Home About us Contact
|
Home About us Contact | ||
|
|
||
Stochastic Dynamics (stochastic + dynamics)
Selected AbstractsConformational studies on a unique bis-sulfated glycolipid using NMR spectroscopy and molecular dynamics simulationsFEBS JOURNAL, Issue 23 2000Naoko Iida-Tanaka The time-averaged solution conformation of a unique bis-sulfated glycolipid (HSO3)2 -2,6Man,-2Glc,-1- sn -2,3- O -alkylglycerol, was studied in terms of the torsional angles of two glycosidic linkages, , (H1-C1-O-Cx) and , (C1-O-Cx-Hx), derived from heteronuclear three-bond coupling constants (3JC,H), and inter-residual proton,proton distances from J-HMBC 2D and ROESY experiments, respectively. The dihedral angles of Glc,1Gro in glycolipids were determined for the first time. The C1-C4 diagonal line of the ,-glucose ring makes an angle of ,,120 ° with the glycerol backbone, suggesting that the ,-glucose ring is almost parallel to the membrane surface in contrast with the perpendicular orientation of the ,-isomer. Furthermore, minimum-energy states around the conformation were estimated by Monte Carlo/stochastic dynamics (MCSD) mixed-mode simulations and the energy minimization with assisted model building and energy refinement (AMBER) force field. The Glc,1Gro linkage has a single minimum-energy structure. On the other hand, three conformers were observed for the Man,2Glc linkage. The flexibility of Man,2Glc was further confirmed by the absence of inter-residual hydrogen bonds which were judged from the temperature coefficients of the chemical shifts, d,/dT (,10,3 p.p.m.·°C,1), of hydroxy protons. The conformational flexibility may facilitate interaction of extracellular substances with both sulfate groups. [source] Robust H, control of uncertain linear impulsive stochastic systemsINTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Issue 13 2008Wu-Hua Chen Abstract This paper develops robust stability theorems and robust H, control theory for uncertain impulsive stochastic systems. The parametric uncertainties are assumed to be time varying and norm bounded. Impulsive stochastic systems can be divided into three cases, namely, the systems with stable/stabilizable continuous-time stochastic dynamics and unstable/unstabilizable discrete-time dynamics, the systems with unstable/unstabilizable continuous dynamics and stable/stabilizable discrete-time dynamics, and the systems in which both the continuous-time stochastic dynamics and the discrete-time dynamics are stable/stabilizable. Sufficient conditions for robust exponential stability and robust stabilization for uncertain impulsive stochastic systems are derived in terms of an average dwell-time condition. Then, a linear matrix inequality-based approach to the design of a robust H, controller for each system is presented. Finally, the numerical examples are provided to demonstrate the effectiveness of the proposed approach. Copyright © 2007 John Wiley & Sons, Ltd. [source] Using individual-based simulations to test the Levins metapopulation paradigmJOURNAL OF ANIMAL ECOLOGY, Issue 2 2002Matt J. Keeling Summary 1,Levins metapopulations have become a standard tool for modelling spatially heterogeneous populations. The acceptance of these models by ecologists may be attributed to their simple structure and their use of presence,absence data. 2,Using structured-metapopulations, which possess stochastic dynamics at the local subpopulation level, the extinction and subsequent recolonization rates can be calculated and compared to those of the classical Levins model. 3,Single-species metapopulations conform to the Levins ideal, validating the widespread use of this conceptual model. However, multispecies systems are shown to deviate in a consistent manner. This deviation, which is explained in terms of a correlation between patch occupancy and average population levels of the species, can be used to identify the signature of enemy,victim interactions. [source] Life table response experiment analysis of the stochastic growth rateJOURNAL OF ECOLOGY, Issue 2 2010Hal Caswell Summary 1.,Life table response experiment (LTRE) analyses decompose treatment effects on a dependent variable (usually, but not necessarily, population growth rate) into contributions from differences in the parameters that determine that variable. 2.,Fixed, random and regression LTRE designs have been applied to plant populations in many contexts. These designs all make use of the derivative of the dependent variable with respect to the parameters, and describe differences as sums of linear approximations. 3.,Here, I extend LTRE methods to analyse treatment effects on the stochastic growth rate log ,s. The problem is challenging because a stochastic model contains two layers of dynamics: the stochastic dynamics of the environment and the response of the vital rates to the state of the environment. I consider the widely used case where the environment is described by a Markov chain. 