5,884 research outputs found
Bouncing Negative-Tension Branes
Braneworlds, understood here as double domain wall spacetimes, can be
described in terms of a linear harmonic function, with kinks at the locations
of the boundary branes. In a dynamical setting, there is therefore the risk
that the boundary brane of negative tension, at whose location the value of the
harmonic function is always lowest, can encounter a zero of this harmonic
function, corresponding to the formation of a singularity. We show that for
certain types of brane-bound matter this singularity can be avoided, and the
negative-tension brane can shield the bulk spacetime from the singularity by
bouncing back smoothly before reaching the singularity. In our analysis we
compare the 5- and 4-dimensional descriptions of this phenomenon in order to
determine the validity of the moduli space approximation.Comment: 20 pages, 1 figure, discussion extended and references added; minor
mistake correcte
Econometrics
Since the last decade we live in a digitalized world where many actions in human and economic life are monitored. This produces a continuous stream of new, rich and high quality data in the form of panels, repeated cross-sections and long time series . These data resources are available to many researchers at a low cost. This new erais fascinating for econometricians who can adress many open economic questions. To do so, new models are developed that call for elaborate estimation techniques. Fast personal computers play an integral part in making it possible to deal with this increased complexity. --
Differential effects of steroids and retinoids on bovine myelopoiesis in vitro
Pregnancy and parturition are associated with physiological changes caused by steroid hormones. Alterations in number, maturity, and function of polymorphonuclear leukocytes observed in dairy cows at parturition suggest a common causative relationship with steroid hormones. This study was designed to investigate the effects of progesterone, 17-beta-estradiol, and hydrocortisone on the proliferation of bovine progenitor cells. An in vitro culturing system was used, and colonies were scored after 7 d of incubation. At low concentrations, 17-beta-estradiol inhibited proliferation of granulocyte progenitor cells. Hydrocortisone reduced growth of granulocyte and monocyte colonies, whereas myelopoiesis was not altered by progesterone. Furthermore, we studied the effect of retinoids on colony formation of bovine bone marrow cells. All-trans- and 9-cis-retinoic acid stimulated growth of granulocyte colonies and inhibited proliferation of the monocyte lineage. The addition of the 13-cis-isomer also increased numbers of granulocyte colony-forming units. This study indicates that steroid hormones may be responsible for alterations in the bovine hematopoietic profiles observed in circulation during the postpartum period. White blood cells, especially polymorphonuclear leukocytes, which are derived from bone marrow, are an important first line defense against mastitis. Therefore, these effects of steroids might contribute to the increased susceptibility of dairy cows to Escherichia coli mastitis. We furthermore hypothesize that an important role might be attributed to retinoic acid in its regulation of bovine myelopoiesis. Modulation of myelopoiesis in favor of the granulocyte lineage during the acute-phase reaction may be an adaptive mechanism designed to increase the capacity of first-line defense to intramammary infections
Scaling Invariance of Fatigue Crack Growth in Gigacycle Loading Regime
The role of the collective behavior of defect ensembles at the crack tip and the laws of fatigue crack propagation in R4 high strength steel have been studied under conditions of symmetric tension–compression gigacycle loading at 20 kHz. At every stage of the fatigue crack growth, replicas from the sample side surface were taken and studied by the method of three dimensional relief profilometry (using NewView interferometer profilometer) so as to study the scaling invariant laws of defect related structure evolution.This study was supported in part by the Russian Foundation for Basic Research, Project nos. 08-01-00699, 09-01-92005-NNS_a, and 09-01-92441-KE-
Children’s Perceptions of Cohesion
The general purpose of the two studies reported here was to examine perceptions of team cohesiveness in children aged 9 to 12 years. In Study 1, focus groups were used to examine individual perceptions of cohesion from the perspective of group integration—the group as a totality. In Study 2, open-ended questionnaires were used to examine individual perceptions of cohesion from the perspective of individual attractions to the group. The results showed that children as young as nine years understand the phenomenon known as cohesion. They can discuss the group as a totality, the characteristics of cohesive and non-cohesive teams, and identify the major factors attracting them to and maintaining their involvement in the group. Also, the ability to clearly distinguish between task and social cohesion is present. The results provide insight into the world of child sport and emphasise the importance of group cohesion, affiliation, and other social constructs in children’s involvement and adherence to sport groups
A comparison of forecasting procedures for macroeconomic series: the contribution of structural break models
This paper compares the forecasting performance of different models which have been proposed for forecasting in the presence of structural breaks. These models differ in their treatment of the break process, the parameters defining the model which applies in each regime and the out-of-sample probability of a break occurring. In an extensive empirical evaluation involving many important macroeconomic time series, we demonstrate the presence of structural breaks and their importance for forecasting in the vast majority of cases. However, we find no single forecasting model consistently works best in the presence of structural breaks. In many cases, the formal modeling of the break process is important in achieving good forecast performance. However, there are also many cases where simple, rolling OLS forecasts perform well.forecasting, change-points, Markov switching, Bayesian inference
Secure Degrees of Freedom of MIMO X-Channels with Output Feedback and Delayed CSIT
We investigate the problem of secure transmission over a two-user multi-input
multi-output (MIMO) X-channel in which channel state information is provided
with one-unit delay to both transmitters (CSIT), and each receiver feeds back
its channel output to a different transmitter. We refer to this model as MIMO
X-channel with asymmetric output feedback and delayed CSIT. The transmitters
are equipped with M-antennas each, and the receivers are equipped with
N-antennas each. For this model, accounting for both messages at each receiver,
we characterize the optimal sum secure degrees of freedom (SDoF) region. We
show that, in presence of asymmetric output feedback and delayed CSIT, the sum
SDoF region of the MIMO X-channel is same as the SDoF region of a two-user MIMO
BC with 2M-antennas at the transmitter, N-antennas at each receiver and delayed
CSIT. This result shows that, upon availability of asymmetric output feedback
and delayed CSIT, there is no performance loss in terms of sum SDoF due to the
distributed nature of the transmitters. Next, we show that this result also
holds if only output feedback is conveyed to the transmitters, but in a
symmetric manner, i.e., each receiver feeds back its output to both
transmitters and no CSIT. We also study the case in which only asymmetric
output feedback is provided to the transmitters, i.e., without CSIT, and derive
a lower bound on the sum SDoF for this model. Furthermore, we specialize our
results to the case in which there are no security constraints. In particular,
similar to the setting with security constraints, we show that the optimal sum
DoF region of the (M,M,N,N)--MIMO X-channel with asymmetric output feedback and
delayed CSIT is same as the DoF region of a two-user MIMO BC with 2M-antennas
at the transmitter, N-antennas at each receiver, and delayed CSIT. We
illustrate our results with some numerical examples.Comment: To Appear in IEEE Transactions on Information Forensics and Securit
A Quantum Analogue of the Algebra
We define a natural quantum analogue for the algebra, and which we
refer to as the algebra, by modding out the Heisenberg algebra
from the quantum affine algebra with level . We discuss
the representation theory of this algebra. In particular, we
exhibit its reduction to a group algebra, and to a tensor product of a group
algebra with a quantum Clifford algebra when , and , and thus, we
recover the explicit constructions of \uq-standard modules as achieved by
Frenkel-Jing and Bernard, respectively. Moreover, for arbitrary nonzero level
, we show that the explicit basis for the simplest -generalized
Verma module as constructed by Lepowsky and primc is also a basis for its
corresponding -module, i.e., it is invariant under the
q-deformation for generic q. We expect this algebra (associated
with \uq at level ), to play the role of a dynamical symmetry in the
off-critical statistical models.Comment: 32 pages, LATEX, minor change
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