803 research outputs found
Exchange Rate Forecasting: Evidence from the Emerging Central and Eastern European Economies
There is a vast literature on exchange rate forecasting focusing on developed economies. Since the early 1990s, many developing economies have liberalized their financial accounts, and become an integral part of the international financial system. A series of financial crises experienced by these emerging market economies ed them to switch to some form of a flexible exchange rate regime, coupled with inflation targeting. These developments, in turn, accentuate the need for exchange rate forecasting in such economies. This paper is a first attempt to compile data from the emerging Central and Eastern European (CEE) economies, to evaluate the performance of versions of the monetary model of exchange rate determination, and time series models for forecasting exchange rates. Forecast performance of these models at various horizons are evaluated against that of a random walk, which, overwhelmingly, was found to be the best exchange rate predictor for developed economies in the previous literature. Following Clark and West (2006, 2007) for forecast performance analysis, we report that in short horizons, structural models and time series models outperform the random walk for the six CEE countries in the data set
In vivo MRI with Concurrent Excitation and Acquisition using Automated Active Analog Cancellation
Magnetic resonance imaging (MRI) provides excellent cross-sectional images of the soft tissues in patients. Unfortunately, MRI is intrinsically slow, it exposes patients to severe acoustic noise levels, and is limited in the visualization of certain tissues such as bone. These limitations are partly caused by the timing structure of the MRI exam which first generates the MR signal by a strong radio-frequency excitation and later acquires the weak MRI signal. Concurrent excitation and acquisition (CEA) can overcome these limitations, but is extremely challenging due to the huge intensity difference between transmit and receive signal (up to 100 dB). To suppress the strong transmit signals during signal reception, a fully automated analog cancellation unit was designed. On a 3 Tesla clinical MRI system we achieved an on-resonance analog isolation of 90 dB between the transmit and receive path, so that CEA images of the head and the extremities could be acquired with an acquisition efficiency of higher than 90% at sound pressure levels close to background noise. CEA with analog cancellation might provide new opportunities for MRI in tissues with very short T2 relaxation times, and it offers a silent and time-efficient MRI acquisition
One Loop Beta Functions in Topologically Massive Gravity
We calculate the running of the three coupling constants in cosmological,
topologically massive 3d gravity. We find that \nu, the dimensionless
coefficient of the Chern-Simons term, has vanishing beta function. The flow of
the cosmological constant and Newton's constant depends on \nu, and for any
positive \nu there exist both a trivial and a nontrivial fixed point.Comment: 44 pages, 16 figure
Kundt spacetimes as solutions of topologically massive gravity
We obtain new solutions of topologically massive gravity. We find the general
Kundt solutions, which in three dimensions are spacetimes admitting an
expansion-free null geodesic congruence. The solutions are generically of
algebraic type II, but special cases are types III, N or D. Those of type D are
the known spacelike-squashed AdS_3 solutions, and of type N are the known AdS
pp-waves or new solutions. Those of types II and III are the first known
solutions of these algebraic types. We present explicitly the Kundt solutions
that are CSI spacetimes, for which all scalar polynomial curvature invariants
are constant, whereas for the general case we reduce the field equations to a
series of ordinary differential equations. The CSI solutions of types II and
III are deformations of spacelike-squashed AdS_3 and the round AdS_3,
respectively.Comment: 30 pages. This material has come from splitting v1 of arXiv:0906.3559
into 2 separate papers. v2: minor changes
Coastal scenic assessment and tourism management in western Cuba
Coastal scenic assessments via checklist tables (18 natural, 8 human related parameters), can provide
benchmark measures regarding desirable/non-desirable beach conditions and also classify sites. The
scenic value for 43 sites in western Cuba was investigated, with the aim of helping managers improve
bathing areas, especially for tourism purposes. Sites were categorised from Class 1, (top grade scenery),
between La Habana and Matanzas, as extremely attractive, with white sand, turquoise water and
additional scenic elements. Class 2 sites were located in Varadero, and their high scores were greatly due
to appropriate human interventions, e.g. beach nourishment and dune restoration works. Classes 3 to 5
(the latter having very poor scenery), included sites with low scores for natural parameters. For these
sites, coastal managers can do little to alleviate scenic impact, apart from addressing the human parameters,
where improvement is possible, e.g. by litter removal together with the present chaotic protective
structures
On Critical Massive (Super)Gravity in adS3
We review the status of three-dimensional "general massive gravity" (GMG) in
its linearization about an anti-de Sitter (adS) vacuum, focusing on critical
points in parameter space that yield generalizations of "chiral gravity". We
then show how these results extend to N=1 super-GMG, expanded about a
supersymmetric adS vacuum, and also to the most general `curvature-squared' N=1
supergravity model.Comment: 10 pages, Proceedings of ERE 2010, Granada, 6-10 september 2010;
reference adde
Higher Derivative Extension of 6D Chiral Gauged Supergravity
Six-dimensional (1,0) supersymmetric gauged Einstein-Maxwell supergravity is
extended by the inclusion of a supersymmetric Riemann tensor squared invariant.
