5,360 research outputs found
Camera motion estimation through planar deformation determination
In this paper, we propose a global method for estimating the motion of a
camera which films a static scene. Our approach is direct, fast and robust, and
deals with adjacent frames of a sequence. It is based on a quadratic
approximation of the deformation between two images, in the case of a scene
with constant depth in the camera coordinate system. This condition is very
restrictive but we show that provided translation and depth inverse variations
are small enough, the error on optical flow involved by the approximation of
depths by a constant is small. In this context, we propose a new model of
camera motion, that allows to separate the image deformation in a similarity
and a ``purely'' projective application, due to change of optical axis
direction. This model leads to a quadratic approximation of image deformation
that we estimate with an M-estimator; we can immediatly deduce camera motion
parameters.Comment: 21 pages, version modifi\'ee accept\'e le 20 mars 200
Dynamical Jahn-Teller Effect and Berry Phase in Positively Charged Fullerene I. Basic Considerations
We study the Jahn-Teller effect of positive fullerene ions C
and C. The aim is to discover if this case, in analogy with the
negative ion, possesses a Berry phase or not, and what are the consequences on
dynamical Jahn-Teller quantization. Working in the linear and spherical
approximation, we find no Berry phase in C, and
presence/absence of Berry phase for coupling of one hole to an
/ vibration. We study in particular the special equal-coupling case
(), which is reduced to the motion of a particle on a 5-dimensional
sphere. In the icosahedral molecule, the final outcome assesses the
presence/absence of a Berry phase of for the hole coupled to
/ vibrations. Some qualitative consequences on ground-state symmetry,
low-lying excitations, and electron emission from C are spelled out.Comment: 31 pages (RevTeX), 3 Postscript figures (uuencoded
Skin antisepsis: it's not only what you use, it's the way that you use it
International audienc
Introduction to RISC-KIT: Resilience-increasing strategies for coasts
Recent and historic low-frequency, high-impact events have demonstrated the flood risks faced by exposed coastal areas in Europe and beyond. These coastal zone risks are likely to increase in the future which requires a re-evaluation of coastal disaster risk reduction (DRR) strategies and a new mix of PMP (prevention, e.g., dike protection; mitigation, e.g., limiting construction in flood-prone areas and eco-system based solutions; and preparedness, e.g., Early Warning Systems, EWS) measures.
In response to these challenges, the RISC-KIT project has delivered a set of open-source and openaccess methods, tools and management approaches to reduce risk and increase resilience to lowfrequency, high-impact hydro-meteorological events in the coastal zone (the “RISC-toolKIT”). These products enhance forecasting, prediction and early warning capabilities, improve the assessment of long-term coastal risk and optimise the mix of PMP-measures.
In this paper an introduction is provided to the objectives, products, applications and lessonslearned of the RISC-KIT project, which are the subjects of this Special Issue. Subsequent papers provide details on the tools and their application on 10 case study sites in Europe
Enhanced Electron Pairing in a Lattice of Berry Phase Molecules
We show that electron hopping in a lattice of molecules possessing a Berry
phase naturally leads to pairing. Our building block is a simple molecular site
model inspired by C, but realized in closer similarity with Na. In
the resulting model electron hopping must be accompanied by orbital operators,
whose function is to switch on and off the Berry phase as the electron number
changes. The effective hamiltonians (electron-rotor and electron-pseudospin)
obtained in this way are then shown to exhibit a strong pairing phenomenon, by
means of 1D linear chain case studies. This emerges naturally from numerical
studies of small -site rings, as well as from a BCS-like mean-field theory
formulation. The pairing may be explained as resulting from the exchange of
singlet pairs of orbital excitations, and is intimately connected with the
extra degeneracy implied by the Berry phase when the electron number is odd.
