Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “trajectory”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 739 records · Page 41Linked to original sources
PubMed2001

Mean-atom-trajectory model for the velocity autocorrelation function of monatomic liquids.

We present a model for the motion of an average atom in a liquid or supercooled liquid state and apply it to calculations of the velocity autocorrelation function Z(t) and diffusion coefficient D. The model trajectory consists of oscillations at a distribution of frequencies characteristic of the normal modes of a single potential valley, interspersed with position- and velocity-conserving transits to similar adjacent valleys. The resulting predictions for Z(t) and D agree remarkably well with molecular dynamics simulations of Na at up to almost three times its melting temperature. Two independent processes in the model relax velocity autocorrelations: (a) dephasing due to the presence of many frequency components, which operates at all temperatures but which produces no diffusion, and (b) the transit process, which increases with increasing temperature and which produces diffusion. Because the model provides a single-atom trajectory in real space and time, including transits, it may be used to calculate all single-atom correlation functions.

Journal Article↗
PubMed2003

Competing tunneling trajectories in a two-dimensional potential with variable topology as a model for quantum bifurcations.

We present a path-integral approach to treat a two-dimensional model of a quantum bifurcation. The model potential has two equivalent minima separated by one or two saddle points, depending on the value of a continuous parameter. Tunneling is, therefore, realized either along one trajectory or along two equivalent paths. The zero-point fluctuations smear out the sharp transition between these two regimes and lead to a certain crossover behavior. When the two saddle points are inequivalent one can also have a first order transition related to the fact that one of the two trajectories becomes unstable. We illustrate these results by numerical investigations. Even though a specific model is investigated here, the approach is quite general and has potential applicability for various systems in physics and chemistry exhibiting multistability and tunneling phenomena.

Journal Article↗
PubMed2005

Quasistationary trajectories of the mean-field XY Hamiltonian model: a topological perspective.

We employ a topological approach to investigate the nature of quasistationary states of the mean-field XY Hamiltonian model. We focus on the quasistationary states reached when the system is initially prepared in a fully magnetized configuration. By means of numerical simulations and analytical considerations, we show that, along the quasistationary trajectories, the system evolves in a manifold of critical points of the potential energy function. Although these critical points are maxima, the large number of directions with marginal stability may be responsible for the slow relaxation dynamics and the trapping of the system in such trajectories.

Journal Article↗
PubMed2006

Detection of confinement and jumps in single-molecule membrane trajectories.

We propose a variant of the algorithm by [R. Simson, E. D. Sheets, and K. Jacobson, Biophys. 69, 989 (1995)]. Their algorithm was developed to detect transient confinement zones in experimental single-particle tracking trajectories of diffusing membrane proteins or lipids. We show that our algorithm is able to detect confinement in a wider class of confining potential shapes than that of Simson et al. Furthermore, it enables to detect not only temporary confinement but also jumps between confinement zones. Jumps are predicted by membrane skeleton fence and picket models. In the case of experimental trajectories of mu-opioid receptors, which belong to the family of G-protein-coupled receptors involved in a signal transduction pathway, this algorithm confirms that confinement cannot be explained solely by rigid fences.

Journal Article↗
PubMed2006

Escape of trajectories from a vase-shaped cavity.

We consider the escape of ballistic trajectories from an open, vase-shaped cavity. Such a system serves as a model for microwaves escaping from a cavity or electrons escaping from a microjunction. Fixing the initial position of a particle and recording its escape time as a function of the initial launch direction, the resulting escape-time plot shows "epistrophic fractal" structure--repeated structure within structure at all levels of resolution with new features appearing in the fractal at longer time scales. By launching trajectories simultaneously in all directions (modeling an outgoing wave), a detector placed outside the cavity would measure a train of escaping pulses. We connect the structure of this chaotic pulse train with the fractal structure of the escape-time plot.

Journal Article↗
PubMed2001

Nonequilibrium defect-unbinding transition: defect trajectories and loop statistics.

In a Ginzburg-Landau model for parametrically driven waves, a transition between a state of ordered and one of disordered spatiotemporal defect chaos is found. To get insight into the breakdown of the order, the defect trajectories are tracked in detail. Since the defects are created and annihilated in pairs, the trajectories form loops in space-time. The probability distribution functions for the size of the loops and the number of defects involved in them undergo a transition from exponential decay in the ordered regime to a power-law decay in the disordered regime. These power laws are also found in a simple lattice model of randomly created defect pairs that diffuse and annihilate upon collision.

Journal Article↗
PubMed2001

Quantum tunneling using entangled classical trajectories.

In this Letter, we present a new method for simulating quantum processes in the context of classical molecular dynamics simulations. The approach is based on solving numerically the quantum Liouville equation in the Wigner representation using ensembles of classical trajectories. Quantum effects arise in this formulation as a breakdown of the statistical independence of the ensemble. New interaction forces between ensemble members are derived, which require the trajectory ensemble representing the state to evolve as an entangled, unified whole. The method is applied to the simulation of quantum tunneling in a one-dimensional model system, yielding excellent agreement with exact quantum calculations.

