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Comparison of Semi-Parametric Shared Frailty Models for Bees’Survival
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作者 Patience Isiaho Daisy Salifu +1 位作者 Samuel Mwalili Henri E. Z. Tonnang 《Journal of Data Analysis and Information Processing》 2024年第2期267-288,共22页
Survival analysis is a fundamental tool in medical science for time-to-event data. However, its application to colony organisms like bees poses challenges due to their social nature. Traditional survival models may no... Survival analysis is a fundamental tool in medical science for time-to-event data. However, its application to colony organisms like bees poses challenges due to their social nature. Traditional survival models may not accurately capture the interdependence among individuals within a colony. Frailty models, accounting for shared risks within groups, offer a promising alternative. This study evaluates the performance of semi-parametric shared frailty models (gamma, inverse normal, and positive stable-in comparison to the traditional Cox model using bees’ survival data). We examined the effect of misspecification of the frailty distribution on regression and heterogeneity parameters using simulation and concluded that the heterogeneity parameter was more sensitive to misspecification of the frailty distribution and choice of initial parameters (cluster size and true heterogeneity parameter) compared to the regression parameter. From the data, parameter estimates for covariates were close for the four models but slightly higher for the Cox model. The shared gamma frailty model provided a better fit to the data in comparison with the other models. Therefore, when focusing on regression parameters, the gamma frailty model is recommended. This research underscores the importance of tailored survival methodologies for accurately analyzing time-to-event data in social organisms. 展开更多
关键词 Correlated Failure Times FRAILTY Survival Analysis Unobserved Heterogeneity
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Subgrid-scale contributions to Lagrangian time correlations in isotropic turbulence 被引量:3
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作者 Jian Zhang Guowei He Lipeng Lu 《Acta Mechanica Sinica》 SCIE EI CAS CSCD 2009年第1期45-49,共5页
The application of large-eddy simulation (LES) to particle-laden turbulence raises such a fundamental question as whether the LES with a subgrid scale (SGS) model can correctly predict Lagrangian time correlations... The application of large-eddy simulation (LES) to particle-laden turbulence raises such a fundamental question as whether the LES with a subgrid scale (SGS) model can correctly predict Lagrangian time correlations (LTCs). Most of the currently existing SGS models are constructed based on the energy budget equations. Therefore, they are able to correctly predict energy spectra, but they may not ensure the correct prediction on the LTCs. Previous researches investigated the effect of the SGS modeling on the Eulerian time correlations. This paper is devoted to study the LTCs in LES. A direct numerical simulation (DNS) and the LES with a spectral eddy viscosity model are performed for isotropic turbulence and the LTCs are calculated using the passive vector method. Both a priori and a posteriori tests are carried out. It is observed that the subgrid;scale contributions to the LTCs cannot be simply ignored and the LES overpredicts the LTCs than the DNS. It is concluded from the straining hypothesis that an accurate prediction of enstrophy spectra is most critical to the prediction of the LTCs. 展开更多
关键词 Isotropic turbulence Large eddy simulation Lagrangian time correlation Enstrophy spectra
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Stratigraphical Time——Correlation and Mass Extinction Event Near Permian——Triassic Boundary in South China 被引量:2
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作者 Xu GuirongChina University of Geosciences , Wuhan 430074 《Journal of Earth Science》 SCIE CAS CSCD 1991年第1期39-49,共11页
Shaw's method used to correlate 40 sections across the Permo-Triassic boundary in South China is applied in the paper. Two steps are adopted to get an Integral Composite Section (ICS) by synthesizing these data : ... Shaw's method used to correlate 40 sections across the Permo-Triassic boundary in South China is applied in the paper. Two steps are adopted to get an Integral Composite Section (ICS) by synthesizing these data : First , South China is divided into five areas and composite section developed for each area . Then the second step . the Changxing composite section is regarded as a composite standard (CSRS) while the ICS is produced by matching the CSRS with composite sections of the other areas. Three biozones in the Changxingian and two biozones in the Griesbachian can be discerned on the basis of computing Z values in the ICS. These biozones are marked by the Z values which quantitatively represent their time ranges ; therefore , they may increase accuracy of stratigraphic time correlation . The mass extinction at the end of the Permian is an abrupt event that is supported by the relative rate of extinction near the P/T boundary . About 90% of invertebrate species died out by the end of the Permian . The duration of the mass extinction is rather short ,approximately 0.018Ma . 展开更多
关键词 South China the Permian - Triassic boundary stratigraphic time - correlation biostratigraphic events biologic events mass extinction .
