3,363 research outputs found
Computerized Searches on Articles Reporting Reading Research: A Closer Look
This article, which draws on examples supplied by previous studies involving fifty articles in the psycholinguistic literature [ 4], [5], [6], explores how the choice of terminology used in database records and in searches may actually impede retrieval, and it suggests compensatory measures
Entire solutions of hydrodynamical equations with exponential dissipation
We consider a modification of the three-dimensional Navier--Stokes equations
and other hydrodynamical evolution equations with space-periodic initial
conditions in which the usual Laplacian of the dissipation operator is replaced
by an operator whose Fourier symbol grows exponentially as \ue ^{|k|/\kd} at
high wavenumbers . Using estimates in suitable classes of analytic
functions, we show that the solutions with initially finite energy become
immediately entire in the space variables and that the Fourier coefficients
decay faster than \ue ^{-C(k/\kd) \ln (|k|/\kd)} for any . The
same result holds for the one-dimensional Burgers equation with exponential
dissipation but can be improved: heuristic arguments and very precise
simulations, analyzed by the method of asymptotic extrapolation of van der
Hoeven, indicate that the leading-order asymptotics is precisely of the above
form with . The same behavior with a universal constant
is conjectured for the Navier--Stokes equations with exponential
dissipation in any space dimension. This universality prevents the strong
growth of intermittency in the far dissipation range which is obtained for
ordinary Navier--Stokes turbulence. Possible applications to improved spectral
simulations are briefly discussed.Comment: 29 pages, 3 figures, Comm. Math. Phys., in pres
Evaluation of beneficial and inhibitory effects of nitrate on nodulation and nitrogen fixation in common bean (Phaseolus vulgaris)
AbstractThe effects of applied nitrate on symbiotic nitrogen fixation in legumes are complex. Both inhibition and promotion of nodulation by nitrate have been observed in a dose‐dependent manner. The objectives of this study were to determine the effects of nitrate at different concentrations on root nodulation in different genotypes in common bean (Phaseolus vulgaris). Six genotypes were inoculated with the same rhizobial strain and grown hydroponically in growth pouches in a growth chamber and exposed to six nitrate concentrations, including 0, 2.5, 5, 10, 15, and 20 mM for 4 weeks. The tested genotypes included three recombinant inbred lines (RILs, 25, 46, and 70) that differed in their responses to nitrogen (based on observations of one field growing season), their parents (Mist and Sanilac—registered varieties), which are different in N‐fixing abilities, and one nonnodulating mutant (R99). Our results showed that small amounts of nitrate (2.5 and 5 mM) promoted nodule formation and increased nodule biomass, compared with plants in the 0 nitrate control treatment. In contrast, nitrate concentrations over 10 mM inhibited nodulation, resulting in reductions in nodule number and nodule biomass. Nodulation was completely inhibited by 15‐mM nitrate in all the genotypes. Regression analyses indicated that 5‐mM nitrate is the optimum concentration for promoting nodulation as measured by the total number of nodules formed, the number of effective nodules formed, and the nodule biomass formed. In contrast, nitrogen fixation was inhibited by all levels of nitrate. No genotypic differences were observed in nodulation among the three RILs and their parental cultivars, but all were significantly different than R99, a nonnodulating mutant
Image reconstruction in optical interferometry: Benchmarking the regularization
With the advent of infrared long-baseline interferometers with more than two
telescopes, both the size and the completeness of interferometric data sets
have significantly increased, allowing images based on models with no a priori
assumptions to be reconstructed. Our main objective is to analyze the multiple
parameters of the image reconstruction process with particular attention to the
regularization term and the study of their behavior in different situations.
The secondary goal is to derive practical rules for the users. Using the
Multi-aperture image Reconstruction Algorithm (MiRA), we performed multiple
systematic tests, analyzing 11 regularization terms commonly used. The tests
are made on different astrophysical objects, different (u,v) plane coverages
and several signal-to-noise ratios to determine the minimal configuration
needed to reconstruct an image. We establish a methodology and we introduce the
mean-square errors (MSE) to discuss the results. From the ~24000 simulations
performed for the benchmarking of image reconstruction with MiRA, we are able
to classify the different regularizations in the context of the observations.
