31,914 research outputs found
Multi-view 3D retrieval using silhouette intersection and multi-scale contour representation
We describe in this paper two methods for 3D shape indexing
and retrieval that we apply on two data collections
of the SHREC - SHape Retrieval Contest 2007: Watertight
models and 3D CAD models. Both methods are based on a
set of 2D multi-views after a pose and scale normalization
of the models using PCA and the enclosing sphere. In all
views we extract the models silhouettes and compare them
pairwise. In the first method the similitude measure is obtained by integrating on the pairs of views the difference between the areas of the silhouettes union and the silhouettes intersection. In the second method we consider the external contour of the silhouettes, extract their convexities and concavities at different scale levels and build a multiscale representation. The pairs of contours are then compared by elastic matching achieved by using dynamic programming. Comparisons of the two methods are shown with their respective strengths and weaknesses
Canonical Transformation Approach to the Ultrafast Non-linear Optical Dynamics of Semiconductors
We develop a theory describing the effects of many-particle Coulomb
correlations on the coherent ultrafast nonlinear optical response of
semiconductors and metals. Our approach is based on a mapping of the nonlinear
optical response of the ``bare'' system onto the linear response of a
``dressed'' system. The latter is characterized by effective time-dependent
optical transition matrix elements, electron/hole dispersions, and interaction
potentials, which in undoped semiconductors are determined by the
single-exciton and two-exciton Green functions in the absence of optical
fields. This mapping is achieved by eliminating the optically-induced charge
fluctuations from the Hamiltonian using a Van Vleck canonical transformation.
It takes into account all many-body contributions up to a given order in the
optical fields as well as important Coulomb-induced quantum dynamics to all
orders in the optical field. Our approach allows us to distinguish between
optical nonlinearities of different origins and provides a physically-intuitive
interpretation of their manifestations in ultrafast coherent nonlinear optical
spectroscopy.Comment: 24 page
CVT/GPL phase 3 integrated testing
The hardware for 20 candidate shuttle program life sciences experiments was installed in the GPL and experiments were conducted during a 5-day simulated mission. The experiments involved humans, primates, rats, chickens, and marigold plants. All experiments were completed to the satisfaction of the experimenters. In addition to the scientific data gathered for each experiment, information was obtained concerning experiment hardware design and integration, experiment procedures, GPL support systems, and test operations. The results of the integrated tests are presented
"Strongly interacting matter in magnetic fields": an overview
This is an introduction to the volume of Lecture Notes in Physics on
"Strongly interacting matter in magnetic fields". The volume combines
contributions written by a number of experts on different aspects of the
problem. The response of QCD matter to intense magnetic fields has attracted a
lot of interest recently. On the theoretical side, this interest stems from the
possibility to explore the plethora of novel phenomena arising from the
interplay of magnetic field with QCD dynamics. On the experimental side, the
interest is motivated by the recent results on the behavior of quark-gluon
plasma in a strong magnetic field created in relativistic heavy ion collisions
at RHIC and LHC. The purpose of this introduction is to provide a brief
overview and a guide to the individual contributions where these topics are
covered in detail.Comment: 12 pages, introduction to "Strongly interacting matter in magnetic
fields", Lect. Notes Phys. 871, 1 (2013), edited by D. Kharzeev, K.
Landsteiner, A. Schmitt, H.-U. Yee; v2: references update
Population synthesis of HII galaxies
We study the stellar population of galaxies with active star formation,
determining ages of the stellar components by means of spectral population
synthesis of their absorption spectra. The data consist of optical spectra of
185 nearby () emission line galaxies. They are mostly HII
galaxies, but we also include some Starbursts and Seyfert 2s, for comparison
purposes. They were grouped into 19 high signal-to-noise ratio template
spectra, according to their continuum distribution, absorption and emission
line characteristics. The templates were then synthesized with a star cluster
spectral base. The synthesis results indicate that HII galaxies are typically
age-composite stellar systems, presenting important contribution from
generations up to as old as 500 Myr. We detect a significant contribution of
populations with ages older than 1 Gyr in two groups of HII galaxies. The age
distributions of stellar populations among Starbursts can vary considerably
despite similarities in the emission line spectra. In the case of Seyfert 2
groups we obtain important contributions of old population, consistent with a
bulge. From the diversity of star formation histories, we conclude that typical
HII galaxies in the local universe are not systems presently forming their
first stellar generation.Comment: 12 pages, 4 figures, MNRAS in pres
Wakefulness affects synaptic and network activity by increasing extracellular astrocyte-derived adenosine
Loss of sleep causes an increase in sleep drive and deficits in hippocampal-dependent memory. Both of these responses are thought to require activation of adenosine A1 receptors (adorA1Rs) and release of transmitter molecules including ATP, which is rapidly converted to adenosine in the extracellular space, from astrocytes in a process termed gliotransmission. Although it is increasingly clear that astrocyte-derived adenosine plays an important role in driving the homeostatic sleep response and the effects of sleep loss on memory (Halassa et al., 2009; Florian et al., 2011), previous studies have not determined whether the concentration of this signaling molecule increases in response to wakefulness. Here, we show that the level of adorA1R activation increases in response to wakefulness in mice (Mus musculus). We found that this increase affected synaptic transmission in the hippocampus and modulated network activity in the cortex. Direct biosensor-based measurement of adenosine showed that the net extracellular concentration of this transmitter increased in response to normal wakefulness and sleep deprivation. Genetic inhibition of gliotransmission prevented this increase and attenuated the wakefulness-dependent changes in synaptic and network regulation by adorA1R. Consequently, we conclude that wakefulness increases the level of extracellular adenosine in the hippocampus and that this increase requires the release of transmitters from astroctyes
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