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Effect on Intensity Correlation Time by Input Signal in a Single-Mode Laser with Bias Signal Modulation 被引量:1
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作者 CHEN Li-Mei CAO Li +1 位作者 WU Da-Jin GE Guo-Qin 《Communications in Theoretical Physics》 SCIE CAS CSCD 2005年第4X期638-642,共5页
The effect on intensity correlation time T by input signal is studied for gain-noise model of a single-mode laser driven by colored pump noise and colored quantum noise with colored cross-correlation with a bias signa... The effect on intensity correlation time T by input signal is studied for gain-noise model of a single-mode laser driven by colored pump noise and colored quantum noise with colored cross-correlation with a bias signal modulation in this paper. By using the linear approximation method, we detect that there exists maximum (i.e., resonance) in the curve of the intensity correlation time T upon bias current io when the noise correlation coefficient λ is positive; and there exists minimum (i.e., suppression) in the T-io curve when λ is negative. And whenλ is zero, T increases monotonously with increasing io. Furthermore, the curve of T upon the signal frequency Ω is also studied. Our study shows that no matter what the value of λ is, there exists minimum (i.e., suppression) in the T-Ω curve. 展开更多
关键词 intensity correlation time bias signal modulation linear approximation method
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The statistical fluctuation of a single-mode laser system driven by coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts
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作者 徐大海 程庆华 +1 位作者 曹力 吴大进 《Chinese Physics B》 SCIE EI CAS CSCD 2006年第10期2324-2331,共8页
Using the linear approximation method, this paper studies the statistical property of a single-mode laser driven by both coloured pump noise with signal modulation and the quantum noise with cross-correlation between ... Using the linear approximation method, this paper studies the statistical property of a single-mode laser driven by both coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts, and calculates the steady-state mean normalized intensity fluctuation and intensity correlation time. It analyses the influences of the modulation signal, the net gain coefficient, the noise and its correlation form on the statistical fluctuation of the laser system respectively. It is found that the coloured pump noise modulated by the signal has a great suppressing action on the statistical fluctuation of the laser system; the pump noise self-correlation time and the specific frequency of modulation signal have the result that the statistical fluctuation tends to zero. Furthermore, the 'colour' correlation of pump noise has much influences on the statistical fluctuation of the laser system. Increasing the intensity of pump noise will augment the statistical fluctuation of the laser system, but the intensity of quantum noise and the coefficient of cross-correlation between its real and imaginary parts have less influence on the statistical fluctuation of the laser system. Therefore, from the conclusions of this paper the statistical property can be known and a theoretical basis for steady operation and output of the laser system can be provided. 展开更多
关键词 signal noise single-mode laser steady-state mean normalized intensity fluctuation intensity correlation time
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Time difference correlation between earthquake lights and seismic ground accelerations
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作者 Juan Antonio Lira Maurizio Mulas 《Applied Geophysics》 SCIE CSCD 2021年第1期9-16,128,共9页
Although earthquake lights have been known since ancient times,it has not been easy to study them.It was not until the 60s that the first photographs of them were taken.During the Peruvian earthquake in 2007,it was po... Although earthquake lights have been known since ancient times,it has not been easy to study them.It was not until the 60s that the first photographs of them were taken.During the Peruvian earthquake in 2007,it was possible to obtain the fi rst fi lm recording on earthquake lights.Likewise,during the earthquakes in Ecuador in 2016 and in Mexico in 2017,two fi lms of the earthquake lights were recorded.These fi lm recordings have helped in the study of earthquake lights,both for their objectivity and for their informational content.Several causal mechanisms have been proposed to explain earthquake lights:piezoelectricity,radon emanation,fluid diffusion,friction-vaporization,positive holes and dipole currents,among others.In this work a time difference correlation between earthquake lights and seismic ground accelerations was found and we use both seismic data and fi lm recordings of earthquake lights to explain its origin.In the discussion section it is suggested that fracturing of rocks manifest itself to some extent in the form of static electricity producing earthquake lights through induction The induction model proposed is new and it can explain the formation of EQL,even if the earth’s crust has layers of large electrical resistivity.The model also explains the formation of seismic lights without the need for special conditions on the earth’s surface or in the atmosphere.A better understanding of the earthquake lights generation process can improve our understanding of seismicity and help in the prediction of earthquakes. 展开更多