Journal Article↗
PubMed2004

Photon trajectories in incoherent atomic radiation trapping as Lévy flights.

Photon trajectories in incoherent radiation trapping for Doppler, Lorentz, and Voigt line shapes under complete frequency redistribution are shown to be Lévy flights. The jump length (r) distributions display characteristic long tails. For the Lorentz line shape, the asymptotic form is a strict power law r(-3/2), while for Doppler the asymptotic is r(-2)(ln(r)(-1/2). For the Voigt profile, the asymptotic form always has a Lorentz character, but the trajectory is a self-affine fractal with two characteristic Hausdorff scaling exponents.

Journal Article↗
PubMed2005

Efficient procedure to compute the microcanonical volume of initial conditions that lead to escape trajectories from a multidimensional potential well.

A procedure is presented for computing the phase space volume of initial conditions for trajectories that escape or "react" from a multidimensional potential well. The procedure combines a phase space transition state theory, which allows one to construct dividing surfaces that are free of local recrossing and that minimize the directional flux, and a classical spectral theorem. The procedure gives the volume of reactive initial conditions in terms of a sum over each entrance channel of the well of the product of the phase space flux across the dividing surface associated with the channel and the mean residence time in the well of trajectories which enter through the channel. This approach is illustrated for HCN isomerization in three dimensions, for which the method is several orders of magnitude more efficient than standard Monte Carlo sampling.

Journal Article↗
PubMed2006

Imaging electron trajectories in a laser-wakefield cavity using betatron X-ray radiation.

We demonstrate that betatron x-ray radiation accurately provides direct imaging of electrons trajectories accelerated in laser wakefields. Experimental far field x-ray beam profiles reveal that electrons can follow similar transverse trajectories with typical excursions of 1.5 microm+/-0.5 microm in the plane of laser polarization and 0.7 microm+/-0.2 microm in the plane perpendicular.

Journal Article↗
PubMed2006

Re-engineering healthcare pipelnes: why trajectory selection is as important as process selection in enabling effective transfer of best practice.

PURPOSE: To demonstrate that effective re-engineering of healthcare pipelines requires selecting both the "best" process (how we shall do it in the future) and the best trajectory for change (how we get from here to there). DESIGN/METHODOLOGY/APPROACH: Exploitation of Braess' Paradox to identify strategic factors necessary to enable change in the re-engineering of NHS healthcare pipelines. FINDINGS: Route to maximising the chance of achieving effective change is displayed via a Johari Window. Each cell is then related to no change (despite significant investment); failure; or successful implementation. RESEARCH LIMITATIONS/IMPLICATIONS: The demonstrator pipeline is an NHS cataract repair supply chain. However, via the "Power of Analogy" concept the NHS scenario is readily related to a substantial number of industrial case studies. PRACTICAL IMPLICATIONS: The need to understand both process and trajectory is the key to effective re-engineering of pipelines. All "actors" in re-engineering programmes should acquire this knowledge and benefit from the new way of doing things. ORIGINALITY/VALUE: Formalises the strategic route to enabling healthcare delivery "best practice".

Health Care Reform↗
PubMed2005

Estimating the pen trajectories of static signatures using hidden Markov models.

Static signatures originate as handwritten images on documents and by definition do not contain any dynamic information. This lack of information makes static signature verification systems significantly less reliable than their dynamic counterparts. This study involves extracting dynamic information from static images, specifically the pen trajectory while the signature was created. We assume that a dynamic version of the static image is available (typically obtained during an earlier registration process). We then derive a hidden Markov model from the static image and match it to the dynamic version of the image. This match results in the estimated pen trajectory of the static image.

Algorithms↗
PubMedDate not supplied

Chronicity and instability of children's peer victimization experiences as predictors of loneliness and social satisfaction trajectories.

The present investigation was conducted to predict children's loneliness and social satisfaction growth curves from changes in their peer victimization status. Toward this aim, 388 children (193 boys, 195 girls) were interviewed at five points: as children entered kindergarten (in the fall) and spring of kindergarten through third grade. At each assessment, data were gathered on the frequency of children's peer victimization and degree of loneliness and social satisfaction. Groups were formed on the basis of timing and duration of children's victimization status. Hierarchical linear modeling was used to test several hypotheses regarding the nature of victimized children's growth curves. For instance, consistent with the Onset Hypothesis, the trajectories that emerged for children who moved from nonvictim to victim classification showed increasing levels of loneliness and decreasing social satisfaction. In contrast, findings for the Cessation Hypothesis were mixed, which suggests that children moving from victim to nonvictim status do not necessarily evidence significant improvements in loneliness or social satisfaction. The somewhat disparate trajectories that emerged for loneliness and social satisfaction are discussed.

