626 research outputs found
Toy Model for Pion Production II: The role of three-particle singularities
The influence of three-particle breakup singularities on s-wave meson
production in nucleon-nucleon collisions is studied within the distorted wave
Born approximation. This study is based on a simple scalar model for the
two-nucleon interaction and the production mechanism. An algorithm for the
exact numerical treatment of the inherent three-body cuts, together with its
straightforward implementation is presented. It is also shown that two
often-used approximations to avoid the calculation of the three-body breakup
are not justified. The possible impact on pion production observables is
discussed.Comment: 14 pages, 6 figure
Tight Combinatorial Generalization Bounds for Threshold Conjunction Rules
Abstract. We propose a combinatorial technique for obtaining tight data dependent generalization bounds based on a splitting and connec-tivity graph (SC-graph) of the set of classifiers. We apply this approach to a parametric set of conjunctive rules and propose an algorithm for effective SC-bound computation. Experiments on 6 data sets from the UCI ML Repository show that SC-bound helps to learn more reliable rule-based classifiers as compositions of less overfitted rules
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Transpacific Transport of Ozone Pollution and the Effect of Recent Asian Emission Increases on Air Quality in North America: An Integrated Analysis Using Satellite, Aircraft, Ozonesonde, and Surface Observations
We use an ensemble of aircraft, satellite, sonde, and surface observations for April–May 2006 (NASA/INTEX-B aircraft campaign) to better understand the mechanisms for transpacific ozone pollution and its implications for North American air quality. The observations are interpreted with a global 3-D chemical transport model (GEOS-Chem). OMI NO2 satellite observations constrain Asian anthropogenic NOx emissions and indicate a factor of 2 increase from 2000 to 2006 in China. Satellite observations of CO from AIRS and TES indicate two major events of Asian transpacific pollution during INTEX-B. Correlation between TES CO and ozone observations shows evidence for transpacific ozone pollution. The semi-permanent Pacific High and Aleutian Low cause splitting of transpacific pollution plumes over the Northeast Pacific. The northern branch circulates around the Aleutian Low and has little impact on North America. The southern branch circulates around the Pacific High and some of that air impacts western North America. Both aircraft measurements and model results show sustained ozone production driven by peroxyacetylnitrate (PAN) decomposition in the southern branch, roughly doubling the transpacific influence from ozone produced in the Asian boundary layer. Model simulation of ozone observations at Mt. Bachelor Observatory in Oregon (2.7 km altitude) indicates a mean Asian ozone pollution contribution of 9±3 ppbv to the mean observed concentration of 54 ppbv, reflecting mostly an enhancement in background ozone rather than episodic Asian plumes. Asian pollution enhanced surface ozone concentrations by 5–7 ppbv over western North America in spring 2006. The 2000–2006 rise in Asian anthropogenic emissions increased this influence by 1–2 ppbv.Earth and Planetary SciencesEngineering and Applied Science
Surface Kinetics and Generation of Different Terms in a Conservative Growth Equation
A method based on the kinetics of adatoms on a growing surface under
epitaxial growth at low temperature in (1+1) dimensions is proposed to obtain a
closed form of local growth equation. It can be generalized to any growth
problem as long as diffusion of adatoms govern the surface morphology. The
method can be easily extended to higher dimensions. The kinetic processes
contributing to various terms in the growth equation (GE) are identified from
the analysis of in-plane and downward hops. In particular, processes
corresponding to the (h -> -h) symmetry breaking term and curvature dependent
term are discussed. Consequence of these terms on the stable and unstable
transition in (1+1) dimensions is analyzed. In (2+1) dimensions it is shown
that an additional (h -> -h) symmetry breaking term is generated due to the
in-plane curvature associated with the mound like structures. This term is
independent of any diffusion barrier differences between in-plane and out
of-plane migration. It is argued that terms generated in the presence of
downward hops are the relevant terms in a GE. Growth equation in the closed
form is obtained for various growth models introduced to capture most of the
processes in experimental Molecular Beam Epitaxial growth. Effect of
dissociation is also considered and is seen to have stabilizing effect on the
growth. It is shown that for uphill current the GE approach fails to describe
the growth since a given GE is not valid over the entire substrate.Comment: 14 pages, 7 figure
Self-similar Solutions to a Kinetic Model for Grain Growth
We prove the existence of self-similar solutions to the Fradkov model for
two-dimensional grain growth, which consists of an infinite number of
nonlocally coupled transport equations for the number densities of grains with
given area and number of neighbours (topological class). For the proof we
introduce a finite maximal topological class and study an appropriate
upwind-discretization of the time dependent problem in self-similar variables.
We first show that the resulting finite dimensional differential system has
nontrivial steady states. Afterwards we let the discretization parameter tend
to zero and prove that the steady states converge to a compactly supported
self-similar solution for a Fradkov model with finitely many equations. In a
third step we let the maximal topology class tend to infinity and obtain
self-similar solutions to the original system that decay exponentially.
Finally, we use the upwind discretization to compute self-similar solutions
numerically.Comment: 25 pages, several figure
Placental growth factor and its potential role in diabetic retinopathy and other ocular neovascular diseases.
