9,718 research outputs found
The Ultrarelativistic Kerr-Geometry and its Energy-Momentum Tensor
The ultrarelativistic limit of the Schwarzschild and the Kerr-geometry
together with their respective energy-momentum tensors is derived. The approach
is based on tensor-distributions making use of the underlying Kerr-Schild
structure, which remains stable under the ultrarelativistic boost.Comment: 16 pages, (AMS-LaTeX), TUW-94-0
Dependence of the 0.5(2e2/h) conductance plateau on the aspect ratio of InAs quantum point contacts with in-plane side gates
The observation of a 0.5 conductance plateau in asymmetrically biased quantum
point contacts with in-plane side gates has been attributed to the onset of
spin-polarized current through these structures. For InAs quantum point
contacts with the same width but longer channel length, there is roughly a
fourfold increase in the range of common sweep voltage applied to the side
gates over which the 0.5 conductance plateau is observed when the QPC aspect
ratio (ratio of length over width of the narrow portion of the structure) is
increased by a factor 3. Non-equilibrium Green s function simulations indicate
that the increase in the size of the 0.5 conductance plateau is due to an
increased importance, over a larger range of common sweep voltage, of the
effects of electron-electron interactions in QPC devices with larger aspect
ratio. The use of asymmetrically biased QPCs with in-plane side gates and large
aspect ratio could therefore pave the way to build robust spin injectors and
detectors for the successful implementation of spin field effect transistorsComment: 30 pages, 9 figure
Escape path complexity and its context dependency in Pacific blue-eyes (Pseudomugil signifer)
The escape trajectories animals take following a predatory attack appear to
show high degrees of apparent 'randomness' - a property that has been described
as 'protean behaviour'. Here we present a method of quantifying the escape
trajectories of individual animals using a path complexity approach. When fish
(Pseudomugil signifer) were attacked either on their own or in groups, we find
that an individual's path rapidly increases in entropy (our measure of
complexity) following the attack. For individuals on their own, this entropy
remains elevated (indicating a more random path) for a sustained period (10
seconds) after the attack, whilst it falls more quickly for individuals in
groups. The entropy of the path is context dependent. When attacks towards
single fish come from greater distances, a fish's path shows less complexity
compared to attacks that come from short range. This context dependency effect
did not exist, however, when individuals were in groups. Nor did the path
complexity of individuals in groups depend on a fish's local density of
neighbours. We separate out the components of speed and direction changes to
determine which of these components contributes to the overall increase in path
complexity following an attack. We found that both speed and direction measures
contribute similarly to an individual's path's complexity in absolute terms.
Our work highlights the adaptive behavioural tactics that animals use to avoid
predators and also provides a novel method for quantifying the escape
trajectories of animals.Comment: 9 page
A Note on the Symmetries of the Gravitational Field of a Massless Particle
It is shown that the metric of a massless particle obtained from boosting the
Schwarzschild metric to the velocity of light, has four Killing vectors
corresponding to an E(2)\times \RR symmetry-group. This is in agreement with
the expectations based on flat-space kinematics but is in contrast to previous
statements in the literature \cite{Schueck}. Moreover, it also goes beyond the
general Jordan-Ehlers-Kundt-(JEK)-classification of gravitational pp-waves as
given in \cite{JEK}.Comment: 10pages, amslatex, TUW-94-12 and UWThPh-1994-2
An Online Decision-Theoretic Pipeline for Responder Dispatch
The problem of dispatching emergency responders to service traffic accidents,
fire, distress calls and crimes plagues urban areas across the globe. While
such problems have been extensively looked at, most approaches are offline.
Such methodologies fail to capture the dynamically changing environments under
which critical emergency response occurs, and therefore, fail to be implemented
in practice. Any holistic approach towards creating a pipeline for effective
emergency response must also look at other challenges that it subsumes -
predicting when and where incidents happen and understanding the changing
environmental dynamics. We describe a system that collectively deals with all
these problems in an online manner, meaning that the models get updated with
streaming data sources. We highlight why such an approach is crucial to the
effectiveness of emergency response, and present an algorithmic framework that
can compute promising actions for a given decision-theoretic model for
responder dispatch. We argue that carefully crafted heuristic measures can
balance the trade-off between computational time and the quality of solutions
achieved and highlight why such an approach is more scalable and tractable than
traditional approaches. We also present an online mechanism for incident
prediction, as well as an approach based on recurrent neural networks for
learning and predicting environmental features that affect responder dispatch.