The relevance of this model to fullerides, to other molecular superconductors,
as well as to present and future experiments, is discussed.Comment: 30 pages, RevTe
Dealing with substantial heterogeneity in Cochrane reviews. Cross-sectional study
<p>Abstract</p> <p>Background</p> <p>Dealing with heterogeneity in meta-analyses is often tricky, and there is only limited advice for authors on what to do. We investigated how authors addressed different degrees of heterogeneity, in particular whether they used a fixed effect model, which assumes that all the included studies are estimating the same true effect, or a random effects model where this is not assumed.</p> <p>Methods</p> <p>We sampled randomly 60 Cochrane reviews from 2008, which presented a result in its first meta-analysis with substantial heterogeneity (I<sup>2 </sup>greater than 50%, i.e. more than 50% of the variation is due to heterogeneity rather than chance). We extracted information on choice of statistical model, how the authors had handled the heterogeneity, and assessed the methodological quality of the reviews in relation to this.</p> <p>Results</p> <p>The distribution of heterogeneity was rather uniform in the whole I<sup>2 </sup>interval, 50-100%. A fixed effect model was used in 33 reviews (55%), but there was no correlation between I<sup>2 </sup>and choice of model (P = 0.79). We considered that 20 reviews (33%), 16 of which had used a fixed effect model, had major problems. The most common problems were: use of a fixed effect model and lack of rationale for choice of that model, lack of comment on even severe heterogeneity and of reservations and explanations of its likely causes. The problematic reviews had significantly fewer included trials than other reviews (4.3 vs. 8.0, P = 0.024). The problems became less pronounced with time, as those reviews that were most recently updated more often used a random effects model.</p> <p>Conclusion</p> <p>One-third of Cochrane reviews with substantial heterogeneity had major problems in relation to their handling of heterogeneity. More attention is needed to this issue, as the problems we identified can be essential for the conclusions of the reviews.</p
Patients want to know about the \u27cardiac blues\u27
BACKGROUND: Much attention has been given to identifying and supporting the minority of patients who develop severe clinical depression after a cardiac event. However, relatively little has been given to supporting the many patients who experience transient but significant emotional disturbance that we term the \u27cardiac blues\u27. OBJECTIVE: The aim of this study was to investigate patients\u27 preferences regarding information provision about cardiac blues. METHODS: One hundred and sixty consecutive cardiac patients admitted to two Victorian hospitals in Australia were interviewed three times over six months. They were asked about emotional issues, including information provision preferences. RESULTS: Four out of five (81%) patients would like to have received information about the cardiac blues, but only a minority received this information. CONCLUSION: Most patients want to know about cardiac blues. The development and evaluation of resources for health professionals and patients to support recovery through cardiac blues appears warranted
Generalized "Quasi-classical" Ground State for an Interacting Two Level System
We treat a system (a molecule or a solid) in which electrons are coupled
linearly to any number and type of harmonic oscillators and which is further
subject to external forces of arbitrary symmetry. With the treatment restricted
to the lowest pair of electronic states, approximate "vibronic"
(vibration-electronic) ground state wave functions are constructed having the
form of simple, closed expressions. The basis of the method is to regard
electronic density operators as classical variables. It extends an earlier
"guessed solution", devised for the dynamical Jahn-Teller effect in cubic
symmetry, to situations having lower (e.g., dihedral) symmetry or without any
symmetry at all. While the proposed solution is expected to be quite close to
the exact one, its formal simplicity allows straightforward calculations of
several interesting quantities, like energies and vibronic reduction (or Ham)
factors. We calculate for dihedral symmetry two different -factors (""
and "") and a -factor. In simplified situations we obtain . The formalism enables quantitative estimates to be made for the dynamical
narrowing of hyperfine lines in the observed ESR spectrum of the dihedral
cyclobutane radical cation.Comment: 28 pages, 4 figure
Electron--Vibron Interactions and Berry Phases in Charged Buckminsterfullerene: Part I
A simple model for electron-vibron interactions on charged
buckminsterfullerene C, , is solved both at weak and
strong couplings. We consider a single vibrational multiplet interacting
with electrons. At strong coupling the semiclassical dynamical
Jahn-Teller theory is valid. The Jahn-Teller distortions are unimodal for
=1,2,4,5 electrons, and bimodal for 3 electrons. The distortions are
quantized as rigid body pseudo--rotators which are subject to geometrical Berry
phases. These impose ground state degeneracies and dramatically change zero
point energies. Exact diagonalization shows that the semiclassical level
degeneracies and ordering survive well into the weak coupling regime. At weak
coupling, we discover an enhancement factor of for the pair binding
energies over their classical values. This has potentially important
implications for superconductivity in fullerides, and demonstrates the
shortcoming of Migdal--Eliashberg theory for molecular crystals.Comment: 29 pages (+7 figures, 3 available upon request), LATEX,
report-number: BM515
Numerical study of the lattice vacancy effects on the single-channel electron transport of graphite ribbons
Lattice vacancy effects on electrical conductance of nanographite ribbon are
investigated by means of the Landauer approach using a tight binding model. In
the low-energy regime ribbons with zigzag boundary provide a single conducting
channel whose origin is connected with the presence of edge states. It is found
that the chemical potential dependence of conductance strongly depends on the
difference () of the number of removed A and B sublattice sites. The
large lattice vacancy with shows zero-conductance dips
in the single-channel region, however, the large lattice vacancy with
has no dip structure in this region. The connection between this
conductance rule and the Longuet-Higgins conjecture is also discussed
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