101,945 research outputs found
On Newton Screening
Screening and working set techniques are important approaches to reducing the
size of an optimization problem. They have been widely used in accelerating
first-order methods for solving large-scale sparse learning problems. In this
paper, we develop a new screening method called Newton screening (NS) which is
a generalized Newton method with a built-in screening mechanism. We derive an
equivalent KKT system for the Lasso and utilize a generalized Newton method to
solve the KKT equations. Based on this KKT system, a built-in working set with
a relatively small size is first determined using the sum of primal and dual
variables generated from the previous iteration, then the primal variable is
updated by solving a least-squares problem on the working set and the dual
variable updated based on a closed-form expression. Moreover, we consider a
sequential version of Newton screening (SNS) with a warm-start strategy. We
show that NS possesses an optimal convergence property in the sense that it
achieves one-step local convergence. Under certain regularity conditions on the
feature matrix, we show that SNS hits a solution with the same signs as the
underlying true target and achieves a sharp estimation error bound with high
probability. Simulation studies and real data analysis support our theoretical
results and demonstrate that SNS is faster and more accurate than several
state-of-the-art methods in our comparative studies
Electrostatics of Vortices in Type II Superconductors
In a type II superconductor the gap variation in the core of a vortex line
induces a local charge modulation. Accounting for metallic screening, we
determine the line charge of individual vortices and calculate the electric
field distribution in the half space above a field penetrated superconductor.
The resulting field is that of an atomic size dipole , is the Bohr radius, acting
on a force microscope in the pico to femto Newton range.Comment: 9 pages, late
The XMM-Newton serendipitous survey. VII. The third XMM-Newton serendipitous source catalogue
Thanks to the large collecting area (3 x ~1500 cm at 1.5 keV) and wide
field of view (30' across in full field mode) of the X-ray cameras on board the
European Space Agency X-ray observatory XMM-Newton, each individual pointing
can result in the detection of hundreds of X-ray sources, most of which are
newly discovered. Recently, many improvements in the XMM-Newton data reduction
algorithms have been made. These include enhanced source characterisation and
reduced spurious source detections, refined astrometric precision, greater net
sensitivity and the extraction of spectra and time series for fainter sources,
with better signal-to-noise. Further, almost 50\% more observations are in the
public domain compared to 2XMMi-DR3, allowing the XMM-Newton Survey Science
Centre (XMM-SSC) to produce a much larger and better quality X-ray source
catalogue. The XMM-SSC has developed a pipeline to reduce the XMM-Newton data
automatically and using improved calibration a new catalogue version has been
produced from XMM-Newton data made public by 2013 Dec. 31 (13 years of data).
Manual screening ensures the highest data quality. This catalogue is known as
3XMM. In the latest release, 3XMM-DR5, there are 565962 X-ray detections
comprising 396910 unique X-ray sources. For the 133000 brightest sources,
spectra and lightcurves are provided. For all detections, the positions on the
sky, a measure of the quality of the detection, and an evaluation of the X-ray
variability is provided, along with the fluxes and count rates in 7 X-ray
energy bands, the total 0.2-12 keV band counts, and four hardness ratios. To
identify the detections, a cross correlation with 228 catalogues is also
provided for each X-ray detection. 3XMM-DR5 is the largest X-ray source
catalogue ever produced. Thanks to the large array of data products, it is an
excellent resource in which to find new and extreme objects.Comment: 23 pages, version accepted for publication in A&
Constraints on Shift-Symmetric Scalar-Tensor Theories with a Vainshtein Mechanism from Bounds on the Time Variation of G
We show that the current bounds on the time variation of the Newton constant
G can put severe constraints on many interesting scalar-tensor theories which
possess a shift symmetry and a nonminimal matter-scalar coupling. This
includes, in particular, Galileon-like models with a Vainshtein screening
mechanism. We underline that this mechanism, if efficient to hide the effects
of the scalar field at short distance and in the static approximation, can in
general not alter the cosmological time evolution of the scalar field. This
results in a locally measured time variation of G which is too large when the
matter-scalar coupling is of order one.Comment: RevTeX4 format; v.2: 5 pages, title changed, matches published
versio
Distributed Contingency Analysis over Wide Area Network among Dispatch Centers
Traditionally, a regional dispatch center uses the equivalent method to deal
with external grids, which fails to reflect the interactions among regions.