Both the original model as well as the Riemann tensor squared invariant are
formulated off-shell and consequently the total action is off-shell invariant
without modification of the supersymmetry transformation rules. In this
formulation, superconformal techniques, in which the dilaton Weyl multiplet
plays a crucial role, are used. It is found that the gauging of the U(1)
R-symmetry in the presence of the higher-order derivative terms does not modify
the positive exponential in the dilaton potential. Moreover, the supersymmetric
Minkowski(4) x S^2 compactification of the original model, without the
higher-order derivatives, is remarkably left intact. It is shown that the model
also admits non-supersymmetric vacuum solutions that are direct product spaces
involving de Sitter spacetimes and negative curvature internal spaces.Comment: 32 pages; typos corrected, footnote in conclusions section adde
RELATION OF LEFT ATRIAL SPONTANEOUS ECHO CONTRAST IN PATIENTS WITH MITRAL STENOSIS TO INFLAMMATORY MARKERS
Numerical Analysis of Three-dimensional Acoustic Cloaks and Carpets
We start by a review of the chronology of mathematical results on the
Dirichlet-to-Neumann map which paved the way towards the physics of
transformational acoustics. We then rederive the expression for the
(anisotropic) density and bulk modulus appearing in the pressure wave equation
written in the transformed coordinates. A spherical acoustic cloak consisting
of an alternation of homogeneous isotropic concentric layers is further
proposed based on the effective medium theory. This cloak is characterised by a
low reflection and good efficiency over a large bandwidth for both near and far
fields, which approximates the ideal cloak with a inhomogeneous and anisotropic
distribution of material parameters. The latter suffers from singular material
parameters on its inner surface. This singularity depends upon the sharpness of
corners, if the cloak has an irregular boundary, e.g. a polyhedron cloak
becomes more and more singular when the number of vertices increases if it is
star shaped. We thus analyse the acoustic response of a non-singular spherical
cloak designed by blowing up a small ball instead of a point, as proposed in
[Kohn, Shen, Vogelius, Weinstein, Inverse Problems 24, 015016, 2008]. The
multilayered approximation of this cloak requires less extreme densities
(especially for the lowest bound). Finally, we investigate another type of
non-singular cloaks, known as invisibility carpets [Li and Pendry, Phys. Rev.
Lett. 101, 203901, 2008], which mimic the reflection by a flat ground.Comment: Latex, 21 pages, 7 Figures, last version submitted to Wave Motion.
OCIS Codes: (000.3860) Mathematical methods in physics; (260.2110)
Electromagnetic theory; (160.3918) Metamaterials; (160.1190) Anisotropic
optical materials; (350.7420) Waves; (230.1040) Acousto-optical devices;
(160.1050) Acousto-optical materials; (290.5839) Scattering,invisibility;
(230.3205) Invisibility cloak
Witten-Nester Energy in Topologically Massive Gravity
We formulate topologically massive supergravity with cosmological constant in
the first order formalism, and construct the Noether supercurrent and
superpotential associated with its local supersymmetry. Using these results, we
construct in ordinary topologically massive gravity the Witten-Nester integral
for conserved charges containing spinors which satisfy a generalized version of
Witten equation on the initial value surface. We show that the Witten-Nester
charge, represented as an integral over the boundary of the initial value
surface produces the Abbott-Deser-Tekin energy for asymptotically anti de
Sitter spacetimes. We consider all values of the Chern-Simons coupling
constant, including the critical value known as the chiral point, and study the
cases of standard Brown-Henneaux boundary conditions, as well as their weaker
version that allow a slower fall-off. Studying the Witten-Nester energy as a
bulk integral over the initial value surface instead, we find a bound on the
energy, and through it the sufficient condition for the positivity of the
energy. In particular, we find that spacetimes of Petrov type N that admit
globally well defined solutions of the generalized Witten equation have
positive energy.Comment: 43 page
- …