30,525 research outputs found
An interior point algorithm for minimum sum-of-squares clustering
Copyright @ 2000 SIAM PublicationsAn exact algorithm is proposed for minimum sum-of-squares nonhierarchical clustering, i.e., for partitioning a given set of points from a Euclidean m-space into a given number of clusters in order to minimize the sum of squared distances from all points to the centroid of the cluster to which they belong. This problem is expressed as a constrained hyperbolic program in 0-1 variables. The resolution method combines an interior point algorithm, i.e., a weighted analytic center column generation method, with branch-and-bound. The auxiliary problem of determining the entering column (i.e., the oracle) is an unconstrained hyperbolic program in 0-1 variables with a quadratic numerator and linear denominator. It is solved through a sequence of unconstrained quadratic programs in 0-1 variables. To accelerate resolution, variable neighborhood search heuristics are used both to get a good initial solution and to solve quickly the auxiliary problem as long as global optimality is not reached. Estimated bounds for the dual variables are deduced from the heuristic solution and used in the resolution process as a trust region. Proved minimum sum-of-squares partitions are determined for the rst time for several fairly large data sets from the literature, including Fisher's 150 iris.This research was supported by the Fonds
National de la Recherche Scientifique Suisse, NSERC-Canada, and FCAR-Quebec
Machine Learning Classification of SDSS Transient Survey Images
We show that multiple machine learning algorithms can match human performance
in classifying transient imaging data from the Sloan Digital Sky Survey (SDSS)
supernova survey into real objects and artefacts. This is a first step in any
transient science pipeline and is currently still done by humans, but future
surveys such as the Large Synoptic Survey Telescope (LSST) will necessitate
fully machine-enabled solutions. Using features trained from eigenimage
analysis (principal component analysis, PCA) of single-epoch g, r and
i-difference images, we can reach a completeness (recall) of 96 per cent, while
only incorrectly classifying at most 18 per cent of artefacts as real objects,
corresponding to a precision (purity) of 84 per cent. In general, random
forests performed best, followed by the k-nearest neighbour and the SkyNet
artificial neural net algorithms, compared to other methods such as na\"ive
Bayes and kernel support vector machine. Our results show that PCA-based
machine learning can match human success levels and can naturally be extended
by including multiple epochs of data, transient colours and host galaxy
information which should allow for significant further improvements, especially
at low signal-to-noise.Comment: 14 pages, 8 figures. In this version extremely minor adjustments to
the paper were made - e.g. Figure 5 is now easier to view in greyscal
Chromosomal Gains and Losses in Uveal Melanomas Detected by Comparative Genomic Hybridization
Eleven uveal melanomas were analyzed using comparative genomic hybridization (CGH). The most abundant genetic changes were loss of chromosome 3, overrepresentation of 6p, loss of 6q, and multiplication of 8q. The smallest overrepresented regions on 6p and 8q were 6pterp21 and 8q24qter, respectively. Several additional gains and losses of chromosome segments were repeatedly observed, the most frequent one being loss of 9p (three cases). Monosomy 3 appeared to be a marker for ciliary body involvement.
CGH data were compared with the results of chromosome banding. Some alterations, e.g., gains of 6p and losses of 6q, were observed with higher frequencies after CGH, while others, e.g., 9p deletions, were detected only by CGH. The data suggest some similarities of cytogenetic alterations between cutaneous and uveal melanoma. In particular, the 9p deletions are of interest due to recent reports about the location of a putative tumor-suppressor gene for cutaneous malignant melanoma in this region
Metal-Insulator-Like Behavior in Semimetallic Bismuth and Graphite
When high quality bismuth or graphite crystals are placed in a magnetic field
directed along the c-axis (trigonal axis for bismuth) and the temperature is
lowered, the resistance increases as it does in an insulator but then
saturates. We show that the combination of unusual features specific to
semimetals, i.e., low carrier density, small effective mass, high purity, and
an equal number of electrons and holes (compensation), gives rise to a unique
ordering and spacing of three characteristic energy scales, which not only is
specific to semimetals but which concomitantly provides a wide window for the
observation of apparent field induced metal-insulator behavior. Using
magnetotransport and Hall measurements, the details of this unusual behavior
are captured with a conventional multi-band model, thus confirming the
occupation by semimetals of a unique niche between conventional metals and
semiconductors.Comment: 4 pages, 4 figs, data and discussion on bismuth added, final
published versio
1/m_Q Corrections to the Heavy-to-Light-Vector Transitions in the HQET
Within the HQET, the heavy to light vector meson transitions are
systematically analyzed to the order of 1/m_Q. Besides the four universal
functions at the leading order, there are twenty-two independent universal form
factors at the order of 1/m_Q. Both the semileptonic decay B->\rho which is
relevant to the |V_{ub}| extraction, and the penguin induced decay B -> K^*
which is important to new physics discovering, depend on these form factors.