We find typical values of the regularization weight. A minimal (u,v) coverage
is required to reconstruct an acceptable image, whereas no limits are found for
the studied values of the signal-to-noise ratio. We also show that
super-resolution can be achieved with increasing performance with the (u,v)
coverage filling. Using image reconstruction with a sufficient (u,v) coverage
is shown to be reliable. The choice of the main parameters of the
reconstruction is tightly constrained. We recommend that efforts to develop
interferometric infrastructures should first concentrate on the number of
telescopes to combine, and secondly on improving the accuracy and sensitivity
of the arrays.Comment: 15 pages, 16 figures; accepted in A&
Constraining Disk Parameters of Be Stars using Narrowband H-alpha Interferometry with the NPOI
Interferometric observations of two well-known Be stars, gamma Cas and phi
Per, were collected and analyzed to determine the spatial characteristics of
their circumstellar regions. The observations were obtained using the Navy
Prototype Optical Interferometer equipped with custom-made narrowband filters.
The filters isolate the H-alpha emission line from the nearby continuum
radiation, which results in an increased contrast between the interferometric
signature due to the H-alpha-emitting circumstellar region and the central
star. Because the narrowband filters do not significantly attenuate the
continuum radiation at wavelengths 50 nm or more away from the line, the
interferometric signal in the H-alpha channel is calibrated with respect to the
continuum channels. The observations used in this study represent the highest
spatial resolution measurements of the H-alpha-emitting regions of Be stars
obtained to date. These observations allow us to demonstrate for the first time
that the intensity distribution in the circumstellar region of a Be star cannot
be represented by uniform disk or ring-like structures, whereas a Gaussian
intensity distribution appears to be fully consistent with our observations.Comment: 23 pages, 14 figures, accepted for publication in A
A measure of centrality based on the spectrum of the Laplacian
We introduce a family of new centralities, the k-spectral centralities.
k-Spectral centrality is a measurement of importance with respect to the
deformation of the graph Laplacian associated with the graph. Due to this
connection, k-spectral centralities have various interpretations in terms of
spectrally determined information.
We explore this centrality in the context of several examples. While for
sparse unweighted networks 1-spectral centrality behaves similarly to other
standard centralities, for dense weighted networks they show different
properties. In summary, the k-spectral centralities provide a novel and useful
measurement of relevance (for single network elements as well as whole
subnetworks) distinct from other known measures.Comment: 12 pages, 6 figures, 2 table
Nonattacking Queens in a Rectangular Strip
The function that counts the number of ways to place nonattacking identical
chess or fairy chess pieces in a rectangular strip of fixed height and variable
width, as a function of the width, is a piecewise polynomial which is
eventually a polynomial and whose behavior can be described in some detail. We
deduce this by converting the problem to one of counting lattice points outside
an affinographic hyperplane arrangement, which Forge and Zaslavsky solved by
means of weighted integral gain graphs.
We extend their work by developing both generating functions and a detailed
analysis of deletion and contraction for weighted integral gain graphs.
For chess pieces we find the asymptotic probability that a random
configuration is nonattacking, and we obtain exact counts of nonattacking
configurations of small numbers of queens, bishops, knights, and nightriders.Comment: 21 pages, 3 figures, preprint of published version copyright Springer
Basel AG 2011, Published online February 15, 2011, submitted March 15, 200
The Nature of the Molecular Environment within 5 pc of the Galactic Center
We present a detailed study of molecular gas in the central 10pc of the
Galaxy through spectral line observations of four rotation inversion
transitions of NH3 made with the VLA. Updated line widths and NH3(1,1)
opacities are presented, and temperatures, column densities, and masses are
derived. We examine the impact of Sgr A East on molecular material at the
Galactic center and find that there is no evidence that the expansion of this
shell has moved a significant amount of the 50 km/s GMC. The western streamer,
however, shows strong indications that it is composed of material swept-up by
the expansion of Sgr A East. Using the mass and kinematics of the western
streamer, we calculate an energy of E=(2-9)x10^{51} ergs for the progenitor
explosion and conclude that Sgr A East was most likely produced by a single
supernova. The temperature structure of molecular gas in the central ~20pc is
also analyzed in detail. We find that molecular gas has a ``two-temperature''
structure similar to that measured by Huttemeister et al. (2003a) on larger
scales. The largest observed line ratios, however, cannot be understood in
terms of a two-temperature model, and most likely result from absorption of
NH3(3,3) emission by cool surface layers of clouds. By comparing the observed
NH3 (6,6)-to-(3,3) line ratios, we disentangle three distinct molecular
features within a projected distance of 2pc from Sgr A*. Gas associated with
the highest line ratios shows kinematic signatures of both rotation and
expansion. The southern streamer shows no significant velocity gradients and
does not appear to be directly associated with either the circumnuclear disk or
the nucleus. The paper concludes with a discussion of the line-of-sight
arrangement of the main features in the central 10pc.Comment: 51 pages, 16 figures, accepted for publication in ApJ. Due to size
limitations, some of the images have been cut from this version. A complete,
color PS or PDF version can be downloaded from
http://www.astro.columbia.edu/~herrnstein/NH3/paper
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