关键词 time diff erence correlation earthquake lights seismic ground accelerations rock fracturing
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Detecting the Relationship Between Summer Rainfall Anomalies in Eastern China and the SSTA in the Global Domain with a New Significance Test Method 被引量:4
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作者 LU Chuhan GUAN Zhaoyong WANG Panxing DUAN Mingken 《Journal of Ocean University of China》 SCIE CAS 2009年第1期15-22,共8页
It is suggested that the multiple samples in a correlation map or a set of correlation maps should be examined with significance tests as per the Bernoulli probability model. Therefore, both the contemporaneous and la... It is suggested that the multiple samples in a correlation map or a set of correlation maps should be examined with significance tests as per the Bernoulli probability model. Therefore, both the contemporaneous and lag correlations of summertime precipitation R in any one of the three regions of Northern China (NC), the Changjiang-Huaihe River Valley (CHRV), and Southern China (SC) with the SSTA in the global domain have been tested in the present article, using our significance test method and the method proposed by Livezey and Chen (1983) respectively. Our results demonstrate that the contemporaneous correlations of sum- mer R in CHRV with the SSTA are larger than those in NC. Significant correlations of SSTA with CHRV R are found to be in some warm SST regions in the tropics, whereas those of SSTA with NC R, which are opposite in sign as compared to the SSTA-CHRVR correlations, are found to be in some regions where the mean SSTs are low. In comparison with the patterns of the contemporaneous correlations, the 1 to 12 month lag correlations between NC R and SSTA, and those between CHRV summer R and SSTA show similar patterns, including the magnitudes and signs, and the spatial distributions of the coefficients. However, the summer rainfall in SC is not well correlated with the SSTA, no matter how long the lag interval is. The results derived from the observations have set up a relationship frame connecting the precipitation anomalies in NC, CHRV, and SC with the SSTA in the global domain, which is critically useful for our understanding and predicting the climate variabilities in different parts of China. Both NC and CHRV summer R are connected with E1 Nifio events, showing a ‘- -'pattern in an E1 Nifio year and a‘+ +' pattern in the subsequent year. Key words summer precipitation; eastern China; global sea surface 展开更多
关键词 temperature contemporaneous correlation time lag correlation significance test for multiple correlation maps
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Dynamic obstacles detection of tram based on laser radar 被引量:3
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作者 KUANG Wen-zhen WU Meng-luo XU Li 《Journal of Measurement Science and Instrumentation》 CAS CSCD 2018年第4期316-320,共5页
The detection of obstacles in a dynamic environment is a hot and difficult problem.A method of autonomously detecting obstacles based on laser radar is proposed as a safety auxiliary structure of tram.The nearest neig... The detection of obstacles in a dynamic environment is a hot and difficult problem.A method of autonomously detecting obstacles based on laser radar is proposed as a safety auxiliary structure of tram.The nearest neighbor method is used for spatial obstacles clustering from laser radar data.By analyzing the characteristics of obstacles,the types of obstacles are determined by time correlation.Experiments were carried out on the developed unmanned aerial vehicle(UAV),and the experimental results verify the effectiveness of the proposed method. 展开更多
关键词 laser radar TRAM dynamic obstacle detection spatial obstacle clustering time correlation nearest neigbor method
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Rescue vehicle allocation problem based on optimal reliable path under uncertainty 被引量:2
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作者 SHEN Liang WANG Fei-ran +2 位作者 HU Lei LYU Xin-yi SHAO Hu 《Journal of Central South University》 SCIE EI CAS CSCD 2022年第11期3779-3792,共14页
