9,101 research outputs found
When does âAll Eggs in One Risky Basketâ Make Sense?
In an important paper comparing expected utility and mean-variance analysis, Feldstein (1969) examined a simple portfolio problem involving just two assets, one riskless and one risky. He concluded there could easily be âplungingâ, that is, investment in the risky asset alone. His background assumptions were that the risky assetâs yield was log normally distributed and that the investorâs attitude to risk was expressible by a logarithmic utility. We look at how conclusions are affected by choice of distribution and utility function. While conclusions can depend on choice of distribution, they are remarkably robust to choice within the range of plausible positive distributions. In contrast, conclusions are sensitive to choice of utility function and we find the key determinant to be how much the investorâs relative risk aversion differs from unity and in what direction. Based on historical stock market returns, our analysis implies that the prevalence of diversification that is observed is consistent with a relative risk aversion coefficient of about 2.5.
Electron scattering and transport in liquid argon
The transport of excess electrons in liquid argon driven out of equilibrium
by an applied electric field is revisited using a multi-term solution of
Boltzmann's equation together with ab initio liquid phase cross-sections
calculated using the Dirac-Fock scattering equations. The calculation of liquid
phase cross-sections extends previous treatments to consider multipole
polarisabilities and a non-local treatment of exchange while the accuracy of
the electron-argon potential is validated through comparison of the calculated
gas phase cross-section with experiment. The results presented highlight the
inadequacy of local treatments of exchange that are commonly used in liquid and
cluster phase cross-section calculations. The multi-term Boltzmann equation
framework accounting for coherent scattering enables the inclusion of the full
anisotropy in the differential cross-section arising from the interaction and
the structure factor, without an a priori assumption of quasi-isotropy in the
velocity distribution function. The model, which contains no free parameters
and accounts for both coherent scattering and liquid phase screening effects,
was found to reproduce well the experimental drift velocities and
characteristic energies.Comment: 32 pages, 16 figures; minor corrections, added 1 figur
Registration of retinal images from Public Health by minimising an error between vessels using an affine model with radial distortions
In order to estimate a registration model of eye fundus images made of an
affinity and two radial distortions, we introduce an estimation criterion based
on an error between the vessels. In [1], we estimated this model by minimising
the error between characteristics points. In this paper, the detected vessels
are selected using the circle and ellipse equations of the overlap area
boundaries deduced from our model. Our method successfully registers 96 % of
the 271 pairs in a Public Health dataset acquired mostly with different
cameras. This is better than our previous method [1] and better than three
other state-of-the-art methods. On a publicly available dataset, ours still
better register the images than the reference method
Hardware and software status of QCDOC
QCDOC is a massively parallel supercomputer whose processing nodes are based
on an application-specific integrated circuit (ASIC). This ASIC was
custom-designed so that crucial lattice QCD kernels achieve an overall
sustained performance of 50% on machines with several 10,000 nodes. This strong
scalability, together with low power consumption and a price/performance ratio
of $1 per sustained MFlops, enable QCDOC to attack the most demanding lattice
QCD problems. The first ASICs became available in June of 2003, and the testing
performed so far has shown all systems functioning according to specification.
We review the hardware and software status of QCDOC and present performance
figures obtained in real hardware as well as in simulation.Comment: Lattice2003(machine), 6 pages, 5 figure
The IRAS 1-Jy Survey of Ultraluminous Infrared Galaxies: I. The sample and Luminosity Function
A complete flux-limited sample of 118 ultraluminous infrared galaxies (ULIGs)
has been identified from the IRAS Faint Source Catalog (FSC). The selection
criteria were a 60 micron flux density greater than 1 Jy in a region of the sky
delta > -40 deg, |b| > 30 deg. All sources were subsequently reprocessed using
coadded IRAS maps in order to obtain the best available flux estimates in all
four IRAS wavelength bands. The maximum observed infrared luminosity is L_ir =
10^{12.90} L_{sun}, and the maximum redshift is z = 0.268. The luminosity
function for ULIGs over the decade luminosity range L_ir = 10^{12} - 10^{13}
L_{sun} can be approximated by a power law Phi (L) ~= L^{-2.35} Mpc^{-3}
mag^{-1}. In the local Universe z < 0.1, the space density of ULIGs appears to
be comparable to or slightly larger than that of optically selected QSOs at
comparable bolometric luminosities. A maximum likelihood test suggests strong
evolution for our sample; assuming density evolution proportional to
(1+z)^{alpha} we find alpha = 7.6+/-3.2. Examination of the two-point
correlation function shows a barely significant level of clustering, xi (r) =
1.6 +/- 1.2, on size scales r ~= 22 h^{-1} Mpc.Comment: 18 pages of text, 10 pages of figures 1 to 6, 6 pages of tables 1 to
3, ApJS accepte
The Fall of the Quasar Population
We derive quantitative predictions of the optical and X-ray luminosity
functions (LF) for QSs in the redshift range . Based on BH paradigm, we
investigate how the accretion is controlled by the surrounding structures, as
these grow hierarchically. We argue that for efficient black hole
fueling is triggered by the encounters of a gas-rich host with its companions
in a group. The dispersion of the dynamical parameters in the encounters
produces a double power-law LF. Strong luminosity evolution (LE) is produced as
these encounters deplete the gas supply in the host; an additional, milder
density evolution obtains since the interactions become progressively rarer as
the groups grow richer but less dense. From the agreement with the optical and
the X-ray data, we conclude that the evolution of the bright quasars is
articulated in two ways. Earlier than the gas-rich protogalaxies grow by
merging, which also induces parallel growth of central holes accreting at
Eddington rates. In the later era of group assemblage the host encounters with
companions drive onto already existing holes further but meager accretion;
these consume the gas in the hosts, and cause supply-limited emissions which
are intermittent, go progressively sub-Eddington and peter out. Then other
fueling processes come to the foreground; we discuss the faint emissions,
especially noticeable in X-rays, which are expected when hosts in the field
cannibalize satellite galaxies with their meager gas contents.Comment: 12 pages Latex + 3 EPS figures, ApJ in press, we have corrected the
previous printing problems with the style \ca
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