We compare our methodology with prior state-of-the-art and existing dispatch
strategies in the field, which show that our approach results in a reduction in
response time with a drastic reduction in computational time.Comment: Appeared in ICCPS 201
Previously Unidentified Changes in Renal Cell Carcinoma Gene Expression Identified by Parametric Analysis of Microarray Data
BACKGROUND. Renal cell carcinoma is a common malignancy that often presents as a metastatic-disease for which there are no effective treatments. To gain insights into the mechanism of renal cell carcinogenesis, a number of genome-wide expression profiling studies have been performed. Surprisingly, there is very poor agreement among these studies as to which genes are differentially regulated. To better understand this lack of agreement we profiled renal cell tumor gene expression using genome-wide microarrays (45,000 probe sets) and compare our analysis to previous microarray studies. METHODS. We hybridized total RNA isolated from renal cell tumors and adjacent normal tissue to Affymetrix U133A and U133B arrays. We removed samples with technical defects and removed probesets that failed to exhibit sequence-specific hybridization in any of the samples. We detected differential gene expression in the resulting dataset with parametric methods and identified keywords that are overrepresented in the differentially expressed genes with the Fisher-exact test. RESULTS. We identify 1,234 genes that are more than three-fold changed in renal tumors by t-test, 800 of which have not been previously reported to be altered in renal cell tumors. Of the only 37 genes that have been identified as being differentially expressed in three or more of five previous microarray studies of renal tumor gene expression, our analysis finds 33 of these genes (89%). A key to the sensitivity and power of our analysis is filtering out defective samples and genes that are not reliably detected. CONCLUSIONS. The widespread use of sample-wise voting schemes for detecting differential expression that do not control for false positives likely account for the poor overlap among previous studies. Among the many genes we identified using parametric methods that were not previously reported as being differentially expressed in renal cell tumors are several oncogenes and tumor suppressor genes that likely play important roles in renal cell carcinogenesis. This highlights the need for rigorous statistical approaches in microarray studies.National Institutes of Healt
The One-dimensional KPZ Equation and the Airy Process
Our previous work on the one-dimensional KPZ equation with sharp wedge
initial data is extended to the case of the joint height statistics at n
spatial points for some common fixed time. Assuming a particular factorization,
we compute an n-point generating function and write it in terms of a Fredholm
determinant. For long times the generating function converges to a limit, which
is established to be equivalent to the standard expression of the n-point
distribution of the Airy process.Comment: 15 page
Development of retinal blood vessel segmentation methodology using wavelet transforms for assessment of diabetic retinopathy
Automated image processing has the potential to assist in the early detection of diabetes, by detecting changes in blood vessel diameter and patterns in the retina. This paper describes the development of segmentation methodology in the processing of retinal blood vessel images obtained using non-mydriatic colour photography. The methods used include wavelet analysis, supervised classifier probabilities and adaptive threshold procedures, as well as morphology-based techniques. We show highly accurate identification of blood vessels for the purpose of studying changes in the vessel network that can be utilized for detecting blood vessel diameter changes associated with the pathophysiology of diabetes. In conjunction with suitable feature extraction and automated classification methods, our segmentation method could form the basis of a quick and accurate test for diabetic retinopathy, which would have huge benefits in terms of improved access to screening people for risk or presence of diabetes
Immune response & modulation of immune response induced in the guineapigs by Mycobacterium avium complex (MAC) & M. fortuitum complex isolates from different sources in the south Indian BCG trial area
A total of 139 guineapigs were used to study the immune response and its modulation induced by
Mycobacterium avium complex (MAC) and M. fortuitum complex strains obtained from different
sources in the south Indian BCG trial area. The guineapigs were divided into groups and some were
directly sensitised/immunised with different MAC strains, M. fortuitum complex strain or BCG and
others were sensitised with MAC or M. fortuitum complex and then immunised with BCG. The
resulting delayed type hypersensitivity (DTH) response in the different groups of guineapigs was
studied by skin tests using PPD-RT23 and PPD-B, and protective response was studied by challenging
the guineapigs with a south Indian low virulent strain of M. tuberculosis and enumerating the
bacilli in spleen at different points of time. The 3 strains of MAC induced similar low levels of DTH
to PPD-RT23 but much higher and varying levels of DTH to PPD-B. MAC strains from soil and
sputum induced different levels of immune modulation during subsequent immunisation with BCG
on the DTH response to PPD-RT23 and PPD-B. At 2 wk after challenge, 23.8, 81 and 90.5 per cent
protection was induced by the standard strain, soil isolate and sputum isolate of MAC, respectively,
while 33.3 per cent protection was induced by the M. fortuitum complex strain compared to the
protection induced by BCG alone. Prior exposure to MAC or M. fortuitum complex did not have
any modulatory effect on the protective immunity due to BCG at this time point. However, at 6
wk after challenge, while the guineapigs immunised with BCG were protected, modulation of the
protective response resulting from BCG was observed in the guineapigs sensitised with MAC and
M. fortuitum from soil
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