Phenomenological implications are discussed.Comment: RevTeX, 9 pages, no figure
Stripe State in the Lowest Landau Level
The stripe state in the lowest Landau level is studied by the density matrix
renormalization group (DMRG) method. The ground state energy and pair
correlation functions are systematically calculated for various
pseudopotentials in the lowest Landau level. We show that the stripe state in
the lowest Landau level is realized only in a system whose width perpendicular
to the two-dimensional electron layer is smaller than the order of magnetic
length.Comment: 4 pages, 6 figures, to appear in J. Phys. Soc. Jpn. vol.73 No.1
(2004
SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with Extreme X-ray Variability
We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS
J. It has a black hole mass of
measured from reverberation mapping (RM), and the black hole is accreting with
a super-Eddington accretion rate. Its XMM-Newton observation in 2015 May
reveals a flux drop by a factor of with respect to the Swift
observation in 2013 May when it showed a typical level of X-ray emission
relative to its UV/optical emission. The lack of correlated UV variability
results in a steep X-ray-to-optical power-law slope () of
-1.97 in the low X-ray flux state, corresponding to an X-ray weakness factor of
36.2 at rest-frame 2 keV relative to its UV/optical luminosity. The mild
UV/optical continuum and emission-line variability also suggest that the
accretion rate did not change significantly. A single power-law model modified
by Galactic absorption describes well the keV spectra of the X-ray
observations in general. The spectral fitting reveals steep spectral shapes
with . We search for active galactic nuclei (AGNs) with such
extreme X-ray variability in the literature and find that most of them are
narrow-line Seyfert 1 galaxies and quasars with high accretion rates. The
fraction of extremely X-ray variable objects among super-Eddington accreting
AGNs is estimated to be . We discuss two possible scenarios,
disk reflection and partial covering absorption, to explain the extreme X-ray
variability of SDSS J. We propose a possible origin for the
partial covering absorber, which is the thick inner accretion disk and its
associated outflow in AGNs with high accretion rates.Comment: 15 pages, 9 figures, accepted for publication in Ap
Effective Mass of the Four Flux Composite Fermion at
We have measured the effective mass () of the four flux composite
fermion at Landau level filling factor (CF), using the
activation energy gaps at the fractional quantum Hall effect (FQHE) states
= 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de
Haas (SdH) oscillations around . We find that the energy gaps show a
linear dependence on the effective magnetic field (), and from this linear dependence we obtain and
a disorder broadening 1 K for a sample of density /cm. The deduced from the temperature dependence of
the SdH effect shows large differences for and . For
, . It scales as with the mass
derived from the data around and shows an increase in as , resembling the findings around . For ,
increases rapidly with increasing and can be described by . This anomalous dependence on is
precursory to the formation of the insulating phase at still lower filling.Comment: 5 pages, 3 figure
Interaction of Individual Skyrmions in Nanostructured Cubic Chiral Magnet
We report the direct evidence of field-dependent character of the interaction
between individual magnetic skyrmions as well as between skyrmions and edges in
B20-type FeGe nanostripes observed by means of high resolution Lorentz
transmission electron microscopy. It is shown that above certain critical
values of external magnetic field the character of such long-range skyrmion
interactions change from attraction to repulsion. Experimentally measured
equilibrium inter-skyrmion and skrymion-edge distances as function of applied
magnetic field shows quantitative agreement with the results of micromagnetic
simulations. Important role of demagnetizing fields and internal symmetry of
three-dimensional magnetic skyrmions are discussed in details.Comment: accepted in PR
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