3,139 research outputs found
A Fast Algorithm for Robust Regression with Penalised Trimmed Squares
The presence of groups containing high leverage outliers makes linear
regression a difficult problem due to the masking effect. The available high
breakdown estimators based on Least Trimmed Squares often do not succeed in
detecting masked high leverage outliers in finite samples.
An alternative to the LTS estimator, called Penalised Trimmed Squares (PTS)
estimator, was introduced by the authors in \cite{ZiouAv:05,ZiAvPi:07} and it
appears to be less sensitive to the masking problem. This estimator is defined
by a Quadratic Mixed Integer Programming (QMIP) problem, where in the objective
function a penalty cost for each observation is included which serves as an
upper bound on the residual error for any feasible regression line. Since the
PTS does not require presetting the number of outliers to delete from the data
set, it has better efficiency with respect to other estimators. However, due to
the high computational complexity of the resulting QMIP problem, exact
solutions for moderately large regression problems is infeasible.
In this paper we further establish the theoretical properties of the PTS
estimator, such as high breakdown and efficiency, and propose an approximate
algorithm called Fast-PTS to compute the PTS estimator for large data sets
efficiently. Extensive computational experiments on sets of benchmark instances
with varying degrees of outlier contamination, indicate that the proposed
algorithm performs well in identifying groups of high leverage outliers in
reasonable computational time.Comment: 27 page
Environmental Effect on the Associations of Background Quasars with Foreground Objects: II. Numerical Simulations
Using numerical simulations of cluster formation in the standard CDM model
(SCDM) and in a low-density, flat CDM model with a cosmological constant
(LCDM), we investigate the gravitational lensing explanation for the reported
associations between background quasars and foreground clusters. Under the
thin-lens approximation and the unaffected background hypothesis , we show that
the recently detected quasar overdensity around clusters of galaxies on scales
of arcminutes cannot be interpreted as a result of the gravitational
lensing by cluster matter and/or by their environmental and projected matter
along the line of sight, which is consistent with the analytical result based
on the observed cluster and galaxy correlations (Wu, et al. 1996). It appears
very unlikely that uncertainties in the modeling of the gravitational lensing
can account for the disagreement between the theoretical predictions and the
observations. We conclude that either the detected signal of the quasar-cluster
associations is a statistical fluke or the associations are are generated by
mechanisms other than the magnification bias.Comment: 15 pages, 5 figures, accepted for publication in Ap
Recommended from our members
Influences of increasing temperature on Indian wheat: quantifying limits to predictability
As climate changes, temperatures will play an increasing role in determining crop yield. Both
climate model error and lack of constrained physiological thresholds limit the predictability of
yield. We used a perturbed-parameter climate model ensemble with two methods of
bias-correction as input to a regional-scale wheat simulation model over India to examine
future yields. This model configuration accounted for uncertainty in climate, planting date,
optimization, temperature-induced changes in development rate and reproduction. It also
accounts for lethal temperatures, which have been somewhat neglected to date. Using
uncertainty decomposition, we found that fractional uncertainty due to temperature-driven
processes in the crop model was on average larger than climate model uncertainty (0.56 versus
0.44), and that the crop model uncertainty is dominated by crop development. Simulations
with the raw compared to the bias-corrected climate data did not agree on the impact on future
wheat yield, nor its geographical distribution. However the method of bias-correction was not
an important source of uncertainty. We conclude that bias-correction of climate model data
and improved constraints on especially crop development are critical for robust impact
predictions
On the Lebesgue measure of Li-Yorke pairs for interval maps
We investigate the prevalence of Li-Yorke pairs for and
multimodal maps with non-flat critical points. We show that every
measurable scrambled set has zero Lebesgue measure and that all strongly
wandering sets have zero Lebesgue measure, as does the set of pairs of
asymptotic (but not asymptotically periodic) points.
If is topologically mixing and has no Cantor attractor, then typical
(w.r.t. two-dimensional Lebesgue measure) pairs are Li-Yorke; if additionally
admits an absolutely continuous invariant probability measure (acip), then
typical pairs have a dense orbit for . These results make use of
so-called nice neighborhoods of the critical set of general multimodal maps,
and hence uniformly expanding Markov induced maps, the existence of either is
proved in this paper as well.
For the setting where has a Cantor attractor, we present a trichotomy
explaining when the set of Li-Yorke pairs and distal pairs have positive
two-dimensional Lebesgue measure.Comment: 41 pages, 3 figure
Structure function of the UV variability of Q0957+561
We present a detailed structure function analysis of the UV variability of
Q0957+561. From new optical observations, we constructed normalized structure
functions of the quasar luminosity at restframe wavelengths of 2100 and 2600
\AA. Old optical records also allow the structure function to be obtained at
2100 \AA, but 10 years ago in the observer's frame. These three structure
functions are then compared to predictions of both simple and relatively
sophisticated (incorporating two independent variable components) Poissonian
models. We do not find clear evidence of a chromatic mechanism of variability.
From the recent data, 100-d time-symmetric and 170-d time-asymmetric flares
are produced at both restframe wavelengths. Taking into account measurements of
time delays and the existence of an EUV/radio jet, reverberation is probably
the main mechanism of variability. Thus, two types of EUV/X-ray fluctuations
would be generated within or close to the jet and later reprocessed by the disc
gas in the two emission rings. The 100-d time-symmetric shots are also
responsible for most of the 2100 \AA variability detected in the old
experiment. However, there is no evidence of asymmetric shots in the old UV
variability. If reverberation is the involved mechanism of variability, this
could mean an intermittent production of high-energy asymmetric fluctuations.
