of graphs. Probabilistic methods
allows us to derive the existence of an infinite number of graphs whose local
minimum degree is linear in their order with constant 0.189 for graphs in
general and 0.110 for bipartite graphs. As regards the computational complexity
of the decision problem associated with the local minimum degree, we show that
it is NP-complete and that there exists no k-approximation algorithm for this
problem for any constant k unless P = NP.Comment: 11 page
Diffuse emission measurement with INTEGRAL/SPI as indirect probe of cosmic-ray electrons and positrons
Significant advances have been made in the understanding of the diffuse
Galactic hard X-ray continuum emission using data from the INTEGRAL
observatory. The diffuse hard power-law component seen with the INTEGRAL/SPI
spectrometer has been identified with inverse-Compton emission from
relativistic (GeV) electrons on the cosmic microwave background and Galactic
interstellar radiation field. In the present analysis, SPI data from 2003 to
2009, with a total exposure time of ~ 10^8 s, are used to derive the Galactic
ridge hard X-ray spatial distribution and spectrum between 20 keV and 2.4 MeV.
Both are consistent with predictions from the GALPROP code. The good agreement
between measured and predicted emission from keV to GeV energies suggests that
the correct production mechanisms have been identified. We discuss the
potential of the SPI data to provide an indirect probe of the interstellar
cosmic-ray electron distribution, in particular for energies below a few GeV.Comment: 39 pages, 11 figures. Accepted for publication in The Astrophysical
Journa
Discovery of a large set of SNP and SSR genetic markers by high-throughput sequencing of pepper (Capsicum annuum)
Genetic markers based on single nucleotide polymorphisms (SNPs) are in increasing demand for genome mapping and fingerprinting of breeding populations in crop plants. Recent advances in high-throughput sequencing provide the opportunity for whole-genome resequencing and identification of allelic variants by mapping the reads to a reference genome. However, for many species, such as pepper (Capsicum annuum), a reference genome sequence is not yet available. To this end, we sequenced the C. annuum cv. "Yolo Wonder" transcriptome using Roche 454 pyrosequencing and assembled de novo 23,748 isotigs and 60,370 singletons. Mapping of 10,886,425 reads obtained by the Illumina GA II sequencing of C. annuum cv. "Criollo de Morclos 334" to the "Yolo Wonder" transcriptome allowed for SNP identification. By setting a threshold value that allows selecting reliable SNPs with minimal loss of information, 11,849 reliable SNPs spread across 5919 isotigs were identified. In addition, 853 single sequence repeats were obtained. This information has been made available online
Simulations of the Microwave Sky and of its ``Observations''
Here follows a preliminary report on the construction of fake millimeter and
sub-millimeter skies, as observed by virtual instruments, e.g. the COBRA/SAMBA
mission, using theoretical modeling and data extrapolations. Our goal is to
create maps as realistic as possible of the relevant physical contributions
which may contribute to the detected signals. This astrophysical modeling is
followed by simulations of the measurement process itself by a given
instrumental configuration. This will enable a precise determination of what
can and cannot be achieved with a particular experimental configuration, and
provide a feedback on how to improve the overall design. It is a key step on
the way to define procedures for the separation of the different physical
processes in the future observed maps. Note that this tool will also prove
useful in preparing and analyzing current (\eg\ balloon borne) Microwave
Background experiments. Keywords: Cosmology -- Microwave Background
Anisotropies.Comment: 6 pages of uuencoded compressed postscript (1.2 Mb uncompressed), to
appear in the proceedings of the meeting "Far Infrared and Sub-millimeter
Space Missions in the Next Decade'', Paris, France, Eds. M. Sauvage, Space
Science Revie
Projection and Galaxy Clustering Fourier Spectra
Second order perturbation theory predicts a specific dependence of the
bispectrum, or three-point correlation function in the Fourier transform
domain, on the shape of the configuration of its three wave vector arguments,
which can be taken as a signature of structure formed by gravitational
instability. Comparing this known dependence on configuration shape with the
weak shape dependence of the galaxy bispectrum has been suggested as an
indication of bias in the galaxy distribution. However, to interpret results
obtained from projected catalogs, we must first understand the effects of
projection on this shape dependence. We present expressions for the projected
power spectrum and bispectrum in both Cartesian and spherical geometries, and
we examine the effects of projection on the predicted bispectrum with
particular attention to the dependence on configuration shape. Except for an
overall numerical factor, for Cartesian projection with characteristic depth
\Dstar there is little effect on the shape dependence of the bispectrum for
wavelengths small compared to \Dstar or projected wavenumbers q \Dstar
\gg 1 . For angular projection, a scaling law is found for spherical harmonic
index , but there is always a mixing of scales over the range of
the selection function. For large it is sufficient to examine a small
portion of the sky.Comment: aastex, 7 figure
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