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Search Results: 1 - 10 of 440364 matches for " J. P. Meyer "
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Prof. J.C. van Rooy: Volksman en kultuurleier.
P. J. Meyer
Koers : Bulletin for Christian Scholarship , 1954, DOI: 10.4102/koers.v22i2.1889
Abstract: Prof. J. C. van Rooy, voorsitter van die Uitvoerende Komitee van die F.A.K. vanaf 1 Oktober 1941 tot 1 Oktober 1951, het as volksman en kultuurleierdie Calvinisties-christelike grondslag en inhoud van die Afrikaanse kultuurstrewe steeds duidelik vertolk, omlyn, doelgerigte leiding daaraan gegee en daarvoor geesdrif verwek. Daarby het hy hom veral beywer vir die eenheid van sy volk as voorwaarde vir die volbrenging van sy God gestelde roeping.
Bestemming van die afrikaanse kultuur
P. J. Meyer
Koers : Bulletin for Christian Scholarship , 1969, DOI: 10.4102/koers.v36i5.1316
Abstract: Kultuur is ’n vierledige geskiedkundige proses wat ’n onskeibare eenheid is van werkers (werker in die wydste sin van die woord), werksaamhede, werke en toe-eienaars, waar die werkers ’n innerlike gemeenskap vorm, ’n gemeenskap wat tegelyk ’n toe-eienaarsgemeenskap is.
A Spring–Dashpot System for Modelling Lung Tumour Motion in Radiotherapy
P. L. Wilson,J. Meyer
Computational and Mathematical Methods in Medicine , 2010, DOI: 10.1080/17486700802616534
Abstract: A 3D system of springs and dashpots is presented to model the motion of a lung tumour during respiration. The main guiding factor in configuring the system is the spatial relationship between abdominal and lung tumour motion. A coupled, non-dimensional triple of ordinary differential equations models the tumour motion when driven by a 3D breathing signal. Asymptotic analysis is used to reduce the system to a single equation driven by a 3D signal, in the limit of small lateral and transverse tumour motions. A numerical scheme is introduced to solve this equation, and tested over wide parameter ranges. Real clinical data is used as input to the model, and the tumour motion output is in excellent agreement with that obtained by a prototype tumour tracking system, with model parameters obtained by optimization. The fully 3D model has the potential to accurately model the motion of any lung tumour given an abdominal signal as input, with model parameters obtained from an internal optimization routine.
Scénarios de pénétration des composés oxygénés en Europe de l'Ouest Scenarios for the Penetration of Oxygenated Compounds in Western Europe
Beaufils J. P.,Meyer C.
Oil & Gas Science and Technology , 2006, DOI: 10.2516/ogst:1987014
Abstract: Scénarios de pénétration des composés oxygénés en Europe de l'Ouest Différents scénarios de progression de la consommation de carburants sans plomb dans l'ensemble des pays européens ont été examinés. Les besoins en composés oxygénés en ont été déduits. Les résultats montrent que les disponibilités additionnées des composés oxygénés (MTBE, M3T2, TBA) permettront de satisfaire la demande maximale potentielle au début des années 1990, mais n'autoriseront ensuite qu'une addition limitée de ces composés; la concentration de chacun d'entre eux restera très inférieure aux limites autorisées. Les décisions de construction de nouvelles unités seraient à prendre avant 1995. Different scenarios are examined for the penetration of unleaded gasoline in the European countries. The requirements of oxygenated compounds have been deduced from these scenarios. The results show that the supply of oxygenated compounds (MTBE, M3T2, TBA) will be able to meet the maximum potential demand in the early 1990s. Subsequently, however, only a limited amount of such compounds can be added. The concentration of each compound will remain considerably lower than the authorized limits. Decisions to build new plants will have to be made before 1995.
Monte Carlo SImulation of Polymers: Coarse-Grained Models
J. Baschnagel,J. P. Wittmer,H. Meyer
Physics , 2004,
Abstract: A coarse-grained simulation model eliminates microscopic degrees of freedom and represents a polymer by a simplified structure. A priori, two classes of coarse-grained models may be distinguished: those which are designed for a specific polymer and reflect the underlying atomistic details to some extent, and those which retain only the most basic features of a polymer chain (chain connectivity, short-range excluded-volume interactions, etc.). In this review we mainly focus on the second class of generic polymer models, while the first class of specific coarse-grained models is only touched upon briefly.
Transat—A Method for Detecting the Conserved Helices of Functional RNA Structures, Including Transient, Pseudo-Knotted and Alternative Structures
Nicholas J. P. Wiebe,Irmtraud M. Meyer
PLOS Computational Biology , 2010, DOI: 10.1371/journal.pcbi.1000823
Abstract: The prediction of functional RNA structures has attracted increased interest, as it allows us to study the potential functional roles of many genes. RNA structure prediction methods, however, assume that there is a unique functional RNA structure and also do not predict functional features required for in vivo folding. In order to understand how functional RNA structures form in vivo, we require sophisticated experiments or reliable prediction methods. So far, there exist only a few, experimentally validated transient RNA structures. On the computational side, there exist several computer programs which aim to predict the co-transcriptional folding pathway in vivo, but these make a range of simplifying assumptions and do not capture all features known to influence RNA folding in vivo. We want to investigate if evolutionarily related RNA genes fold in a similar way in vivo. To this end, we have developed a new computational method, Transat, which detects conserved helices of high statistical significance. We introduce the method, present a comprehensive performance evaluation and show that Transat is able to predict the structural features of known reference structures including pseudo-knotted ones as well as those of known alternative structural configurations. Transat can also identify unstructured sub-sequences bound by other molecules and provides evidence for new helices which may define folding pathways, supporting the notion that homologous RNA sequence not only assume a similar reference RNA structure, but also fold similarly. Finally, we show that the structural features predicted by Transat differ from those assuming thermodynamic equilibrium. Unlike the existing methods for predicting folding pathways, our method works in a comparative way. This has the disadvantage of not being able to predict features as function of time, but has the considerable advantage of highlighting conserved features and of not requiring a detailed knowledge of the cellular environment.
