1,229 research outputs found
Do Birds of a Feather Flock Together? Immigration Flows and Cultural Clustering in Host Countries
This paper presents a simple theoretical framework in which immigrants have a relative incentive to cluster in host countries where cultural characteristics and imperfect information sustain the segmentation of the labor market and a higher wage in foreign communities. The hypothesis is tested on a panel of immigration flows to OECD countries. The pull effect of cultural communities is supported and it is found that the minimum size of a given cultural community is around 5% of the foreign population. It is also found that the pull effect weakens as the community grows as predicted by the theoretical framework.International migrations; relative incomes; cultural clustering
(DH) Noise and Signal scaling factors in Digital Holography in week illumination: relationship with Shot Noise
We have performed off axis heterodyne holography with very weak illumination
by recording holograms of the object with and without object illumination in
the same acquisition run. We have experimentally studied, how the reconstructed
image signal (with illumination) and noise background (without) scale with the
holographic acquisition and reconstruction parameters that are the number of
frames, and the number of pixels of the reconstruction spatial filter. The
first parameter is related to the frequency bandwidth of detection in time, the
second one to the bandwidth in space. The signal to background ratio varies
roughly like the inverse of the bandwidth in time and space. We have also
compared the noise background with the theoretical shot noise background
calculated by Monte Carlo simulation. The experimental and Monte Carlo noise
background agree very well together
Phase-resolved heterodyne holographic vibrometry with a strobe local oscillator
We report a demonstration of phase-resolved vibrometry, in which out-of-plane
sinusoidal motion is assessed by heterodyne holography. In heterodyne
holography, the beam in the reference channel is an optical local oscillator
(LO). It is frequency-shifted with respect to the illumination beam to enable
frequency conversion within the sensor bandwidth. The proposed scheme
introduces a strobe LO, where the reference beam is frequency-shifted and
modulated in amplitude, to alleviate the issue of phase retrieval. The strobe
LO is both tuned around the first optical modulation side band at the vibration
frequency, and modulated in amplitude to freeze selected mechanical vibration
states sequentially. The phase map of the vibration can then be derived from
the demodulation of successive vibration states
Holographic microscopy reconstruction in both object and image half spaces with undistorted 3D grid
We propose an holographic microscopy reconstruction method, which propagates
the hologram, in the object half space, in the vicinity of the object. The
calibration yields reconstructions with an undistorted reconstruction grid i.e.
with orthogonal , and axis and constant pixels pitch. The method is
validated with an USAF target imaged by a 60 microscope objective,
whose holograms are recorded and reconstructed for different USAF locations
along the longitudinal axis: -75 to +75 m. Since the reconstruction
numerical phase mask, the reference phase curvature and MO form an afocal
device, the reconstruction can be interpreted as occurring equivalently in the
object or in image half space
High numerical aperture holographic microscopy reconstruction with extended z range
An holographic microscopy reconstruction method compatible with high
numerical aperture microscope objective (MO) up to NA=1.4 is proposed. After
off axis and reference field curvature corrections, and after selection of the
+1 grating order holographic image, a phase mask that transforms the optical
elements of the holographic setup into an afocal device is applied in the
camera plane. The reconstruction is then made by the angular spectrum method.
The field is first propagated in the image half space from the camera to the
afocal image of the MO optimal plane (plane for which MO has been designed) by
using a quadratic kernel. The field is then propagated from the MO optimal
plane to the object with the exact kernel. Calibration of the reconstruction is
made by imaging a calibrated object like an USAF resolution target for
different positions along . Once the calibration is done, the reconstruction
can be made with an object located in any plane . The reconstruction method
has been validated experimentally with an USAF target imaged with a NA=1.4
microscope objective. Near-optimal resolution is obtained over an extended
range (m) of locations
Two-step distortion-free reconstruction scheme for holographic microscopy
We propose a three-dimensional holographic reconstruction procedure
applicable with no a priori knowledge about the recording conditions enabling
distortion-free three-dimensional object reconstruction
Some exact results on the matter star-product in the half-string formalism
We show that the D25 sliver wavefunction, just as the D-instanton sliver,
factorizes when expressed in terms of half-string coordinates. We also
calculate analytically the star-product of two zero-momentum eigenstates of
using the vertex in the oscillator basis, thereby showing that the
star-product in the matter sector can indeed be seen as multiplication of
matrices acting on the space of functionals of half strings. We then use the
above results to establish that the matrices , conjectured by
Rastelli, Sen and Zwiebach to be left and right projectors on the sliver, are
indeed so.Comment: 27 pages; footnote adde
Arbitrary Four String Scattering at High Energy and Fixed Angle
We calculate, using the group theoretic approach to string theory, the tree
and one loop scattering of four open and closed arbitrary bosonic string
states. In the limit of high energy, but fixed angle, the multi-string vertex
at tree and one loop levels that we find takes a very simple form. We propose,
and present arguments for, a form for the high energy multi-string vertex at
all loops; in particular we give a path integral derivation of this vertex. Our
results agree with those of Gross and Mende for tachyon scattering amplitudes,
but those for any other string scattering are substantially different from that
discussed in reference [5]. We also develop some of the technology used in the
group theoretic method to compute loop corrections.Comment: Plain tex, 55 pages, 2 figures. A new section is added, giving a path
integral derivation of four arbitrary string high energy scattering, in
agreement with the results given earlier. Typos corrected, and some
discussion in section 7 clarifie
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