219,039 research outputs found
Behavior of tachyon in string cosmology based on gauged WZW model
We investigate a string theoretic cosmological model in the context of the
gauged Wess-Zumino-Witten model. Our model is based on a product of non-compact
coset space and a spectator flat space; . We
extend the formerly studied semiclassical consideration with infinite Kac-Moody
level to a finite one. In this case, the tachyon field appears in the
effective action, and we solve the Einstein equation to determine the behavior
of tachyon as a function of time. We find that tachyon field dominates over
dilaton field in early times. In particular, we consider the energy conditions
of the matter fields consisting of the dilaton and the tachyon which affect the
initial singularity. We find that not only the strong energy but also the null
energy condition is violated.Comment: 10 figure
Transport phenomenology for a holon-spinon fluid
We propose that the normal-state transport in the cuprate superconductors can
be understood in terms of a two-fluid model of spinons and holons. In our
scenario, the resistivity is determined by the properties of the holons while
magnetotransport involves the recombination of holons and spinons to form
physical electrons. Our model implies that the Hall transport time is a measure
of the electron lifetime, which is shorter than the longitudinal transport
time. This agrees with our analysis of the normal-state data. We predict a
strong increase in linewidth with increasing temperature in photoemission. Our
model also suggests that the AC Hall effect is controlled by the transport
time.Comment: 4 pages, 1 postscript figure. Uses RevTeX, epsf, multico
boosting in kernel regression
In this paper, we investigate the theoretical and empirical properties of
boosting with kernel regression estimates as weak learners. We show that
each step of boosting reduces the bias of the estimate by two orders of
magnitude, while it does not deteriorate the order of the variance. We
illustrate the theoretical findings by some simulated examples. Also, we
demonstrate that boosting is superior to the use of higher-order kernels,
which is a well-known method of reducing the bias of the kernel estimate.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ160 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
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