Child↗
PubMedDate not supplied

Self-efficacy beliefs as shapers of children's aspirations and career trajectories.

This prospective study tested with 272 children a structural model of the network of sociocognitive influences that shape children's career aspirations and trajectories. Familial socioeconomic status is linked to children's career trajectories only indirectly through its effects on parents' perceived efficacy and academic aspirations. The impact of parental self-efficacy and aspirations on their children's perceived career efficacy and choice is, in turn, entirely mediated through the children's perceived efficacy and academic aspirations. Children's perceived academic, social, and self-regulatory efficacy influence the types of occupational activities for which they judge themselves to be efficacious both directly and through their impact on academic aspirations. Perceived occupational self-efficacy gives direction to the kinds of career pursuits children seriously consider for their life's work and those they disfavor. Children's perceived efficacy rather than their actual academic achievement is the key determinant of their perceived occupational self-efficacy and preferred choice of worklife. Analyses of gender differences reveal that perceived occupational self-efficacy predicts traditionality of career choice.

Achievement↗
PubMedDate not supplied

Interparental discord and adolescent adjustment trajectories: the potentiating and protective role of intrapersonal attributes.

This prospective study of 360 adolescent-mother dyads examined whether associations between marital discord and trajectories of adolescent depressive symptoms and delinquency varied as a function of three intrapersonal attributes: temperament, childhood behavior problems, and perceived family support. Difficult temperament (i.e., dysrhythmicity, poor task orientation) potentiated the effects of marital discord on adolescent trajectories of adjustment, whereas heightened perceptions of family support protected adolescents from the adverse effects of marital discord. Adolescents with behavior problem histories were initially less vulnerable to marital discord; however, the high levels of depressive symptoms exhibited by adolescents with childhood behavior problems persisted over time only when they were exposed to elevated marital discord. The effects of the moderators differed in terms of duration and course.

Adaptation, Psychological↗
PubMedDate not supplied

Sibling differentiation: sibling and parent relationship trajectories in adolescence.

Studied here were the links between sibling differences in trajectories of change in the qualities of parent-child relationships and the qualities of sibling relationships across a 2-year period in adolescence. Participants were first- and second-born siblings (M age = 14.94 years for firstborns and M age = 12.46 years for secondborns) from 185 predominantly White, working and middle-class families. In home interviews, siblings reported on their dyadic family relationships. For reports of parent-child warmth but not parent-child conflict, results were consistent with sibling differentiation theory: Increasing differences between siblings over time in parent-child warmth were linked to trajectories of increasing warmth and decreasing conflict in the sibling relationship as reported by firstborns, and increasing warmth in the sibling relationship as reported by secondborns. The findings support the view that sibling differentiation may be a strategy for managing sibling conflict and rivalry.

Adolescent↗
PubMedDate not supplied

Parents do matter: trajectories of change in externalizing and internalizing problems in early adolescence.

This study examined the relative influence of three parenting behaviors (support, behavioral control, and psychological control) and deviant peers on trajectories of externalizing and internalizing problems in early adolescence. A white, working-to-middle-class sample of adolescents and their mothers and fathers in two-earner families participated in a 32-year longitudinal study (N = 109 families). The study began when the adolescents were in sixth grade (M age = 11.5 years). Analyses showed that parents' firm behavioral control seemed to halt the upward trajectory in externalizing problems among adolescents with deviant peers. Initial levels of internalizing problems were higher among adolescents with parents who reported lower levels of behavioral control and among adolescents with deviant peers. This study suggests that parenting exerts an important influence in adolescents' lives and may do so even in the face of potentially negative peer influence.

Adolescent↗
PubMed2004

Functional mapping of quantitative trait loci underlying growth trajectories using a transform-both-sides logistic model.

The incorporation of developmental control mechanisms of growth has proven to be a powerful tool in mapping quantitative trait loci (QTL) underlying growth trajectories. A theoretical framework for implementing a QTL mapping strategy with growth laws has been established. This framework can be generalized to an arbitrary number of time points, where growth is measured, and becomes computationally more tractable, when the assumption of variance stationarity is made. In practice, however, this assumption is likely to be violated for age-specific growth traits due to a scale effect. In this article, we present a new statistical model for mapping growth QTL, which also addresses the problem of variance stationarity, by using a transform-both-sides (TBS) model advocated by Carroll and Ruppert (1984, Journal of the American Statistical Association 79, 321-328). The TBS-based model for mapping growth QTL cannot only maintain the original biological properties of a growth model, but also can increase the accuracy and precision of parameter estimation and the power to detect a QTL responsible for growth differentiation. Using the TBS-based model, we successfully map a QTL governing growth trajectories to a linkage group in an example of forest trees. The statistical and biological properties of the estimates of this growth QTL position and effect are investigated using Monte Carlo simulation studies. The implications of our model for understanding the genetic architecture of growth are discussed.

Biometry↗