Consideration of the travel time variation for rescue vehicles is significant in the field of emergency management research.Because of uncertain factors,such as the weather or OD(origin-destination)variations caused b... Consideration of the travel time variation for rescue vehicles is significant in the field of emergency management research.Because of uncertain factors,such as the weather or OD(origin-destination)variations caused by traffic accidents,travel time is a random variable.In emergency situations,it is particularly necessary to determine the optimal reliable route of rescue vehicles from the perspective of uncertainty.This paper first proposes an optimal reliable path finding(ORPF)model for rescue vehicles,which considers the uncertainties of travel time,and link correlations.On this basis,it investigates how to optimize rescue vehicle allocation to minimize rescue time,taking into account travel time reliability under uncertain conditions.Because of the non-additive property of the objective function,this paper adopts a heuristic algorithm based on the K-shortest path algorithm,and inequality techniques to tackle the proposed modified integer programming model.Finally,the numerical experiments are presented to verify the accuracy and effectiveness of the proposed model and algorithm.The results show that ignoring travel time reliability may lead to an over-or under-estimation of the effective travel time of rescue vehicles on a particular path,and thereby an incorrect allocation scheme. 展开更多
关键词 heuristic algorithm travel time correlation optimal reliable path rescue vehicle allocation traffic network
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Reduced combustion time model for methane in gas turbine flow fields 被引量:1
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作者 Mouna Lamnaouer Robert C. Ryder +1 位作者 Andreja Brankovic Eric L. Petersen 《Journal of Natural Gas Chemistry》 EI CAS CSCD 2009年第2期145-155,共11页
Computational fluid dynamics (CFD) modeling of the complex processes that occur within the burner of a gas turbine engine has become a critical step in the design process. However, due to computer limitations, it is... Computational fluid dynamics (CFD) modeling of the complex processes that occur within the burner of a gas turbine engine has become a critical step in the design process. However, due to computer limitations, it is very difficult to completely couple the fluid mechanics solver with the full combustion chemistry. Therefore, simplified chemistry models are required, and the topic of this research was to provide reduced chemistry models for CH4/O2 gas turbine flow fields to be integrated into CFD codes for the simulation of flow fields of natural gas-fueled burners. The reduction procedure for the CH4/O2 model utilized a response modeling technique wherein the full mechanism was solved over a range of temperatures, pressures, and mixture ratios to establish the response of a particular variable, namely the chemical reaction time. The conditions covered were between 1000 and 2500 K for temperature, 0.1 and 2 for equivalence ratio in air, and 0.1 and 50 atm for pressure. The kinetic time models in the form of ignition time correlations are given in Arrhenius-type formulas as functions of equivalence ratio, temperature, and pressure; or fuel-to-air ratio, temperature, and pressure. A single ignition time model was obtained for the entire range of conditions, and separate models for the low-temperature and high-temperature regions as well as for fuel-lean and rich cases were also derived. Predictions using the reduced model were verified using results from the full mechanism and empirical correlations from experiments. The models are intended for (but not limited to) use in CFD codes for flow field simulations of gas turbine combustors in which initial conditions and degree of mixedness of the fuel and air are key factors in achieving stable and robust combustion processes and acceptable emission levels. The chemical time model was utilized successfully in CFD simulations of a generic gas turbine combustor with four different cases with various levels of fuel-air premixing. 展开更多
关键词 CH4 ignition time correlations reduced kinetic model
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Several Kinematic Data Processing Methods for Time-Correlated Observations 被引量:4
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作者 Bofeng LI Zhetao ZHANG 《Journal of Geodesy and Geoinformation Science》 2019年第4期1-9,共9页
Time correlations always exist in modern geodetic data,and ignoring these time correlations will affect the precision and reliability of solutions.In this paper,several methods for processing kinematic time-correlated... Time correlations always exist in modern geodetic data,and ignoring these time correlations will affect the precision and reliability of solutions.In this paper,several methods for processing kinematic time-correlated observations are studied.Firstly,the method for processing the time-correlated observations is expanded and unified.Based on the theory of maximum a posteriori estimation,the third idea is proposed after the decorrelation transformation and differential transformation.Two types of situations with and without common parameters are both investigated by using the decorrelation transformation,differential transformation and maximum a posteriori estimation solutions.Besides,the characteristics and equivalence of above three methods are studied.Secondly,in order to balance the computational efficiency in real applications and meantime effectively capture the time correlations,the corresponding reduced forms based on the autocorrelation function are deduced.Finally,with GPS real data,the correctness and practicability of derived formulae are evaluated. 展开更多
