39,023 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
On the frequency and remnants of Hypernovae
Under the hypothesis that some fraction of massive stellar core collapses
give rise to unusually energetic events, termed hypernovae, I examine the
required rates assuming some fraction of such events yield gamma ray bursts. I
then discuss evidence from studies of pulsars and r-process nucleosynthesis
that independently suggests the existence of a class of unusually energetic
events. Finally I describe a scenario which links these different lines of
evidence as supporting the hypernova hypothesis.Comment: TeX, To appear in ApJ Letter
Resonant absorption at the vortex-core states in d-wave superconductors
We predict a resonant microwave absorption on collective vortex modes in a
superclean d-wave superconductor. Energies of the collective modes are
multiples of the distance between the exact quantum levels of bound states in
the vortex core at lower temperatures and involve delocalized states for higher
temperatures. We calculate the vortex mass in a d-wave superconductor as a
response to a slow acceleration of the vortex. The universal flux-flow regime
predicted by N. Kopnin and G. Volovik [Phys. Rev. Lett. 79, 1377 (1997)] is
discussed in more detail.Comment: RevTex file, 10 page
Comparability of the performance of in-line computer vision for geometrical verification of parts, produced by Additive Manufacturing
Estimate solar contribution to the global surface warming using the ACRIM TSI satellite composite
We study, by using a wavelet decomposition methodology, the solar signature
on global surface temperature data using the ACRIM total solar irradiance
satellite composite by Willson and Mordvinov. These data present a
+0.047%/decade trend between minima during solar cycles 21-23 (1980-2002). We
estimate that the ACRIM upward trend might have minimally contributed
10-30% of the global surface temperature warming over the period
1980-2002
Real-Time Data Processing in the Muon System of the D0 Detector
This paper presents a real-time application of the 16-bit fixed point Digital
Signal Processors (DSPs), in the Muon System of the D0 detector located at the
Fermilab Tevatron, presently the world's highest-energy hadron collider. As
part of the Upgrade for a run beginning in the year 2000, the system is
required to process data at an input event rate of 10 KHz without incurring
significant deadtime in readout. The ADSP21csp01 processor has high I/O
bandwidth, single cycle instruction execution and fast task switching support
to provide efficient multisignal processing. The processor's internal memory
consists of 4K words of Program Memory and 4K words of Data Memory. In addition
there is an external memory of 32K words for general event buffering and 16K
words of Dual Port Memory for input data queuing. This DSP fulfills the
requirement of the Muon subdetector systems for data readout. All error
handling, buffering, formatting and transferring of the data to the various
trigger levels of the data acquisition system is done in software. The
algorithms developed for the system complete these tasks in about 20
microseconds per event.Comment: 4 pages, Presented and published at the 11th IEEE NPSS Real Time
Conference, held at Santa Fe, New Mexico, USA, from June 14-18, 199
The short-time self-diffusion coefficient of a sphere in a suspension of rigid rods
The short--time self diffusion coefficient of a sphere in a suspension of
rigid rods is calculated in first order in the rod volume fraction. For low rod
concentrations the correction to the Einstein diffusion constant of the sphere
is a linear function of the rod volume fraction with the slope proportional to
the equilibrium averaged mobility diminution trace of the sphere interacting
with a single freely translating and rotating rod. The two--body hydrodynamic
interactions are calculated using the so--called bead model in which the rod is
replaced by a stiff linear chain of touching spheres. The interactions between
spheres are calculated numerically using the multipole method. Also an
analytical expression for the diffusion coefficient as a function of the rod
aspect ratio is derived in the limit of very long rods. We show that in this
limit the correction to the Einstein diffusion constant does not depend on the
size of the tracer sphere. The higher order corrections depending on the
applied model are computed numerically. An approximate expression is provided,
valid for a wide range of aspect ratios.Comment: 11 pages, 6 figure
Parity Violation in Neutrino Transport and the Origin of Pulsar Kicks
In proto-neutron stars with strong magnetic fields, the neutrino-nucleon
scattering/absorption cross sections depend on the direction of neutrino
momentum with respect to the magnetic field axis, a manifestation of parity
violation in weak interactions. We study the deleptonization and thermal
cooling (via neutrino emission) of proto-neutron stars in the presence of such
asymmetric neutrino opacities. Significant asymmetry in neutrino emission is
obtained due to multiple neutrino-nucleon scatterings. For an ordered magnetic
field threading the neutron star interior, the fractional asymmetry in neutrino
emission is about , corresponding to a pulsar kick velocity
of about km/s for a total radiated neutrino energy of
erg.Comment: AASTeX, 10 pages including 2 ps figures; ApJ Letter in press (March
10, 1998). Shortened to agree with the published versio
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