880 research outputs found
Variance Function Estimation
We develops a general theory for variance function estimation in regression. Most methods in common use are included in our development. The general qualitative conclusions are these. First, most variance function estimation procedures can be looked upon as regressions with responses' being transformations of absolute residuals from a preliminary fit or sample standard deviations from replicates at a design point. Our conclusion is that the former is typically more efficient, but not uniformly so. Secondly, for variance function estimates based on transformations of absolute residuals, we show that efficiency is a monotone function of the efficiency of the fit from which the residuals are formed, at least for symmetric errors. Our conclusion Is that one should iterate so that the residuals are based on generalized least squares. Finally, robustness issues are of even more importance here than in estimation of a regression function for the mean. The loss of efficiency of the standard method away from the normal distribution is much more rapid than in the regression problem
Robust Pricing with Refunds
Before purchase, a buyer of an experience good learns about the product's fit using various information sources, including some of which the seller may be unaware of. The buyer, however, can conclusively learn the fit only after purchasing and trying out the product. We show that the seller can use a simple mechanism to best take advantage of the buyer's post-purchase learning to maximize his guaranteed-profit. We show that this mechanism combines a generous refund, which performs well when the buyer is relatively informed, with non-refundable random discounts, which work well when the buyer is relatively uninformed. JEL: D82, C79, D4
Recent developments in the Thomson Parabola Spectrometer diagnostic for laser-driven multi-species ion sources
Noncommutative Quantum Mechanics and Seiberg-Witten Map
In order to overcome ambiguity problem on identification of mathematical
objects in noncommutative theory with physical observables, quantum mechanical
system coupled to the NC U(1) gauge field in the noncommutative space is
reformulated by making use of the unitarized Seiberg-Witten map, and applied to
the Aharonov-Bohm and Hall effects of the NC U(1) gauge field. Retaining terms
only up to linear order in the NC parameter \theta, we find that the AB
topological phase and the Hall conductivity have both the same formulas as
those of the ordinary commutative space with no \theta-dependence.Comment: 7 pages, no figures, uses revtex4; 8 pages, conclusion changed,
Appendix adde
Noncommutativity and Lorentz Violation in Relativistic Heavy Ion Collisions
The experimental detection of the effects of noncommuting coordinates in
electrodynamic phenomena depends on the magnitude of |\theta B|, where \theta
is the noncommutativity parameter and B a background magnetic field. With the
present upper bound on \theta, given by \theta_{\rm bound} \simeq 1/(10 {\rm
TeV})^2, there was no large enough magnetic field in nature, including those
observed in magnetars, that could give visible effects or, conversely, that
could be used to further improve \theta_{\rm bound}. On the other hand,
recently it has been proposed that intense enough magnetic fields should be
produced at the beginning of relativistic heavy ion collisions. We discuss here
lepton pair production by free photons as one kind of signature of
noncommutativity and Lorentz violation that could occur at RHIC or LHC. This
allows us to obtain a more stringent bound on \theta, given by 10^{-3}
\theta_{\rm bound}, if such "exotic" events do not occur.Comment: Five pages, no figures
Discussing Quantum Aspects of Higher-Derivative 3D-Gravity in the First-Order Formalism
In this paper, we reassess the issue of deriving the propagators and
identifying the spectrum of excitations associated to the vielbein and spin
connection of (1+2)-D gravity in the presence of dynamical torsion, while
working in the first-order formulation. A number of peculiarities is pointed
out whenever the Chern-Simons term is taken into account along with a
combination of bilinear terms in the torsion tensor. We present a procedure to
derive the full set of propagators, based on an algebra of enlarged spin-type
operators, and we discuss under which conditions the poles of the tree-level
2-point functions correspond to physical excitations that do not conflict with
causality and unitarity
Noncommutativity, generalized uncertainty principle and FRW cosmology
We consider the effects of noncommutativity and the generalized uncertainty
principle on the FRW cosmology with a scalar field. We show that, the
cosmological constant problem and removability of initial curvature singularity
find natural solutions in this scenarios.Comment: 8 pages, to appear in IJT
Pure kinetic k-essence as the cosmic speed-up
In this paper, we consider three types of k-essence. These k-essence models
were presented in the parametric forms. The exact analytical solutions of the
corresponding equations of motion are found. It is shown that these k-essence
models for the presented solutions can give rise to cosmic acceleration.Comment: 10 pages, typos corrected, main results remain the same, minor
changes to match IJTP accepted versio
Modified gravity in a viscous and non-isotropic background
We study the dynamical evolution of an model of gravity in a viscous
and anisotropic background which is given by a Bianchi type-I model of the
Universe. We find viable forms of gravity in which one is exactly the
Einsteinian model of gravity with a cosmological constant and other two are
power law models. We show that these two power law models are stable
with a suitable choice of parameters. We also examine three potentials which
exhibit the potential effect of models in the context of scalar tensor
theory. By solving different aspects of the model and finding the physical
quantities in the Jordan frame, we show that the equation of state parameter
satisfy the dominant energy condition. At last we show that the two power law
models behave like quintessence model at late times and also the shear
coefficient viscosity tends to zero at late times.Comment: 7 pages, 2 figure
Parametrization of Born-Infeld Type Phantom Dark Energy Model
Applying the parametrization of dark energy density, we can construct
directly independent-model potentials. In Born-Infeld type phantom dark energy
model, we consider four special parametrization equation of state parameter.
The evolutive behavior of dark energy density with respect to red-shift ,
potentials with respect to and are shown mathematically. Moreover,
we investigate the effect of parameter upon the evolution of the
constructed potential with respect to . These results show that the
evolutive behavior of constructed Born-Infeld type dark energy model is quite
different from those of the other models.Comment: 5 pages, 4 figures, Accepted for publication in Astrophysics & Space
Scienc
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