22,096 research outputs found
Fat handles and phase portraits of Non Singular Morse-Smale flows on S^3 with unknotted saddle orbits
In this paper we build Non-singular Morse-Smale flows on S^3 with unknotted
and unlinked saddle orbits by identifying fat round handles along their
boundaries. This way of building the flows enables to get their phase
portraits. We also show that the presence of heteroclinic trajectories imposes
an order in the round handle decomposition of these flows; this order is total
for NMS flows composed of one repulsive, one attractive and n unknotted saddle
orbits, for n >1.Comment: 15 page
New Results for the MAP Problem in Bayesian Networks
This paper presents new results for the (partial) maximum a posteriori (MAP)
problem in Bayesian networks, which is the problem of querying the most
probable state configuration of some of the network variables given evidence.
First, it is demonstrated that the problem remains hard even in networks with
very simple topology, such as binary polytrees and simple trees (including the
Naive Bayes structure). Such proofs extend previous complexity results for the
problem. Inapproximability results are also derived in the case of trees if the
number of states per variable is not bounded. Although the problem is shown to
be hard and inapproximable even in very simple scenarios, a new exact algorithm
is described that is empirically fast in networks of bounded treewidth and
bounded number of states per variable. The same algorithm is used as basis of a
Fully Polynomial Time Approximation Scheme for MAP under such assumptions.
Approximation schemes were generally thought to be impossible for this problem,
but we show otherwise for classes of networks that are important in practice.
The algorithms are extensively tested using some well-known networks as well as
random generated cases to show their effectiveness.Comment: A couple of typos were fixed, as well as the notation in part of
section 4, which was misleading. Theoretical and empirical results have not
change
Error Propagation in the Hypercycle
We study analytically the steady-state regime of a network of n error-prone
self-replicating templates forming an asymmetric hypercycle and its error tail.
We show that the existence of a master template with a higher non-catalyzed
self-replicative productivity, a, than the error tail ensures the stability of
chains in which m<n-1 templates coexist with the master species. The stability
of these chains against the error tail is guaranteed for catalytic coupling
strengths (K) of order of a. We find that the hypercycle becomes more stable
than the chains only for K of order of a2. Furthermore, we show that the
minimal replication accuracy per template needed to maintain the hypercycle,
the so-called error threshold, vanishes like sqrt(n/K) for large K and n<=4
The Infrared-X-ray continuum correlation in Active Galactic Nuclei
The correlation between the soft X-ray and near infrared emission from AGN is
analysed using composite models by the code SUMA. We find new evidences for
differences in ranges of parameters which characterize the NLR of Seyfert
galaxies and LINERs. Results obtained by modelling the Einstein and the ROSAT
samples of galaxies are in full agreement. In order to fit the infrared and
X-ray continua, an eta factor is defined, which accounts for the emitting area
of the cloud. If the infrared emission is due to bremsstrahlung and comes from
the same cloud producing the soft X-rays, the eta values obtained from both
emissions must be the same. Therefore, if eta_IR < eta_soft-X there must be a
strong contribution of soft X-rays from the active centre. From the eta values
we expect to identify the objects that could present strong variability. \Comment: 11 pages,13 figures, in press in MNRAS. in press in MNRA
D-branes, Orientifolds and K-theory
The complete D-brane spectrum in ZZ[squared] orientifolds is computed. Stable non-BPS D-branes with both integral and torsion charges are found. The relation to K-theory is discussed and a new K-theory relevant to orientifolds is suggested
Redundancy of stereoscopic images: Experimental Evaluation
With the recent advancement in visualization devices over the last years, we
are seeing a growing market for stereoscopic content. In order to convey 3D
content by means of stereoscopic displays, one needs to transmit and display at
least 2 points of view of the video content. This has profound implications on
the resources required to transmit the content, as well as demands on the
complexity of the visualization system. It is known that stereoscopic images
are redundant, which may prove useful for compression and may have positive
effect on the construction of the visualization device. In this paper we
describe an experimental evaluation of data redundancy in color stereoscopic
images. In the experiments with computer generated and real life and test
stereo images, several observers visually tested the stereopsis threshold and
accuracy of parallax measuring in anaglyphs and stereograms as functions of the
blur degree of one of two stereo images and color saturation threshold in one
of two stereo images for which full color 3D perception with no visible color
degradations is maintained. The experiments support a theoretical estimate that