The role of vascular endothelial growth factor (VEGF), including in retinal vascular diseases, has been well studied, and pharmacological blockade of VEGF is the gold standard of treatment for neovascular age-related macular degeneration, retinal vein occlusion and diabetic macular oedema. Placental growth factor (PGF, previously known as PlGF), a homologue of VEGF, is a multifunctional peptide associated with angiogenesis-dependent pathologies in the eye and non-ocular conditions. Animal studies using genetic modification and pharmacological treatment have demonstrated a mechanistic role for PGF in pathological angiogenesis. Inhibition decreases neovascularization and microvascular abnormalities across different models, including oxygen-induced retinopathy, laser-induced choroidal neovascularization and in diabetic mice exhibiting retinopathies. High levels of PGF have been found in the vitreous of patients with diabetic retinopathy. Despite these strong animal data, the exact role of PGF in pathological angiogenesis in retinal vascular diseases remains to be defined, and the benefits of PGF-specific inhibition in humans with retinal neovascular diseases and macular oedema remain controversial. Comparative effectiveness research studies in patients with diabetic retinal disease have shown that treatment that inhibits both VEGF and PGF may provide superior outcomes in certain patients compared with treatment that inhibits only VEGF. This review summarizes current knowledge of PGF, including its relationship to VEGF and its role in pathological angiogenesis in retinal diseases, and identifies some key unanswered questions about PGF that can serve as a pathway for future basic, translational and clinical research
Machine Learning in Automated Text Categorization
The automated categorization (or classification) of texts into predefined
categories has witnessed a booming interest in the last ten years, due to the
increased availability of documents in digital form and the ensuing need to
organize them. In the research community the dominant approach to this problem
is based on machine learning techniques: a general inductive process
automatically builds a classifier by learning, from a set of preclassified
documents, the characteristics of the categories. The advantages of this
approach over the knowledge engineering approach (consisting in the manual
definition of a classifier by domain experts) are a very good effectiveness,
considerable savings in terms of expert manpower, and straightforward
portability to different domains. This survey discusses the main approaches to
text categorization that fall within the machine learning paradigm. We will
discuss in detail issues pertaining to three different problems, namely
document representation, classifier construction, and classifier evaluation.Comment: Accepted for publication on ACM Computing Survey
Statistics of the gravitational force in various dimensions of space: from Gaussian to Levy laws
We discuss the distribution of the gravitational force created by a
Poissonian distribution of field sources (stars, galaxies,...) in different
dimensions of space d. In d=3, it is given by a Levy law called the Holtsmark
distribution. It presents an algebraic tail for large fluctuations due to the
contribution of the nearest neighbor. In d=2, it is given by a marginal
Gaussian distribution intermediate between Gaussian and Levy laws. In d=1, it
is exactly given by the Bernouilli distribution (for any particle number N)
which becomes Gaussian for N>>1. Therefore, the dimension d=2 is critical
regarding the statistics of the gravitational force. We generalize these
results for inhomogeneous systems with arbitrary power-law density profile and
arbitrary power-law force in a d-dimensional universe
Supporting thinking on sample sizes for thematic analyses: a quantitative tool
Thematic analysis is frequently used to analyse qualitative data in psychology, healthcare, social research and beyond. An important stage in planning a study is determining how large a sample size may be required, however current guidelines for thematic analysis are varied, ranging from around 2 to over 400 and it is unclear how to choose a value from the space in between. Some guidance can also not be applied prospectively. This paper introduces a tool to help users think about what would be a useful sample size for their particular context when investigating patterns across participants. The calculation depends on (a) the expected population theme prevalence of the least prevalent theme, derived either from prior knowledge or based on the prevalence of the rarest themes considered worth uncovering, e.g. 1 in 10, 1 in 100; (b) the number of desired instances of the theme; and (c) the power of the study. An adequately powered study will have a high likelihood of finding sufficient themes of the desired prevalence. This calculation can then be used alongside other considerations. We illustrate how to use the method to calculate sample size before starting a study and achieved power given a sample size, providing tables of answers and code for use in the free software, R. Sample sizes are comparable to those found in the literature, for example to have 80% power to detect two instances of a theme with 10% prevalence, 29 participants are required. Increasing power, increasing the number of instances or decreasing prevalence increases the sample size needed. We do not propose this as a ritualistic requirement for study design, but rather as a pragmatic supporting tool to help plan studies using thematic analysis
Eigenvalue Problem in Two Dimensions for an Irregular Boundary II: Neumann Condition
We formulate a systematic elegant perturbative scheme for determining the
eigenvalues of the Helmholtz equation (\bigtriangledown^{2} + k^{2}){\psi} = 0
in two dimensions when the normal derivative of {\psi} vanishes on an irregular
closed curve. Unique feature of this method, unlike other perturbation schemes,
is that it does not require a separate formalism to treat degeneracies.
Degenerate states are handled equally elegantly as the non-degenerate ones. A
real parameter, extracted from the parameters defining the irregular boundary,
serves as a perturbation parameter in this scheme as opposed to earlier schemes
where the perturbation parameter is an artificial one. The efficacy of the
proposed scheme is gauged by calculating the eigenvalues for elliptical and
supercircular boundaries and comparing with the results obtained numerically.
We also present a simple and interesting semi-empirical formula, determining
the eigenspectrum of the 2D Helmholtz equation with the Dirichlet or the
Neumann condition for a supercircular boundary. A comparison of the
eigenspectrum for several low-lying modes obtained by employing the formula
with the corresponding numerical estimates shows good agreement for a wide
range of the supercircular exponent.Comment: 26 pages, 12 figure
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