This paper proposes a distributed N-1 contingency analysis (DCA) solution,
where dispatch centers join a coordinated computation using their private data
and computing resources. A distributed screening method is presented to
determine the Critical Contingency Set (DCCS) in DCA. Then, the distributed
power flow is formulated as a set of boundary equations, which is solved by a
Jacobi-Free Newton-GMRES (JFNG) method. During solving the distributed power
flow, only boundary conditions are exchanged. Acceleration techniques are also
introduced, including reusing preconditioners and optimal resource scheduling
during parallel processing of multiple contingencies. The proposed method is
implemented on a real EMS platform, where tests using the Southwest Regional
Grid of China are carried out to validate its feasibility.Comment: 5 pages, 6 figures, 2017 IEEE PES General Meetin
Gravitational Stability and Screening Effect from D Extra Timelike Dimensions
We study (3+1)+D dimensional spacetime, where D extra dimensions are
timelike. Compactification of the D timelike dimensions leads to tachyonic
Kaluza-Klein gravitons. We calculate the gravitational self-energies of massive
spherical bodies due to the tachyonic exchange, discuss their stability, and
find that the gravitational force is screened in a certain number of the extra
dimensions. We also derive the exact relationship between the Newton constants
in the full 4+D dimensional spacetime with the D extra times and the ordinary
Newton constant of our 4 dimensional world.Comment: harvmac, 20 pages, typos corrected, refs. added and correcte
Semismooth Newton Coordinate Descent Algorithm for Elastic-Net Penalized Huber Loss Regression and Quantile Regression
We propose an algorithm, semismooth Newton coordinate descent (SNCD), for the
elastic-net penalized Huber loss regression and quantile regression in high
dimensional settings. Unlike existing coordinate descent type algorithms, the
SNCD updates each regression coefficient and its corresponding subgradient
simultaneously in each iteration. It combines the strengths of the coordinate
descent and the semismooth Newton algorithm, and effectively solves the
computational challenges posed by dimensionality and nonsmoothness. We
establish the convergence properties of the algorithm. In addition, we present
an adaptive version of the "strong rule" for screening predictors to gain extra
efficiency. Through numerical experiments, we demonstrate that the proposed
algorithm is very efficient and scalable to ultra-high dimensions. We
illustrate the application via a real data example
XMM-Newton observation of the deep minimum state of PG 2112+059: A spectrum dominated by reflection from the accretion disk?
We analyse a 75ks XMM-Newton observation of PG 2112+059 performed in November
2005 and compare it with a 15ks XMM-Newton observation taken in May 2003. PG
2112+059 was found in a deep minimum state as its 0.2-12 keV flux decreased by
a factor of 10 in comparison to the May 2003 observation. During the deep
minimum state the spectra show strong emission in excess of the continuum in
the 3-6 keV region. The excess emission corresponds to an EW = 26.1 keV whereas
its shape resembles that of heavily absorbed objects. The spectra of both
observations of PG 2112+059 can be explained statistically by a combination of
two absorbers where one shows a high column density, , and the other high ionisation parameters. As the ionisation
parameter of the high flux state, , is lower than
the value found for the deep minimum state, ,
either the absorbers are physically different or the absorbing material is
moving with respect to the X-ray source. The spectra can also be explained by a
continuum plus X-ray ionised reflection on the accretion disk, seen behind a
warm absorber. The ionisation parameter of the high state () is higher than the ionisation parameter of the deep minimum state
(), as expected for a stationary absorber. The
values found for the ionisation parameters are in the range typical for AGNs.
The spectra observed during the deep minimum state are reflection dominated and
show no continuum emission. These can be understood in the context of light
bending near the supermassive black hole as predicted by Minutti and Fabian.Comment: 13 pages, 3 figures, A&A latex, accepted for publication in Astronomy
& Astrophysic
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