7,655 research outputs found
A Convex Feature Learning Formulation for Latent Task Structure Discovery
This paper considers the multi-task learning problem and in the setting where
some relevant features could be shared across few related tasks. Most of the
existing methods assume the extent to which the given tasks are related or
share a common feature space to be known apriori. In real-world applications
however, it is desirable to automatically discover the groups of related tasks
that share a feature space. In this paper we aim at searching the exponentially
large space of all possible groups of tasks that may share a feature space. The
main contribution is a convex formulation that employs a graph-based
regularizer and simultaneously discovers few groups of related tasks, having
close-by task parameters, as well as the feature space shared within each
group. The regularizer encodes an important structure among the groups of tasks
leading to an efficient algorithm for solving it: if there is no feature space
under which a group of tasks has close-by task parameters, then there does not
exist such a feature space for any of its supersets. An efficient active set
algorithm that exploits this simplification and performs a clever search in the
exponentially large space is presented. The algorithm is guaranteed to solve
the proposed formulation (within some precision) in a time polynomial in the
number of groups of related tasks discovered. Empirical results on benchmark
datasets show that the proposed formulation achieves good generalization and
outperforms state-of-the-art multi-task learning algorithms in some cases.Comment: ICML201
Limits on the AGN activities in X-ray underluminous galaxy groups
We have observed four X-ray underluminous groups of galaxies using the Giant
Meterwave RadioTelescope. The groups NGC 524, 720, 3607, and 4697 are
underluminous in relation to the extrapolation of the Lx - T relation from rich
clusters and do not show any evidence of current AGN activities that can
account for such a departure. The GMRT observations carried out at low
frequencies (235 and 610 MHz) were aimed at detecting low surface brightness,
steep-spectrum sources indicative of past AGN activities in these groups. No
such radio emissions were detected in any of these four groups. The
corresponding upper limits on the total energy in relativistic particles is
about 3 X 10 erg. This value is more than a factor of 100 less than that
required to account for the decreased X-ray luminosities (or, enhanced
entropies) of these four groups in the AGN-heating scenario. Alternatively, the
AGN activity must have ceased about 4 Gyr ago, allowing the relativistic
particles to diffuse out to such a large extent (about 250 kpc) that their
radio emission could have been undetected by the current observations. If the
latter scenario is correct, the ICM was pre-heated before the assembly of
galaxy clusters.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter
Assessment of the dynamics of microparasite infections in genetically homogeneous and heterogeneous populations using a stochastic epidemic model
A molecular dynamics simulation of DNA damage induction by ionizing radiation
We present a multi-scale simulation of early stage of DNA damages by the
indirect action of hydroxyl (OH) free radicals generated by electrons
and protons. The computational method comprises of interfacing the Geant4-DNA
Monte Carlo with the ReaxFF molecular dynamics software. A clustering method
was employed to map the coordinates of OH-radicals extracted from the
ionization track-structures onto nano-meter simulation voxels filled with DNA
and water molecules. The molecular dynamics simulation provides the time
evolution and chemical reactions in individual simulation voxels as well as the
energy-landscape accounted for the DNA-OH chemical reaction that is
essential for the first principle enumeration of hydrogen abstractions,
chemical bond breaks, and DNA-lesions induced by collection of ions in clusters
less than the critical dimension which is approximately 2-3 \AA. We show that
the formation of broken bonds leads to DNA base and backbone damages that
collectively propagate to DNA single and double strand breaks. For illustration
of the methodology, we focused on particles with initial energy of 1 MeV. Our
studies reveal a qualitative difference in DNA damage induced by low energy
electrons and protons. Electrons mainly generate small pockets of
OH-radicals, randomly dispersed in the cell volume. In contrast,
protons generate larger clusters along a straight-line parallel to the
direction of the particle. The ratio of the total DNA double strand breaks
induced by a single proton and electron track is determined to be 4
in the linear scaling limit. The tool developed in this work can be used in the
future to investigate the relative biological effectiveness of light and heavy
ions that are used in radiotherapy.Comment: 7 pages, 7 figures, accepted for publication in Physics in Medicine
and Biolog
CP Violation and Dark Matter
A brief review is given of the effects of CP violation on the direct
detection of neutralinos in dark matter detectors. We first summarize the
current developments using the cancellation mechanism which allows for the
existence of large CP violating phases consistent with experimental limits on
the electron and on the neutron electric dipole moments in a broad class of
SUSY, string and D brane models. We then discuss their effects on the
scattering of neutralinos from quarks and on the event rates. It is found that
while CP effects on the event rates can be enormous such effects are reduced
significantly with the imposition of the EDM constraints. However, even with
the inclusion of the EDM constraints the effects are still very significant and
should be included in a precision prediction of event rates in any SUSY, string
or D brane model.Comment: Based on an invited talk at the conference "Sources and Detection of
Dark Matter in the Universe", at Marina del Rey, CA, Feb. 23-25, 2000; 12
pages, Latex including 2 figure
Excess entropy, Diffusivity and Structural Order in liquids with water-like anomalies
The excess entropy, Se, defined as the difference between the entropies of
the liquid and the ideal gas under identical density and temperature
conditions, is shown to be the critical quantity connecting the structural,
diffusional and density anomalies in water-like liquids. Based on simulations
of silica and the two-scale ramp liquids, water-like density and diffusional
anomalies can be seen as consequences of a characteristic non-monotonic density
dependence of Se. The relationship between excess entropy, the order metrics
and the structural anomaly can be understood using a pair correlation
approximation to Se.Comment: 9 pages, 5 figues in ps forma
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