one has to add, to data required to reproduce one of two stereoscopic images,
only several percents of that amount of data in order to achieve stereoscopic
perception
The current status of dental education and the dental profession in Chile
IndexaciĂłn: Scopus.Objective: To describe the current situation of the dental profession in Chile, including training and workforce issues. Material and Methods: Data were collected from different national institutions, which included information regarding number of current registered dentists, university of graduation, geographical distribution, professional position, additional specialty certifications obtained, the number and characteristics of dental surgeons who work in the public and private sectors, the traditional character of the university, the accreditation status of the undergraduate dental programs and the general population number. Results: Currently there are 32 schools of Dentistry in Chile, of which 21 have their quality certified. There are 19,100 Chilean dentists and 1,727 foreign dentists registered. The number of graduates from private universities has increased significantly. Currently, 2,164 dentists work for MINSAL. Less than a third hold a specialty certification. Forty-five percent of the dental specialists obtained their certification from universities. The current professional ratio is 104 dentists per 100,000 habitants. Conclusion: The number of dentists in Chile has increased progressively during the last years, mainly associated with the opening of new dental schools. Only 28% of the Chilean dental schools have certified their quality for the total duration of the undergraduate program. Regarding the workforce, there is a public/private and geographical inequities in dentistsâ distribution.http://revista.uepb.edu.br/index.php/pboci/article/view/3875/pd
First-passage times in multi-scale random walks: the impact of movement scales on search efficiency
An efficient searcher needs to balance properly the tradeoff between the
exploration of new spatial areas and the exploitation of nearby resources, an
idea which is at the core of scale-free L\'evy search strategies. Here we study
multi-scale random walks as an approximation to the scale- free case and derive
the exact expressions for their mean-first passage times in a one-dimensional
finite domain. This allows us to provide a complete analytical description of
the dynamics driving the asymmetric regime, in which both nearby and faraway
targets are available to the searcher. For this regime, we prove that the
combination of only two movement scales can be enough to outperform both
balistic and L\'evy strategies. This two-scale strategy involves an optimal
discrimination between the nearby and faraway targets, which is only possible
by adjusting the range of values of the two movement scales to the typical
distances between encounters. So, this optimization necessarily requires some
prior information (albeit crude) about targets distances or distributions.
Furthermore, we found that the incorporation of additional (three, four, ...)
movement scales and its adjustment to target distances does not improve further
the search efficiency. This allows us to claim that optimal random search
strategies in the asymmetric regime actually arise through the informed
combination of only two walk scales (related to the exploitative and the
explorative scale, respectively), expanding on the well-known result that
optimal strategies in strictly uninformed scenarios are achieved through L\'evy
paths (or, equivalently, through a hierarchical combination of multiple
scales)
Interband polarized absorption in InP polytypic superlattices
Recent advances in growth techniques have allowed the fabrication of
semiconductor nanostructures with mixed wurtzite/zinc-blende crystal phases.
Although the optical characterization of these polytypic structures is well
eported in the literature, a deeper theoretical understanding of how crystal
phase mixing and quantum confinement change the output linear light
polarization is still needed. In this paper, we theoretically investigate the
mixing effects of wurtzite and zinc-blende phases on the interband absorption
and in the degree of light polarization of an InP polytypic superlattice. We
use a single 88 kp Hamiltonian that describes both crystal
phases. Quantum confinement is investigated by changing the size of the
polytypic unit cell. We also include the optical confinement effect due to the
dielectric mismatch between the superlattice and the vaccum and we show it to
be necessary to match experimental results. Our calculations for large wurtzite
concentrations and small quantum confinement explain the optical trends of
recent photoluminescence excitation measurements. Furthermore, we find a high
sensitivity to zinc-blende concentrations in the degree of linear polarization.
This sensitivity can be reduced by increasing quantum confinement. In
conclusion, our theoretical analysis provides an explanation for optical trends
in InP polytypic superlattices, and shows that the interplay of crystal phase
mixing and quantum confinement is an area worth exploring for light
polarization engineering.Comment: 9 pages, 6 figures and 1 tabl
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