关键词 time correlation kinematic solution decorrelation transformation differential transformation maximum a posteriori estimation
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Research on overlapping structures and evolution properties of co-citation network 被引量:2
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作者 Shiji CHEN Xiaolin ZHANG 《Chinese Journal of Library and Information Science》 2013年第1期1-13,共13页
Purpose: This paper intends to explore methodologies and indicators for the analysis of overlapping structures and evolution properties in a co-citation network, and provide reference for overlapping structure analysi... Purpose: This paper intends to explore methodologies and indicators for the analysis of overlapping structures and evolution properties in a co-citation network, and provide reference for overlapping structure analysis of other scientific networks.Design/methodology/approach: The Q-value variance is defined to achieve overlapping structures of different levels in the scientific networks. At the same time, analyses for time correlation variance and subject correlation variance are used to present the formation of overlapping structures in scientific networks. As a test, a co-citation network of highly cited papers on Molecular Biology & Genetics from Essential Science Indicator(ESI) is taken as an example for an empirical analysis.Findings: Our research showed that the Q-value variance is effective for achieving the desired overlapping structures. Meanwhile, the time correlation variance and subject correlation variance are equally useful for uncovering the evolution progress of scientific research, and the properties of overlapping structures in the research of co-citation network as well.Research limitations: In this paper, the theoretical analysis and verification of time and subject correlation variances are still at its initial stage. Further studies in this regard need to take actual evolution of research areas into consideration.Practical implications: Evolution properties of overlapping structures pave the way for overlapping and evolution analysis of disciplines or areas, this study is of practical value for the planning of scientific and technical innovation.Originality/value: This paper proposes an analytical method of time correlation variance and subject correlation variance based on the evolution properties of overlapping structures, which would provide the foundation for the evolution analysis of disciplines and interdisciplinary research. 展开更多
关键词 Overlapping structure Co-citation network Q-value variance Time correlation variance Subject correlation variance
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Simulation Study on the Structure and Dynamics of Water in Sodium Tetrafluoroborate/Water
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作者 Guo-cai Tian Jian Li Yi-xin Hua 《Chinese Journal of Chemical Physics》 SCIE CAS CSCD 2009年第5期460-466,I0001,共8页
The microstructure, IR spectrum, as well as rotation dynamics of water molecule in sodium tetrafluoroborate (NaBF4)/water mixture at room temperatures were studied with molecular dynamics simulation. Different conce... The microstructure, IR spectrum, as well as rotation dynamics of water molecule in sodium tetrafluoroborate (NaBF4)/water mixture at room temperatures were studied with molecular dynamics simulation. Different concentrations of water (6.25%, 25.0%, 50.0%, 75.0%, 90.0%, and 99.6%) in NaBF4/water mixture were simulated to understand the structure and dynamics. It was shown that water molecules tend to be isolated from each other in mixtures with more ions than water molecules in both liquids. With increase of the molar fraction of water in the mixture, the rotation bands and the bending bands of water display red shift whereas the O-H stretch bands show blue shift, and the decay of the reorientation correlation function becomes slower. This suggests that the molecules are hindered and their motions are difficult and slow, due to the hydrogen-bond interactions and the inharmonic interactions between the interor intra-molecular modes. 展开更多
关键词 Sodium tetrafluoroborate/water mixture Structure and dynamics IR spectrum Reorientation dynamics Time correlation function Molecular dynamics simulation
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Noise Controlling Methods in Frequency Selection Detecting Radar
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作者 Shi Xin-zhi Shi Zhen-hua Wen Bi-yang 《Wuhan University Journal of Natural Sciences》 EI CAS 2001年第4期807-811,共5页
