2,548 research outputs found
CVaR minimization by the SRA algorithm
Using the risk measure CV aR in �nancial analysis has become
more and more popular recently. In this paper we apply CV aR for portfolio optimization. The problem is formulated as a two-stage stochastic programming model, and the SRA algorithm, a recently developed heuristic algorithm, is applied for minimizing CV aR
Maximum Mass-Radius Ratios for Charged Compact General Relativistic Objects
Upper limits for the mass-radius ratio and total charge are derived for
stable charged general relativistic matter distributions. For charged compact
objects the mass-radius ratio exceeds the value 4/9 corresponding to neutral
stars. General restrictions for the redshift and total energy (including the
gravitational contribution) are also obtained.Comment: 6 pages, 2 figures, RevTex. To appear in Europhys. Let
Renormalization Group Approach to Causal Viscous Cosmological Models
The renormalization group method is applied to the study of homogeneous and
flat Friedmann-Robertson-Walker type Universes, filled with a causal bulk
viscous cosmological fluid. The starting point of the study is the
consideration of the scaling properties of the gravitational field equations,
of the causal evolution equation of the bulk viscous pressure and of the
equations of state. The requirement of scale invariance imposes strong
constraints on the temporal evolution of the bulk viscosity coefficient,
temperature and relaxation time, thus leading to the possibility of obtaining
the bulk viscosity coefficient-energy density dependence. For a cosmological
model with bulk viscosity coefficient proportional to the Hubble parameter, we
perform the analysis of the renormalization group flow around the scale
invariant fixed point, therefore obtaining the long time behavior of the scale
factor.Comment: 19 pages. RevTeX4. Revised version. Accepted in Classical and Quantum
Gravit
A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air
We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140–150 nmol mol-1, which is higher than today's values. CO mole fractions rose by 10–15 nmol mol-1 from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a ˜ 30 nmol mol-1 decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe
Exact anisotropic brane cosmologies
We present exact solutions of the gravitational field equations in the
generalized Randall-Sundrum model for an anisotropic brane with Bianchi type I
and V geometry, with perfect fluid and scalar fields as matter sources. Under
the assumption of a conformally flat bulk (with vanishing Weyl tensor) for a
cosmological fluid obeying a linear barotropic equation of state the general
solution of the field equations can be expressed in an exact parametric form
for both Bianchi type I and V space-times. In the limiting case of a stiff
cosmological fluid with pressure equal to the energy density, for a Bianchi
type I Universe the solution of the field equations are obtained in an exact
analytic form. Several classes of scalar field models evolution on the brane
are also considered, corresponding to different choices of the scalar field
potential. For all models the behavior of the observationally important
parameters like shear, anisotropy and deceleration parameter is considered in
detail.Comment: revised version to appear in PR
Charged anisotropic matter with linear equation of state
We consider the general situation of a compact relativistic body with
anisotropic pressures in the presence of the electromagnetic field. The
equation of state for the matter distribution is linear and may be applied to
strange stars with quark matter. Three classes of new exact solutions are found
to the Einstein-Maxwell system. This is achieved by specifying a particular
form for one of the gravitational potentials and the electric field intensity.
We can regain anisotropic and isotropic models from our general class of
solution. A physical analysis indicates that the charged solutions describe
realistic compact spheres with anisotropic matter distribution. The equation of
state is consistent with dark energy stars and charged quark matter
distributions. The masses and central densities correspond to realistic stellar
objects in the general case when anisotropy and charge are present.Comment: 17 pages, To appear in Class. Quantum Gra
The importance and clinical relevance of surfaces in tissue culture
Cell and tissue culture has evolved from the use of simple glassware for the propagation of cells and tissues into a comprehensive platform for interrogating complex biological systems, directing cell fate and deriving products with clinical and therapeutic value. However, despite significant advances, current in vitro culture approaches remain limited in their capacity to model the clinical/biological complexities of disease, in part at least due to the deficiencies of existing culture materials. The challenge is therefore to identify innovative materials-based solutions that have greater control over cells in vitro, while better representing biological systems in vivo. Such platforms would be suitable for biomarker discovery and tissue engineering applications. This review examines the development of tissue culture materials, advances in our understanding of cell-surface interactions and the application of this knowledge towards the development of new approaches for better examining biological events
On the formation of Wigner molecules in small quantum dots
It was recently argued that in small quantum dots the electrons could
crystallize at much higher densities than in the infinite two-dimensional
electron gas. We compare predictions that the onset of spin polarization and
the formation of Wigner molecules occurs at a density parameter to the results of a straight-forward diagonalization of the Hamiltonian
matrix
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
Diffusive Spreading of Chainlike Molecules on Surfaces
We study the diffusion and submonolayer spreading of chainlike molecules on
surfaces. Using the fluctuating bond model we extract the collective and tracer
diffusion coefficients D_c and D_t with a variety of methods. We show that
D_c(theta) has unusual behavior as a function of the coverage theta. It first
increases but after a maximum goes to zero as theta go to one. We show that the
increase is due to entropic repulsion that leads to steep density profiles for
spreading droplets seen in experiments. We also develop an analytic model for
D_c(theta) which agrees well with the simulations.Comment: 3 pages, RevTeX, 4 postscript figures, to appear in Phys. Rev.
Letters (1996
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