Search results for "wt"
showing 10 items of 5424 documents
Modeling Posidonia oceanica growth data: from linear to generalized linear mixed models
2010
The statistical analysis of annual growth of Posidonia oceanica is traditionally carried out through Gaussian linear models applied to untransformed, or log-transformed, data. In this paper, we claim that there are good reasons for re-considering this established practice, since real data on annual growth often violate the assumptions of Gaussian linear models, and show that the class of Generalized Linear Models (GLMs) represents a useful alternative for handling such violations. By analyzing Sicily PosiData-1, a real dataset on P. oceanica growth data gathered in the period 2000–2002 along the coasts of Sicily, we find that in the majority of cases Normality is rejected and the effect of …
Phase transformation kinetics in d-dimensional grains-containing systems: diffusion-type model
1998
Abstract An analytical approach to the phase transformation in d-dimensional grains-containing complex systems is offered. It is based on considering the mechanism of surface material exchange among neighbouring grains as the so-called state-dependent diffusion process, where the diffusion function is related to the magnitude of the grain boundary. The approach proposed deals with the kinetics of that ensemble under circumstances of a volume increase of the new phase or microstructure. Probabilistic characteristics of the process are derived and analyzed. A comparison with 2D modelling of similar kind is presented for the 3D case, and some possible practical realizations of the situation un…
Social capital and economic growth in Europe: nonlinear trends and heterogeneous regional effects
2016
After two decades of academic debate on the social capital-growth nexus, discussion still remains open. Most of the literature so far, however, has followed the one-size-its-all approach, neglecting that the great disparities across geographical units might have implications in this relationship. This article analyzes the role of two social capital indicators on the growth of 237 European regions in the period 1995–2007 by implementing a set of both parametric and non- parametric regressions. Whereas the former impose a linear functional form for the parameters, the latter relax this assumption providing a flexible frame in which the functional form is given by the data. The technique also …
Stochastic model for the epitaxial growth of two-dimensional islands in the submonolayer regime
2016
The diffusion-based growth of islands composed of clusters of metal atoms on a substrate is considered in the aggregation regime. A stochastic approach is proposed to describe the dynamics of island growth based on a Langevin equation with multiplicative noise. The distribution of island sizes, obtained as a solution of the corresponding Fokker-Planck equation, is derived. The time-dependence of island growth on its fractal dimension is analysed. The effect of mobility of the small islands on the growth of large islands is considered. Numerical simulations are in a good agreement with theoretical results.
Estimating growth charts via nonparametric quantile regression: a practical framework with application in ecology.
2013
We discuss a practical and effective framework to estimate reference growth charts via regression quantiles. Inequality constraints are used to ensure both monotonicity and non-crossing of the estimated quantile curves and penalized splines are employed to model the nonlinear growth patterns with respect to age. A companion R package is presented and relevant code discussed to favour spreading and application of the proposed methods.
A spatial analysis of Italian unemployment differences
2008
Using spatial econometric models, this paper focuses attention on the spatial structure of provincial unemployment disparities of Italian provinces for the year 2003. On the basis of findings from the economic literature and of the available socio-economic data, various model specifications including supply- and demand-side variables are tested. Further we use ESDA analysis as equivalent to integration analysis on time series; therefore it is applied on each variable, dependent and independent, involved in the statistical model. The suggestions of ESDA lead us to the most adequate statistical model, which estimates indicate that there is a significant degree of neighbouring effect (i.e. pos…
Escape Times in Fluctuating Metastable Potential and Acceleration of Diffusion in Periodic Fluctuating Potentials
2004
The problems of escape from metastable state in randomly flipping potential and of diffusion in fast fluctuating periodic potentials are considered. For the overdamped Brownian particle moving in a piecewise linear dichotomously fluctuating metastable potential we obtain the mean first-passage time (MFPT) as a function of the potential parameters, the noise intensity and the mean rate of switchings of the dichotomous noise. We find noise enhanced stability (NES) phenomenon in the system investigated and the parameter region of the fluctuating potential where the effect can be observed. For the diffusion of the overdamped Brownian particle in a fast fluctuating symmetric periodic potential w…
Growth of a colloidal crystallite of hard spheres
1997
Abstract We examine the growth of a single nucleus of hard spheres in a super-saturated colloidal dispersion of hard spheres. A model developed by Bruce Ackerson and Klaus Schatzel based on a Wilson-Frenkel growth law is used. Our emphasis is on the profile of the radial density distribution around the growing (but still spherically symmetric) grain and its Fourier transform, the grain's form factor, which can be observed under small scattering angles in a dynamic light scattering experiment. Depending on the value of the supersaturation we can identify two limiting cases of different growth exponents and density profiles: one is the Frank theory of diffusion-limited growth and the other is…
Rise and fall of historic tram networks: Logistic approximation and discontinuous events
2019
Abstract A logistic approximation was used to describe, in terms of total length (L) and population (H) variables, the growth and decay of historic transportation systems. Three successive stages, separated for sharp discontinuities were detected for several European tramway and metro systems, corresponding to a fast initial growth followed by an intermediate step of slow growth and a final stage of rapid decay. A common, generalized behaviour was obtained in the L/H vs. H variations relative to critical values of L and H parameters defined from the maximum in the L/H ratio.
Synthesis and biological evaluation of cyclic derivatives of combretastatin A-4 containing group 14 elements
2018
Several tricyclic compounds inspired by the structure of combretastatin A-4 and bearing group 14 elements have been synthesized by homocoupling lithiated aryl fragments followed by ring-closing metathesis. These tricyclic compounds and their diolefin precursors were evaluated for their antiproliferative action on the tumor cell lines HT-29, MCF-7, HeLa and A-549 and on the non-tumor cell line HEK-293. In addition, their effects on the cell cycle were also measured. The tricyclic compounds show antiproliferative activity similar to that of combretastatin A-4, even though they are not so active in arresting the cell cycle. However, some diolefin precursors are able to cause accumulation of ce…