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Search Results: 1 - 10 of 325376 matches for " S Vamos "
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Detrended Fluctuation Analysis of Autoregressive Processes
V. V. Morariu,L. Buimaga-Iarinca,C. Vamos,S. Soltuz
Physics , 2007,
Abstract: Autoregressive processes (AR) have typical short-range memory. Detrended Fluctuation Analysis (DFA) was basically designed to reveal long range correlation in non stationary processes. However DFA can also be regarded as a suitable method to investigate both long-range and short range correlation in non-stationary and stationary systems. Applying DFA to AR processes can help understanding the non uniform correlation structure of such processes. We systematically investigated a first order autoregressive model AR(1) by DFA and established the relationship between the interaction constant of AR(1) and the DFA correlation exponent. The higher the interaction constant the higher is the short range correlation exponent. They are exponentially related. The investigation was extended to AR(2) processes. The presence of a distant positive interaction in addition to a near by interaction will increase the correlation exponent and the range of correlation while the effect of a distant negative interaction will decrease significantly only the range of interaction. This analysis demonstrate the possibility to identify and AR(1) model in an unknown DFA plot or to distinguish among AR(1) and AR(2) models. The analysis was performed on medium long series of 1000 terms.
Seismic hemispheric asymmetry induced by Earth's inner core decentering
Calin Vamos,Nicolae Suciu
Physics , 2011,
Abstract: In a first approximation the Earth's interior has an isotropic structure with a spherical symmetry. Over the last decades the geophysical observations have revealed, at different spatial scales, the existence of several perturbations from this basic structure. Some of them are situated in the neighborhood of the inner core boundary (ICB). One of the best documented perturbations is the asymmetry at the top of the inner core (ATIC) characterized by faster seismic wave velocity in the eastern hemisphere than in the western hemisphere. All existing explanations are based on a hemispheric variation of the material properties near ICB inside the inner core. Using numerical simulations of the seismic ray propagation, we show that the ATIC can be explained as well by the displacement of the inner core towards east in the equatorial plane tens of kilometers from the Earth's center, without modifying the spherical symmetry in the upper inner core. The hypothesis of a displaced inner core is also sustained by other observed hemispheric asymmetries at the top of the inner core and at the bottom of the outer core. A displaced inner core would have major implications for many mechanical, thermal, and magnetic phenomena in the Earth's interior.
Geophysical implications of a decentered inner core
Calin Vamos,Nicolae Suciu
Physics , 2014,
Abstract: In a first approximation, the Earth's interior has an isotropic structure with a spherical symmetry. Over the last decades the geophysical observations have revealed, at different spatial scales, the existence of several perturbations from this basic structure. In this paper we discuss the hemispheric perturbations induced to this basic structure if the inner core is displaced from the center of mass of the Earth. Using numerical simulations of the observed hemispheric asymmetry of the seismic waves traveling through the upper inner core, with faster arrival times and higher attenuation in the Eastern Hemisphere, we estimate that the present position of the inner core is shifted by tens of kilometers from the Earth's center eastward in the equatorial plane. If the only forces acting on the inner core were the gravitational forces, then its equilibrium position would be at the Earth's center and the estimated displacement would not be possible. We conjecture that, due to interactions with the flow and the magnetic field inside the outer core, the inner core is in a permanent chaotic motion. To support this hypothesis we analyze more than ten different geophysical phenomena consistent with an inner core motion dominated by time scales from hundreds to thousands of years.
Influence of deterministic trend on the estimated parameters of GARCH(1,1) model
Calin Vamos,Maria Craciun
Physics , 2007,
Abstract: The log returns of financial time series are usually modeled by means of the stationary GARCH(1,1) stochastic process or its generalizations which can not properly describe the nonstationary deterministic components of the original series. We analyze the influence of deterministic trends on the GARCH(1,1) parameters using Monte Carlo simulations. The statistical ensembles contain numerically generated time series composed by GARCH(1,1) noise superposed on deterministic trends. The GARCH(1,1) parameters characteristic for financial time series longer than one year are not affected by the detrending errors. We also show that if the ARCH coefficient is greater than the GARCH coefficient, then the estimated GARCH(1,1) parameters depend on the number of monotonic parts of the trend and on the ratio between the trend and the noise amplitudes.
Community-Based Risk Reduction in Zambia
D.L. Jones, S.M. Weiss, D. Waldrop-Valverde, N. Chitalu, M. Mumbi and S. Vamos
The Open Health Services and Policy Journal , 2008, DOI: 10.2174/1874924000801010045]
Abstract: Following the trial of a sexual risk reduction intervention conducted at the University Teaching Hospital (UTH) in Lusaka, Zambia, this pilot study sought to evaluate the feasibility of conducting the intervention at the Community Health Center (CHC) level. UTH staff implemented assessments and the intervention while CHC staff provided logistic and administrative support. HIV seropositive women (CHC n = 200; UTH n = 612) attended group sessions in which male partners were randomized to a three-session or one-session group intervention arm. At baseline, consistent use of male and female condoms differed between sites (HIV+ UTH, 73%, CHC, 88%, HIV– UTH, 42%, CHC 65%); both sites increased combined condom use at 6 months post baseline and maintained increases over baseline at 12 months. Participants did not differ between sites at baseline on condom attitudes, HIV knowledge or self efficacy. At 12 months post baseline, both sites had improved in attitudes, knowledge and efficacy and participant retention was lower at the UTH site (77% versus 82%). Inconsistent sexual barrier users increased to consistent use at both sites after 6 months (HIV positive UTH, 96%, CHC, 99%, HIV negative UTH, 84%, CHC 100%). At 12 months, HIV negative CHC participants maintained higher levels of condom use in comparison with UTH participants (F = 7.17, p = .001). Results illustrate the feasibility and efficacy of conducting group sexual risk reduction interventions in the Zambian community, and the potential for the use of group interventions in conjunction with existing CHC Voluntary Counseling and Testing (VCT) programs.