The old records are also consistent with the presence of very short-lifetime
(10 d) symmetric flares, which may represent additional evidence of time
evolution. We also discuss the quasar structure that emerges from the
variability scenario.Comment: 7 pages, 4 figures. Accepted for publication in A&A (based on the
brightness records at http://arxiv.org/abs/0810.4619
Habitat filtering determines spatial variation of macroinvertebrate community traits in northern headwater streams
Although our knowledge of the spatial distribution of stream organisms has been increasing rapidly in the last decades, there is still little consensus about trait-based variability of macroinvertebrate communities within and between catchments in near-pristine systems. Our aim was to examine the taxonomic and trait based stability vs. variability of stream macroinvertebrates in three high-latitude catchments in Finland. The collected taxa were assigned to unique trait combinations (UTCs) using biological traits. We found that only a single or a highly limited number of taxa formed a single UTC, suggesting a low degree of redundancy. Our analyses revealed significant differences in the environmental conditions of the streams among the three catchments. Linear models, rarefaction curves and beta-diversity measures showed that the catchments differed in both alpha and beta diversity. Taxon- and trait-based multivariate analyses also indicated that the three catchments were significantly different in terms of macroinvertebrate communities. All these findings suggest that habitat filtering, i.e., environmental differences among catchments, determines the variability of macroinvertebrate communities, thereby contributing to the significant biological differences among the catchments. The main implications of our study is that the sensitivity of trait-based analyses to natural environmental variation should be carefully incorporated in the assessment of environmental degradation, and that further studies are needed for a deeper understanding of trait-based community patterns across near-pristine streams
Palaeomagnetic field intensity measurements from the 2.6 Ga Yandinilling dyke swarm (Western Australia)
SUMMARY
Precambrian palaeointensity measurements provide fundamental constraints on the evolution of the deep Earth. Core evolution models predict trends in dipole moment on billion-year timescales that can be tested by palaeomagnetic records. Here, we report new palaeointensity results from the recently identified âŒ2.62âGa Yandinilling dyke swarm of the Yilgarn Craton, Western Australia, and consider them alongside published measurements spanning 500âMyr across the late Archaean to earliest Proterozoic. Rock magnetic and scanning electron microscopy analysis confirm that the magnetic mineralogy is fine-grained magnetite, appearing mostly as exsolved lamellae with ilmenite. Six sites produced acceptable palaeointensity estimates from thermal and microwave IZZI protocol Thellier experiments and from double-heating technique Shaw experiments. These site mean values of 9â26 ”T translate to virtual dipole moments of 11â44 ZAm2 that are considerably lower than today's dipole moment of âŒ80 ZAm2 and the value predicted for this time period by some thermal evolution models. Their average (median = 41 ZAm2) is, however, similar to the long-term average during both of the intervals 2300â2800 Ma (median = 44 ZAm2; N = 103) and 10â500 Ma (median 41 ZAm2; N = 997). While there is little evidence for a substantial net change in average dipole moment between the late Archaean and Phanerozoic, there is preliminary evidence that its variance has increased between the two intervals. This lower variance more than two billion years ago supports the idea that the geodynamo, even while not producing a stronger magnetic field, was more stable on average at the ArchaeanâProterozoic transition than it is today.</jats:p
All-fibre heterogeneously-integrated frequency comb generation using silicon core fibre.
Originally developed for metrology, optical frequency combs are becoming increasingly pervasive in a wider range of research topics including optical communications, spectroscopy, and radio or microwave signal processing. However, application demands in these fields can be more challenging as they require compact sources with a high tolerance to temperature variations that are capable of delivering flat comb spectra, high power per tone, narrow linewidth and high optical signal-to-noise ratio. This work reports the generation of a flat, high power frequency comb in the telecom band using a 17âmm fully-integrated silicon core fibre as a parametric mixer. Our all-fibre, cavity-free source combines the material benefits of planar waveguide structures with the advantageous properties of fibre platforms to achieve a 30 nm bandwidth comb source containing 143 tones with 30 dB OSNR over the entire spectral region
Increased Programmed Death-1 Molecule Expression in Cytomegalovirus Disease and Acute Graft-versus-Host Disease after Allogeneic Hematopoietic Cell Transplantation
To study the role of the programmed death-1 molecule (PD-1) in cytomegalovirus (CMV) infection and disease after allogeneic hematopoietic cell transplantation (HCT), 206 subjects were followed prospectively for immune response to CMV and assigned to 3 groups based on CMV outcome. The subjects were analyzed retrospectively for PD-1 expression in cryopreserved CD4+ and CD8+T cells collected at days 40, 90, 120, 150, 180, and 360 posttransplantation. HCT recipients with CMV disease (n=14) were compared with recipients with prolonged CMV infection, but no CMV disease (median duration of infection, 3 months; n=14) and with controls with no CMV infection who received similar transplants (n=22). The CMV disease group had a significantly higher mean fluorescein intensity of PD-1 in CD4+ (P < .05) and CD8+ (PÂ < .05) lymphocytes at all time points studied. PD-1 expression also was significantly elevated in those with severe acute graft-versus-host disease (aGVHD), including the no-viremia group. The data suggest that PD-1 is induced by aGVHD even in the absence of CMV infection. This enhanced PD-1 expression during severe aGVHD and with CMV reactivation could explain the known role of aGVHD as a risk factor for CMV disease
- âŠ