Based on time correlation characteristic, width correlation characteristic and frequency correlation characteristic of detecting pulses, several methods are introduced to control random or periodic noise whose width i... Based on time correlation characteristic, width correlation characteristic and frequency correlation characteristic of detecting pulses, several methods are introduced to control random or periodic noise whose width is narrower than 1 ms or wider than 3 ms in Frequency Selection Detecting Radar System. The software flow chart and the results of the experiment are also given. 展开更多
关键词 noise time correlation filter width correlation filter frequency correlation filter
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Two Methods to Solve the Ionospheric Electron Concentration Horizontal Gradient at Chongqing
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作者 Chong Yan-wen, Huang Tian-xi, Zhao Zheng-yu, Xie Shu-guo, Yao Yong-gang College of Electronic Information, Wuhan University, Wuhan 430072, China 《Wuhan University Journal of Natural Sciences》 EI CAS 2000年第3期320-322,共3页
The electron concentration horizontal gradient vector of the ionosphere and its south-north and east-west components over Chongqing station are analyzed and calculated, using the first approximation, time correlation ... The electron concentration horizontal gradient vector of the ionosphere and its south-north and east-west components over Chongqing station are analyzed and calculated, using the first approximation, time correlation and space correlation and another approach introduced. And then, the validity of the two methods is analyzed and compared. 展开更多
关键词 horizontal gradient of ionospheric electron concentration horizontal gradient vector space correlation time correlation
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An EPR Study on Intramolecular Spin Exchange and Rotational Mobility of Nitroxides Linked by a Long-chain
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作者 Yoshimi Sueishi Katsumi Noritake LIU Yingjin 《Chemical Research in Chinese Universities》 SCIE CAS CSCD 1999年第2期82-89,共8页
The effects of pressure and solvent on the intramolecular spin exchange of biradicals having two nitroxide fragments linked by a long flexible chain were studied by means of highpressure EPR technique.It was found tha... The effects of pressure and solvent on the intramolecular spin exchange of biradicals having two nitroxide fragments linked by a long flexible chain were studied by means of highpressure EPR technique.It was found that the intramolecular exchange interaction between nitroxides of biradical took place through the direct contact between them.By analyzing the observed EPR spectra,we have estimated the ratio(in/out)value of the average lifetime of the radical fragments inside a cage(in)to that outside the cage(out).The in/out values decreased with decreasing temperature and increasing pressure.The results suggest that the nearly cyclic conformation in a cage is favorable in solution.Further,the rotational correlation time of individual nitroxide was estimated from the anisotropic EPR signal,and the information on the segmental motion of the nitroxide group in biradical was obtained. 展开更多
关键词 EPR Spin exchange Pressure effect Rotational correlation time exchange Pressure effect Rotational correlation time
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Influence of the colored noise on determining the period of a torsion pendulum
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作者 罗杰 占文泽 +2 位作者 巫伟皇 邵成刚 王典洪 《Chinese Physics B》 SCIE EI CAS CSCD 2016年第8期81-85,共5页
Based on statistical properties, two typical models are considered to calculate the uncertainties for some random noise sequences on the period extraction of a torsion pendulum, which is important and instructive in t... Based on statistical properties, two typical models are considered to calculate the uncertainties for some random noise sequences on the period extraction of a torsion pendulum, which is important and instructive in the measurement of gravitational constant G with the time-of-swing method. An expression of the uncertainty for the period measurement is obtained, which is dependent on the ratio ?t/(1/λ) where ?t is the interval of the sample time and 1/λ is the length of the correlation time. The result of processing experimental data shows that as the interval of the sample time ?t gradually shortens, the uncertainty of the period becomes smaller, and further when the ratio ?t/(1/λ) is less than 1, the uncertainty remains substantially unchanged. 展开更多
关键词 torsion pendulum Gaussian colored noise UNCERTAINTY correlation time
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Times Series Applied to Study Vitamin D Seasonality in Argentina
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作者 José Bavio Carina Fernández +1 位作者 Patricia Fernández Beatriz Marrón 《Applied Mathematics》 2021年第7期546-555,共10页
