3,886 research outputs found
How many independent bets are there?
The benefits of portfolio diversification is a central tenet implicit to
modern financial theory and practice. Linked to diversification is the notion
of breadth. Breadth is correctly thought of as the number of in- dependent bets
available to an investor. Conventionally applications us- ing breadth
frequently assume only the number of separate bets. There may be a large
discrepancy between these two interpretations. We uti- lize a simple
singular-value decomposition (SVD) and the Keiser-Gutman stopping criterion to
select the integer-valued effective dimensionality of the correlation matrix of
returns. In an emerging market such as South African we document an estimated
breadth that is considerably lower than anticipated. This lack of
diversification may be because of market concentration, exposure to the global
commodity cycle and local currency volatility. We discuss some practical
extensions to a more statistically correct interpretation of market breadth,
and its theoretical implications for both global and domestic investors.Comment: Less technical rewrite. 12 Pages, 6 Figures (.eps
Behavioral Changes in Aging but Not Young Mice after Neonatal Exposure to the Polybrominated Flame Retardant DecaBDE
BACKGROUND: After several decades of commercial use, the flame-retardant chemicals polybrominated diphenyl ethers (PBDEs) and their metabolites are pervasive environmental contaminants and are detected in the human body. Decabrominated diphenyl ether (decaBDE) is currently the only PBDE in production in the United States. OBJECTIVES: Little is known about the health effects of decaBDE. In the present study we examined the effects of neonatal decaBDE exposure on behavior in mice at two ages. METHODS: Neonatal male and female C57BL6/J mice were exposed to a daily oral dose of 0, 6, or 20 mg/kg decaBDE from postnatal days 2 through 15. Two age groups were examined: a cohort that began training during young adulthood and an aging cohort of littermates that began training at 16 months of age. Both cohorts were tested on a series of operant procedures that included a fixed-ratio I schedule of reinforcement, a fixed-interval (FI) 2-min schedule, and a light-dark visual discrimination. RESULTS: We observed minimal effects on the light-dark discrimination in the young cohort, with no effects on the other tasks. The performance of the aging cohort was significantly affected by decaBDE. On the FI schedule, decaBDE exposure increased the overall response rate. On the light-dark discrimination, older treated mice learned the task more slowly, made fewer errors on the first-response choice of a trial but more perseverative errors after an initial error, and had lower latencies to respond compared with controls. Effects were observed in both dose groups and sexes on various measures. CONCLUSIONS: These findings suggest that neonatal decaBDE exposure produces effects on behavioral tasks in older but not younger animals. The behavioral mechanisms responsible for the pattern of observed effects may include increased impulsivity, although further research is required
Stress dependent thermal pressurization of a fluid-saturated rock
Temperature increase in saturated porous materials under undrained conditions
leads to thermal pressurization of the pore fluid due to the discrepancy
between the thermal expansion coefficients of the pore fluid and of the solid
matrix. This increase in the pore fluid pressure induces a reduction of the
effective mean stress and can lead to shear failure or hydraulic fracturing.
The equations governing the phenomenon of thermal pressurization are presented
and this phenomenon is studied experimentally for a saturated granular rock in
an undrained heating test under constant isotropic stress. Careful analysis of
the effect of mechanical and thermal deformation of the drainage and pressure
measurement system is performed and a correction of the measured pore pressure
is introduced. The test results are modelled using a non-linear
thermo-poro-elastic constitutive model of the granular rock with emphasis on
the stress-dependent character of the rock compressibility. The effects of
stress and temperature on thermal pressurization observed in the tests are
correctly reproduced by the model
Development of flocculation models for improving water treatment
In water treatment the flocculation of particles into larger aggregates or ‘flocs’ allows impurities or solid material to be removed more easily by sedimentation.
One example is where soluble phosphorus is removed by dosing with ferric sulphate to form a precipitate containing the phosphorus. Dosing usually requires rapid mixing of the ferric sulphate followed by gentle mixing to encourage flocculation.
The use of Computational Fluid Dynamics (CFD) allows multiphase systems to be modelled and the effect of design changes to be studied in order to optimise water treatment. The development of a framework for modelling flocculation provides the possibility to optimise water treatment processes involving the flocculation of particulates.
The paper describes in greater detail the physical models and verification and validation cases undertaken in order to develop the framework and a case study in which CFD was used to optimise the mixing performance of a flocculation tank
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