137 research outputs found
Statistical mixing and aggregation in Feller diffusion
We consider Feller mean-reverting square-root diffusion, which has been
applied to model a wide variety of processes with linearly state-dependent
diffusion, such as stochastic volatility and interest rates in finance, and
neuronal and populations dynamics in natural sciences. We focus on the
statistical mixing (or superstatistical) process in which the parameter related
to the mean value can fluctuate - a plausible mechanism for the emergence of
heavy-tailed distributions. We obtain analytical results for the associated
probability density function (both stationary and time dependent), its
correlation structure and aggregation properties. Our results are applied to
explain the statistics of stock traded volume at different aggregation scales.Comment: 16 pages, 3 figures. To be published in Journal of Statistical
Mechanics: Theory and Experimen
On a generalised model for time-dependent variance with long-term memory
The ARCH process (R. F. Engle, 1982) constitutes a paradigmatic generator of
stochastic time series with time-dependent variance like it appears on a wide
broad of systems besides economics in which ARCH was born. Although the ARCH
process captures the so-called "volatility clustering" and the asymptotic
power-law probability density distribution of the random variable, it is not
capable to reproduce further statistical properties of many of these time
series such as: the strong persistence of the instantaneous variance
characterised by large values of the Hurst exponent (H > 0.8), and asymptotic
power-law decay of the absolute values self-correlation function. By means of
considering an effective return obtained from a correlation of past returns
that has a q-exponential form we are able to fix the limitations of the
original model. Moreover, this improvement can be obtained through the correct
choice of a sole additional parameter, . The assessment of its validity
and usefulness is made by mimicking daily fluctuations of SP500 financial
index.Comment: 6 pages, 4 figure
Host-Tailored Sensors for Leucomalachite Green Potentiometric Measurements
A new biomimetic sensor for leucomalachite green host-guest interactions and potentiometric transduction is presented. The artificial host was imprinted in methacrylic acid or acrylamido-2-methyl-1-propanesulfonic acid-based polymers. Molecularly imprinted particles were dispersed in 2-nitrophenyloctyl ether and trapped in poly(vinyl chloride). The potentiometric sensors exhibited a near-Nernstian response in steady state evaluations, with slopes and detection limits ranging from 45.8 to 81.2 mV and 0.28 to 1.01 , respectively. They were independent from the pH of test solutions within 3 to 5. Good selectivity was observed towards drugs that may contaminate water near fish cultures, such as oxycycline, doxycycline, enrofloxacin, trimethoprim, creatinine, chloramphenicol, and dopamine. The sensors were successfully applied to field monitoring of leucomalachite green in river samples. The method offered the advantages of simplicity, accuracy, applicability to colored and turbid samples, and automation feasibility
The dynamics of financial stability in complex networks
We address the problem of banking system resilience by applying
off-equilibrium statistical physics to a system of particles, representing the
economic agents, modelled according to the theoretical foundation of the
current banking regulation, the so called Merton-Vasicek model. Economic agents
are attracted to each other to exchange `economic energy', forming a network of
trades. When the capital level of one economic agent drops below a minimum, the
economic agent becomes insolvent. The insolvency of one single economic agent
affects the economic energy of all its neighbours which thus become susceptible
to insolvency, being able to trigger a chain of insolvencies (avalanche). We
show that the distribution of avalanche sizes follows a power-law whose
exponent depends on the minimum capital level. Furthermore, we present evidence
that under an increase in the minimum capital level, large crashes will be
avoided only if one assumes that agents will accept a drop in business levels,
while keeping their trading attitudes and policies unchanged. The alternative
assumption, that agents will try to restore their business levels, may lead to
the unexpected consequence that large crises occur with higher probability
Process enhancement at near neutral pH of a homogeneous photo-Fenton reaction using ferricarboxylate complexes: Application to oxytetracycline degradation
This work demonstrates the application at near neutral pH of a photo-Fenton reaction mediated by ferricarboxylates on the treatment of aqueous solutions containing the antibiotic Oxytetracycline (OTC) under solar irradiation. The formation of a Fe:OTC complex after Fe2+ oxidation to Fe3+, in the presence of H2O2, showed the inconvenience of using the conventional Fe2+/H2O2/UV-Vis process at near neutral pH levels, as the complex is retained in the filter. To overcome this, a Fe3+/Oxalate/UV-Vis or Fe3+/Citrate/H2O2/UV-Vis process was proposed. The higher tendency of Fe3+ to form complexes with carboxylates avoids the formation of Fe:OTC complexes and allows for proper OTC detection along reaction times. The photo-Fenton process itself is improved by the extension of the iron solubility to higher and more practical pH values, by the increase of the quantum yield of Fe2+ production and by presenting stronger radiation absorption at wavelengths up to 580 nm. In this way, process efficiency was evaluated for different variables such as Fe3+ concentration, pH, temperature and irradiance, using a compound parabolic collector (CPC) photoreactor at lab-scale under simulated solar radiation. Reaction rates were compared in the presence of different inorganic anions and humic acids, and in two different real wastewater matrixes. Results obtained in a CPC pilot-scale plant under natural solar light, using an iron/oxalate molar rati
Competition and fragmentation: a simple model generating lognormal-like distributions
The current distribution of language size in terms of speaker population is
generally described using a lognormal distribution. Analyzing the original real
data we show how the double-Pareto lognormal distribution can give an
alternative fit that indicates the existence of a power law tail. A simple
Monte Carlo model is constructed based on the processes of competition and
fragmentation. The results reproduce the power law tails of the real
distribution well and give better results for a poorly connected topology of
interactions.Comment: 14 pages, 11 figure
Occupational stress in health professionals: a study with Portuguese Nurses
Este trabalho analisa o estresse ocupacional em 286 enfermeiros de hospitais e centros de saúde portugueses. Avaliaram-se as fontes de estresse, o burnout, os problemas de saúde física, a satisfação e a realização profissional. Os resultados apontaram 30% de enfermeiros com experiências significativas de estresse e 15% com problemas de exaustão emocional. As análises de regressão múltipla apontaram maior capacidade preditiva das dimensões de estresse na exaustão emocional, na saúde física, na satisfação e na realização profissional. As análises comparativas evidenciaram maiores problemas de stresse e reacções mais negativas ao trabalho nas mulheres, nos enfermeiros mais novos e com menor experiência, nos trabalhadores com contratos a prazo, nos profissionais que realizam trabalho por turnos e nos que trabalham mais horas.This work analyses occupational stress in 286 nurses from hospitals and health care centres in Portugal. The
following dimensions were evaluated: stress, burnout, physical health problems, satisfaction and professional fulfilment.
Results revealed significant stress experiences in 30% of the professionals, and emotional exhaustion problems in 15%.
Multiple regression analysis pointed out stress as an important predictor of emotional exhaustion, physical health, satisfaction
and professional fulfilment. Comparative analysis suggested more occupational stress and professional negative experiences
in the following groups: female nurses, younger and less experienced nurses, those with short-term working contracts, nurses
working on a shift-basis system, and nurses working during long hours.(undefined
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