In this study, we analyze how vitamin D (VD) serum levels flow with latitude and throughout seasons of the year within a population sample over three years, taking into account that VD is mainly photosynthesized in th... In this study, we analyze how vitamin D (VD) serum levels flow with latitude and throughout seasons of the year within a population sample over three years, taking into account that VD is mainly photosynthesized in the skin from sun exposure. Vitamin D levels have been measured in 80,763 patients during 2013, 2014, and 2015. To accomplish the objectives, we first perform some inference tests like two-way Analysis of Variance (ANOVA) followed by post-hoc tests. Secondly, we develop time series techniques including cross correlation calculations. Least than 10% of the sample had healthy VD levels, which should be a fact of public health major concern. The effect of the interaction between the two factors, zones and seasons, was proved by ANOVA. The mean values which are significantly different were determined by post hoc test. Furthermore, we find that mean serum VD levels, measured as 25-hydroxy-VD, follow a seasonal lag pattern of 9 weeks, a delay for minimum and maximum values after the respective equinoxes and daily sunlight duration. Reliable estimates of the population are provided in the present study, since one of the strengths is its huge sample size. We have quantitatively characterized the seasonality of serum vitamin D levels in the Argentine and the seasonal lag pattern has been determined for the study region. 展开更多
关键词 Vitamin D Status Population Study Time Series correlation Interaction Effects
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Understanding Dependent Competing Risks: A Simulation Study to Illustrate the Relationship Between Cause-Specific Hazard and Marginal Hazard
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作者 Zihang Lu Jenna Sykes Melania Pintilie 《Journal of Statistical Science and Application》 2016年第2期96-107,共12页
In the analysis of competing risk data, the observed effect of a covariate can be obtained via a Fine and Gray sub-distribution hazard ratio. Sometimes, it is also desirable to obtain the virtual effect of a covariate... In the analysis of competing risk data, the observed effect of a covariate can be obtained via a Fine and Gray sub-distribution hazard ratio. Sometimes, it is also desirable to obtain the virtual effect of a covariate as if the competing risks were non-existent. Under the latent failure time scenario, when the event of interest and the competing risk event are independent, the cause-specific hazard ratio obtained from the Cox model where the competing events are censored represents the ratio of the marginal hazards and can be interpreted as the virtual effect of the covariate. However, when the two events are not independent, the cause-specific hazard ratio is not the ratio of the marginal hazards as the ratio depends not only on the marginal hazards but also on the correlation between the competing risk and the event of interest. Using simulation, we investigated the degree to which the cause-specific hazard ratio changes relative to the marginal hazard with this correlation. It was found that the discrepancy between the cause-specific hazard ratio and the theoretical marginal hazard ratio increased as the proportion of competing risk events and the correlation between the events increased (〉0.2). Depending on the direction of the correlation, the cause-specific hazard ratio can over- or under-estimate the marginal hazard ratio. Using real-life datasets, we show how these results can be used to make inferences on the virtual effects. 展开更多
关键词 competing risk correlated event times cause-specific hazard ratio marginal hazard ratio
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Effects of correlation time between noises on the noise enhanced stability phenomenon in an asymmetric bistable system 被引量:1
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作者 Chun Li Zheng-Lin Jia Dong-Cheng Mei 《Frontiers of physics》 SCIE CSCD 2015年第1期95-101,共7页
The effects of the correlation time τ between noises on the noise-enhanced stability (NES) phe- nomenon in an asymmetric bistable system driven by cross-correlated noise are investigated. The ex- pressions for the ... The effects of the correlation time τ between noises on the noise-enhanced stability (NES) phe- nomenon in an asymmetric bistable system driven by cross-correlated noise are investigated. The ex- pressions for the average escape time from the left metastable state TL and from the right metastable state TR are derived. The results indicate that: i) The NES effect is suppressed as the correlation time τ increases for two metastable states; ii) The increase in τ speeds up the escape process from the right state for positively correlated noise, whereas its role is reverses for negatively correlated; iii) In the escape process from the left state, the role of τ is opposite to that in escape from the right state. 展开更多
关键词 asymmetric bistable system noise correlation time noise enhanced stability
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