21,676 research outputs found
Bounds and extremal domains for Robin eigenvalues with negative boundary parameter
We present some new bounds for the first Robin eigenvalue with a negative
boundary parameter. These include the constant volume problem, where the bounds
are based on the shrinking coordinate method, and a proof that in the fixed
perimeter case the disk maximises the first eigenvalue for all values of the
parameter. This is in contrast with what happens in the constant area problem,
where the disk is the maximiser only for small values of the boundary
parameter. We also present sharp upper and lower bounds for the first
eigenvalue of the ball and spherical shells.
These results are complemented by the numerical optimisation of the first
four and two eigenvalues in 2 and 3 dimensions, respectively, and an evaluation
of the quality of the upper bounds obtained. We also study the bifurcations
from the ball as the boundary parameter becomes large (negative).Comment: 26 pages, 20 figure
Extinction in the Coma of Comet 17P/Holmes
On 2007 October 29 the outbursting comet 17P/Holmes passed within 0.79 arcsec
of a background star. We recorded the event using optical, narrowband
photometry and detect a 3% to 4% dip in stellar brightness bracketing the time
of closest approach to the comet nucleus. The detected dimming implies an
optical depth tau~0.04 at 1.5 arcsec from the nucleus and an optical depth
towards the nucleus center tau_n<13.3. At the time of our observations, the
coma was optically thick only within rho<~0.01 arcsec from the nucleus. By
combining the measured extinction and the scattered light from the coma we
estimate a dust red geometric albedo p_d=0.006+/-0.002 at 16 deg phase angle.
Our measurements place the most stringent constraints on the extinction optical
depth of any cometary coma.Comment: 5 pages, 3 figures, 2 table. Accepted for publication in ApJ
Instability results for the damped wave equation in unbounded domains
We extend some previous results for the damped wave equation in bounded
domains in Euclidean spaces to the unbounded case. In particular, we show that
if the damping term is of the form with bounded taking on
negative values on a set of positive measure, then there will always exist
unbounded solutions for sufficiently large positive .
In order to prove these results, we generalize some existing results on the
asymptotic behaviour of eigencurves of one-parameter families of Schrodinger
operators to the unbounded case, which we believe to be of interest in their
own right.Comment: LaTeX, 19 pages; to appear in J. Differential Equation
Physics of quantum light emitters in disordered photonic nanostructures
Nanophotonics focuses on the control of light and the interaction with matter
by the aid of intricate nanostructures. Typically, a photonic nanostructure is
carefully designed for a specific application and any imperfections may reduce
its performance, i.e., a thorough investigation of the role of unavoidable
fabrication imperfections is essential for any application. However, another
approach to nanophotonic applications exists where fabrication disorder is used
to induce functionalities by enhancing light-matter interaction. Disorder leads
to multiple scattering of light, which is the realm of statistical optics where
light propagation requires a statistical description. We review here the recent
progress on disordered photonic nanostructures and the potential implications
for quantum photonics devices.Comment: Review accepted for publication in Annalen der Physi
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