324,575 research outputs found
The Lyman <span class='mathrm'>α</span> and Lyman <span class='mathrm'>β</span> lines in solar coronal streamers
No abstract available
Multitraining support vector machine for image retrieval
Relevance feedback (RF) schemes based on support vector machines (SVMs) have been widely used in content-based image retrieval (CBIR). However, the performance of SVM-based RF approaches is often poor when the number of labeled feedback samples is small. This is mainly due to 1) the SVM classifier being unstable for small-size training sets because its optimal hyper plane is too sensitive to the training examples; and 2) the kernel method being ineffective because the feature dimension is much greater than the size of the training samples. In this paper, we develop a new machine learning technique, multitraining SVM (MTSVM), which combines the merits of the cotraining technique and a random sampling method in the feature space. Based on the proposed MTSVM algorithm, the above two problems can be mitigated. Experiments are carried out on a large image set of some 20 000 images, and the preliminary results demonstrate that the developed method consistently improves the performance over conventional SVM-based RFs in terms of precision and standard deviation, which are used to evaluate the effectiveness and robustness of a RF algorithm, respectively
factorization of exclusive processes
We prove factorization theorem in perturbative QCD (PQCD) for exclusive
processes by considering and . The relevant form factors are expressed as the convolution of hard
amplitudes with two-parton meson wave functions in the impact parameter
space, being conjugate to the parton transverse momenta . The point is
that on-shell valence partons carry longitudinal momenta initially, and acquire
through collinear gluon exchanges. The -dependent two-parton wave
functions with an appropriate path for the Wilson links are gauge-invariant.
The hard amplitudes, defined as the difference between the parton-level
diagrams of on-shell external particles and their collinear approximation, are
also gauge-invariant. We compare the predictions for two-body nonleptonic
meson decays derived from factorization (the PQCD approach) and from
collinear factorization (the QCD factorization approach).Comment: 11 pages, REVTEX, 5 figure
On the foundations of cancer modelling: selected topics, speculations, & perspectives
This paper presents a critical review of selected topics related to the modelling of cancer onset, evolution and growth, with the aim of illustrating, to a wide applied mathematical readership, some of the novel mathematical problems in the field. This review attempts to capture, from the appropriate literature, the main issues involved in the modelling of phenomena related to cancer dynamics at all scales which characterise this highly complex system: from the molecular scale up to that of tissue. The last part of the paper discusses the challenge of developing a mathematical biological theory of tumour onset and evolution
Symmetry breaking and manipulation of nonlinear optical modes in an asymmetric double-channel waveguide
We study light-beam propagation in a nonlinear coupler with an asymmetric
double-channel waveguide and derive various analytical forms of optical modes.
The results show that the symmetry-preserving modes in a symmetric
double-channel waveguide are deformed due to the asymmetry of the two-channel
waveguide, yet such a coupler supports the symmetry-breaking modes. The
dispersion relations reveal that the system with self-focusing nonlinear
response supports the degenerate modes, while for self-defocusingmedium the
degenerate modes do not exist. Furthermore, nonlinear manipulation is
investigated by launching optical modes supported in double-channel waveguide
into a nonlinear uniform medium.Comment: 10 page
Are Tidal Effects Responsible for Exoplanetary Spin-Orbit Alignment?
The obliquities of planet-hosting stars are clues about the formation of
planetary systems. Previous observations led to the hypothesis that for
close-in giant planets, spin-orbit alignment is enforced by tidal interactions.
Here, we examine two problems with this hypothesis. First, Mazeh and coworkers
recently used a new technique -- based on the amplitude of starspot-induced
photometric variability -- to conclude that spin-orbit alignment is common even
for relatively long-period planets, which would not be expected if tides were
responsible. We re-examine the data and find a statistically significant
correlation between photometric variability and planetary orbital period that
is qualitatively consistent with tidal interactions. However it is still
difficult to explain quantitatively, as it would require tides to be effective
for periods as long as tens of days. Second, Rogers and Lin argued against a
particular theory for tidal re-alignment by showing that initially retrograde
systems would fail to be re-aligned, in contradiction with the observed
prevalence of prograde systems. We investigate a simple model that overcomes
this problem by taking into account the dissipation of inertial waves and the
equilibrium tide, as well as magnetic braking. We identify a region of
parameter space where re-alignment can be achieved, but it only works for
close-in giant planets, and requires some fine tuning. Thus, while we find both
problems to be more nuanced than they first appeared, the tidal model still has
serious shortcomings.Comment: 12 pages, 9 figures. Accepted for publication in Ap
- …