4.,As the parameters describing the environmental Markov chain do not appear explicitly in the calculation of log ,s, derivatives cannot be calculated. The solution presented here combines derivatives for the vital rates with an alternative (and older) approach, due to Kitagawa and Keyfitz, that calculates contributions in a way analogous to the calculation of main effects in statistical models. 5.,The resulting LTRE analysis decomposes log ,s into contributions from differences in: (i) the stationary distribution of environmental states, (ii) the autocorrelation pattern of the environment, and (iii) the stage-specific vital rate responses within each environmental state. 6.,As an example, the methods are applied to a stage-classified model of the prairie plant Lomatium bradshawii in a stochastic fire environment. 7.,Synthesis. The stochastic growth rate is an important parameter describing the effects of environmental fluctuations on population viability. Like any growth rate, it responds to differences in environmental factors. Without a decomposition analysis there is no way to attribute differences in the stochastic growth rate to particular parts of the life cycle or particular aspects of the stochastic environment. The methods presented here provide such an analysis, extending the LTRE analyses already available for deterministic environments. [source] SUSTAINABLE YIELDS IN FISHERIES: UNCERTAINTY, RISK-AVERSION, AND MEAN-VARIANCE ANALYSISNATURAL RESOURCE MODELING, Issue 3 2010CHRISTIAN-OLIVER EWALD Abstract We consider a model of a fishery in which the dynamics of the unharvested fish population are given by the stochastic logistic growth equation Similar to the classical deterministic analogon, we assume that the fishery harvests the fish population following a constant effort strategy. In the first step, we derive the effort level that leads to maximum expected sustainable yield, which is understood as the expectation of the equilibrium distribution of the stochastic dynamics. This replaces the nonzero fixed point in the classical deterministic setup. In the second step, we assume that the fishery is risk averse and that there is a tradeoff between expected sustainable yield and uncertainty measured in terms of the variance of the equilibrium distribution. We derive the optimal constant effort harvesting strategy for this problem. In the final step, we consider an approach that we call the mean-variance analysis to sustainable fisheries. Similar as in the now classical mean-variance analysis in finance, going back to Markowitz [1952], we study the problem of maximizing expected sustainable yields under variance constraints, and with this, minimizing the variance, e.g., risk, under guaranteed minimum expected sustainable yields. We derive explicit formulas for the optimal fishing effort in all four problems considered and study the effects of uncertainty, risk aversion, and mean reversion speed on fishing efforts. [source] Climate patterns and the stochastic dynamics of migratory birdsOIKOS, Issue 3 2002Niclas Jonzén We analyse time series data of 17 bird species trapped at Ottenby Bird Observatory, Sweden, during spring migration 1972,1999. The species have similar demography but respond differently to variation in the North Atlantic Oscillation (NAO) , a strong determinant of winter climate in the northern Hemisphere. Species wintering in northern Europe, compared to species having winter quarters in the Mediterranean area, tend to respond positively to variation in NAO. The variation within each group is high due to wide-ranging winter-distribution in many species, probably smoothing out the effect of spatial variation in NAO. Whereas mild winters (high NAO) is benign for many , but not all , birds wintering in northern Europe, the effect of drier-than-normal conditions in the Mediterranean area during high NAO index winters are uncertain. The work presented here goes beyond simple correlative studies and help identifying which species that are most affected by variation in winter climate. This is a first important step that calls for a more mechanistic approach when analysing possible changes to climate change. [source] A semi-Markov model of disease recurrence in insured dogsAPPLIED STOCHASTIC MODELS IN BUSINESS AND INDUSTRY, Issue 5 2007Xikui Wang Abstract We use a semi-Markov model to analyse the stochastic dynamics of disease occurrence of dogs insured in Canada from 1990 to 1999, and the probability pattern of death from illness. After statistically justifying the use of a stochastic model, we demonstrate that a stationary first-order semi-Markov process is appropriate for analysing the available data set. The probability transition function is estimated and its stationarity is tested statistically. Homogeneity of the semi-Markov model with respect to important covariates (such as geographic location, insurance plan, breed and age) is also statistically examined. We conclude with discussions and implications of our results in veterinary contents. Copyright © 2007 John Wiley & Sons, Ltd. [source] | ||||||||||