Search Results

You are looking at 91 - 100 of 5,035 items for :

  • "variations" x
  • All content x
Clear All

Abstract  

The method described to determine the neutron fluence is based on the plot of the isotopic variation of Cd and Gd subjected to neutron irradiation in a research reactor. The isotopic ratios are measured by thermal ionization mass spectrometry. The results indicate that the fluence values obtained, using the variation in the ratios114Cd/113Cd,156Gd/155Gd and158Gd/157Gd show standard deviations varying from 0.3 to 6.6%. These values agree with the extrapolated values calculated using the short time Au activation method. The method appears to be useful for determining paleo neutron flux in natural samples and irradiated fuels.

Restricted access

Melanin-concentrating hormone (MCH), the neuropeptide produced mainly in the hypothalamus, plays an operative role in regulating food intake and the sleep/wake cycle. Considering that these physiological functions pursue diurnal variations, we checked whether the total hypothalamic MCH level depends on the time of the day. The aggregated MCH peptide content of the whole MCH neuron population was significantly higher at the end of the sleeping period (lights on), than at the end of the active period (lights off). This result, together with earlier observations, indicates that in contrast to the MCH gene expression, the level of MCH peptide is object of circadian variation in the hypothalamus.

Open access

Hubbell (2001) proposes that random demographic processes (i.e., neutral dynamics) can explain observed levels of variation in the richness and abundance of species within and among communities. Hubbell's neutral models have drawn attention because they reproduce several characteristic features of natural communities. But neutral models are criticized for ignoring nonrandom processes known to cause species densities to fluctuate. We parameterized neutral models using the population counts of 64 species of aquatic invertebrates collected from 49 discrete rock pools over a 13 year period.We used Hubbell's numerical modeling approach to evaluate the effect of natural population fluctuations on the parameter settings. We also analyzed the effect of observed variation on the species proportional abundance predicted by neutral models. We find that observed levels of variation in abundance are much higher than predicted by neutral models, forcing estimates of themigration probability, m, and fundamental biodiversity parameter, ?, to fluctuateover time. Much of the observed variation is mediated by predator-prey interactions. Low predator densities are associated with fewer species and less even relative abundances of species, resulting in lower estimates of m and ? comparedto periods of high predator densities. Our results show that by assuming an identical survival probability for all species, neutral models misrepresent substantial aspects of community dynamics.

Restricted access

By using the partial ordering method, a more general type of Ekeland's variational principle and Caristi's coincidence theorem for set-valued mappings in probabilistic metric spaces are given in this paper. In addition, we give also a directly simple proof of the equivalence between theses theorems in probabilistic metric spaces.

Restricted access

The multi-dimensional analysis of biodiversity addresses several components, including the number of species, as well as their ecological and evolutionary identities. This approach can lead to key biodiversity patterns that remain covered when only species richness is studied. The convergence of the Nearctic and Neotropical biogeographic regions in Mexico suggests spatial variation in biodiversity patterns. In this study we analysed species richness, functional diversity, and phylogenetic diversity of mammal assemblages in relation to abiotic variables and their variation across the biogeographic regions. Our results showed contrasting patterns among biodiversity dimensions: 1) species richness followed the typical global pattern of a gradual increase from the Nearctic towards the Neotropical region, 2) phylogenetic diversity was highest in the Transitional region; and 3) functional diversity was highest in the Neotropical region, but not different between the Nearctic and Transitional regions. Abiotic variables had a significant relation with variation in biodiversity dimensions, but they had a higher contribution to explain functional diversity than species richness and phylogenetic diversity. The decoupled patterns of functional and phylogenetic diversity among the biogeographical regions highlight the complementarity between these dimensions and the importance of transitional regions, and suggest that several historical, environmental and ecological processes may be structuring mammal assemblages. Likewise, different approaches should be followed in accordance to variation of functional and phylogenetic traits among regions.

Restricted access

Abstract  

Lead concentrations of atmospheric particulate matter were measured systematically by X-ray fluorescence method at three sites of Debrecen city over long periods. The time dependence of Pb concentration level in air was investigated and compared with literature data. High variation was found in lead concentration values with respect to place and time.

Restricted access

A method for determining the variation of the kinetic parametersA, E andn is proposed, and is exemplified by a study of PVC degradation, with the chlorine partially substituted by benzene. Because of the complexity of the method, a digital computer has been used.

Restricted access
Acta Geodaetica et Geophysica Hungarica
Authors: P. Varga, Z. Bus, B. Süle, A. Schreider, C. Bizouard, D. Gambis, and C. Denis

In its first part this work focuses on connection of length of day (LOD) with centered and eccentric geomagnetic dipole fields described with the use of Gaussian coefficients derived from global geomagnetic observations is discussed for the epoch 1900–2000. The statistical comparison of temporal variation of earth magnetic and astronomical data shows close correlation of geomagnetic dipole momentum M 0 and ΔLOD. It should be mentioned that the time-correlation is closer when the centred geomagnetic dipole is used for statistical modelling. In the same time no relation was found between ΔLOD and the orientation variations of the geomagnetic dipole.In the second part of present study the connection of geomagnetic field and the LOD is investigated in geological time-scale. A significant ΔLOD was found which coincides in time with the geomagnetic Mesozoic low. The reason of this coincidence is enigmatic.

Restricted access

Khan, M. K., On the Rate of Convergence of Bernstein Power Series for Functions of Bounded Variation, Journal of Approximation Theory 57 (1989), 90-103. MR 90d :41043 On the Rate of

Restricted access

Abstract  

The concentrations of nine trace elements in segments of individual human head hairs have been determined by neutron activation analysis. Concentration variations have been studied along individual hairs, among hairs plucked from various locations on the scalp, among hairs in different phases of the growth cycle, and before and after hair cleaning. Recent atomic absorption analysis data indicate strong concentration fluctuations in successive single hair segments of 2 to 5 mm in length. The results are discussed in a forensic context.

Restricted access