Sexual risk reduction among Zambian couples
DL Jones, N Chitalu, P Ndubani, M Mumbi, SM Weiss, O Villar-Loubet, S Vamos, D Waldrop-Valverde
SAHARA J (Journal of Social Aspects of HIV/AIDS Research Alliance) , 2009,
Abstract: Zambia has over 1 million HIV infections nationwide and an urban prevalence rate of 23%. This study compared the impact of male involvement in multiple and single session risk reduction interventions among inconsistent condom users in Zambia and the role of serostatus among HIV-seropositive and serodiscordant couples. Couples (N=392) were randomised into intervention arms. Among inconsistent condom users at baseline (N=83), condom use increased in both conditions and this increase was maintained over a 12- month period. At 12 months, seronegative men in the multiple session condition increased sexual barrier (male and female condoms) use in comparison with those in the single session condition (F=16.13, p=0.001) while seropositive individuals increased sexual barrier use regardless of condition. Results illustrate the importance of both single and multiple session risk reduction counselling among seronegative men in serodiscordant couples in Zambia, and highlight the differing perception of risk between seropositive and serodiscordant persons.
Order 1 autoregressive process of finite length
Calin Vamos,Stefan M. Soltuz,Maria Craciun
Physics , 2007,
Abstract: The stochastic processes of finite length defined by recurrence relations request additional relations specifying the first terms of the process analogously to the initial conditions for the differential equations. As a general rule, in time series theory one analyzes only stochastic processes of infinite length which need no such initial conditions and their properties are less difficult to be determined. In this paper we compare the properties of the order 1 autoregressive processes of finite and infinite length and we prove that the time series length has an important influence mainly if the serial correlation is significant. These different properties can manifest themselves as transient effects produced when a time series is numerically generated. We show that for an order 1 autoregressive process the transient behavior can be avoided if the first term is a Gaussian random variable with standard deviation equal to that of the theoretical infinite process and not to that of the white noise innovation.
Nicolae Suciu,Calin Vamos,Harry Vereecken,Peter Knabner
Mathematics and its Applications : Annals of the Academy of Romanian Scientists , 2011,
Abstract: The Global Random Walk algorithm (GRW) performs a simultaneoustracking on a fixed grid of huge numbers of particles at costscomparable to those of a single-trajectory simulation by the traditional Particle Tracking (PT) approach. Statistical ensembles of GRW simulations of a typical advection-dispersion process in groundwater systems with randomly distributed spatial parameters are used to obtain reliable estimations of the input parameters for the upscaled transport model and of their correlations, input-output correlations, as well as full probability distributions of the input and output parameters.
Role of retinoic receptors in lung carcinogenesis
Krisztina Bogos, Ferenc Renyi-Vamos, Gabor Kovacs, Jozsef Tovari, Balazs Dome
Journal of Experimental & Clinical Cancer Research , 2008, DOI: 10.1186/1756-9966-27-18
Abstract: In this review we have compiled basic and recent knowledge regarding the role of retinoid receptors in lung carcinogenesis. Sensitive and appropriate biological tools are necessary for screening the risk population and monitoring the efficacy of chemoprevention. Investigation of retinoid receptors is important and may contribute to the establishment of new strategies in chemoprevention for high-risk patients and in the treatment of lung cancer.Despite antismoking efforts and advances in therapy, lung cancer remains the leading cause of cancer-related death worldwide [1].Multi-step carcinogenesis has been described as "a gradual accumulation of genetic and epigenetic aberrations resulting in the deregulation of cellular homeostasis" [2]. There is a similarity between bronchial lesions found in carcinogen-treated animals (squamous metaplasia) and the histological changes affecting the bronchial epithelia of humans or animals deficient in vitamin A. Restoration to a normal histological state occurs after vitamin A repletion, which in experimental models has also been shown to confer protection against pro-carcinogens.The term retinoid (first coined by Sporn in 1976) generally refers to naturally occurring and synthetic vitamin A (retinol) metabolites and analogs [3].Several studies have shown that vitamin A/retinoids are physiological regulators of embryonic development, vision, reproduction, bone formation, haematopoesis, differentiation, proliferation and apoptosis. Pharmacologically, they have been recognized as modulators of cell growth, differentiation and apoptosis. Furthermore they have been shown to suppress carcinogenesis in various organs (e.g. oral cancer, skin, bladder, lung, prostate and breast cancers) in experimental animals [4]. Clinically, retinoids reverse premalignant human epithelial lesions and prevent lung, liver and breast cancer and second primary tumors in the head and neck [5].It is now generally thought that the effects of retinoids are mainl
Autoregressive description of biological phenomena
Vasile V Morariu,Calin Vamos,Alexadru Pop,Stefan M Soltuz,Luiza Buimaga-Iarinca,Oana Zainea
Physics , 2008,
Abstract: Many natural phenomena can be described by power-laws. A closer look at various experimental data reveals more or less significant deviations from a 1/f spectrum. We exemplify such cases with phenomena offered by molecular biology, cell biophysics, and cognitive psychology. Some of these cases can be described by first order autoregressive (AR) models or by higher order AR models which are short range correlation models. The calculations are checked against astrophysical data which were fitted to a an AR model by a different method. We found that our fitting method of the data give similar results for the astrhophysical data and therefore applied the method for examples mentioned above. Our results show that such phenomena can be described by first or higher order of AR models. Therefore such examples are described by short range correlation properties while they can be easily confounded with long range correlation phenomena.
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