Testing comparative phylogeographic models of marine vicariance and dispersal using a hierarchical Bayesian approach
Michael J Hickerson, Christopher P Meyer
BMC Evolutionary Biology , 2008, DOI: 10.1186/1471-2148-8-322
Abstract: Given the Marquesan data, we find strong evidence of simultaneous colonization across all seven cowrie gastropod endemics co-distributed in the Marquesas. In contrast, the lower sample sizes in the Hawaiian data lead to greater uncertainty associated with the Hawaiian estimates. Although, the hyper-parameter estimates point to soft vicariance in a subset of the 11 Hawaiian taxon-pairs, the hyper-prior and hyper-posterior are too similar to make a definitive conclusion. Both results are not inconsistent with what is known about the geologic history of the archipelagos. Simulations verify that our method can successfully distinguish these two histories across a wide range of conditions given sufficient sampling.Although soft vicariance and colonization are likely to produce similar genetic patterns when a single taxon-pair is used, our hierarchical Bayesian model can potentially detect if either history is a dominant process across co-distributed taxon-pairs. As comparative phylogeographic datasets grow to include > 100 co-distributed taxon-pairs, the HABC approach will be well suited to dissect temporal patterns in community assembly and evolution, thereby providing a bridge linking comparative phylogeography with community ecology.Allopatric speciation is an enigma in many marine organisms because larval dispersal can potentially connect disjoint populations and thereby prevent the reproductive and morphological divergence that arises from prolonged isolation [1-7]. This is especially enigmatic in the Indo-Pacific region where many species range freely across this expanse without evidence for barriers to genetic exchange. This marine region harbors the planet's highest species diversity and endemism of marine fauna, and with the absence of explicit barriers, some have pushed controversial models of sympatric speciation to explain this elevated diversity [8,9]. Even the proposed competing models of geographic speciation in the Indo-Pacific remain contentious and gene
The Origin of Present Day Cosmic Rays: Fresh SN Ejecta or Interstellar Medium Material ? I Cosmic Ray Composition and SN Nucleosynthesis. A Conflict with the Early Galactic Evolution of Be ?
J. P. Meyer,D. C. Ellison
Physics , 1999,
Abstract: The composition of present day cosmic rays (CRs) is inconsistent with a significant acceleration of SN ejecta material (even with a preferential acceleration of ejecta grain material). Current CRs must result mainly from the acceleration of ISM ("solar mix") and circumstellar (22Ne- and 12C-rich WR wind) gas and grain material. The CR source composition, indeed, shows no anomaly related to SN nucleosynthesis: (i) The CR source FeNi/MgSiCa ratios have precisely solar mix values, while FeNi are mainly synthesized in SN Ia's, and MgSiCa in SN II's. If CR's originate in SN ejecta, this requires tight conditions on the acceleration efficiencies of the various SN Ia's and SN II's. (ii) The non-deficiency of the main-s-process elements, not made in any SN, relative to all elements made in SNae. (iii) All CR source isotope ratios are consistent with solar mix (except for the 22Ne and 12C excesses, WR wind). (iv) The absence of 59Ni in CRs implies that the time delay between the SN nucleosynthesis and their acceleration is > 10^5 yr. (v) The physics of SNR's and of CR shock acceleration implies that the acceleration of interior ejecta material is comparatively insignificant (Ellison & Meyer, this volume). Predominant acceleration of current CRs out of superbubble material also seems implausible. These conclusions regarding current CRs do not necessarily conflict with the linear evolution of Be/H in the early Galaxy. With the near absence of heavy elements in the early Galactic ISM, indeed, the acceleration of even a minute amount of freshly processed material must have then played a dominant role for the generation of Be from C and O. The "Be indicator" is blind to a possibly dominant early Galactic CR component originating in the ISM then composed of virtually pure H and He.
The Origin of Present Day Cosmic Rays: Fresh SN Ejecta or Interstellar Medium Material ? II Physics of the SNR shock wave acceleration
D. C. Ellison,J. P. Meyer
Physics , 1999,
Abstract: Assuming that refractory elements in cosmic rays originate in dust grains, we examine the viability of cosmic ray origin models wherein the bulk of present day cosmic rays are accelerated out of fresh supernova ejecta material before it mixes with the average interstellar medium (ISM). We conclude that the fresh ejecta scenarios that have been proposed thus far have serious flaws, and are unable to account for known properties of present day cosmic rays. These flaws include: (1) the small fraction of ejecta to ISM mass processed by the forward supernova remnant (SNR) shock; (2) the difficulty fresh ejecta grains have in reaching the forward shock in isolated SNRs, and the small expected sputtering yield, especially ahead of the shock, even if grains do reach the forward shock; (3) the implausibility that fresh ejecta material can dominate cosmic ray production in diffuse superbubbles; and (4) the lack of a connection in fresh ejecta models between the production of cosmic ray refractory and volatile elements. We conclude that the near linear increase in Be abundance with metallicity observed in old, halo stars cannot imply that a significant fraction of the cosmic rays seen today come from fresh supernova ejecta. This conclusion is supported by the analysis of the present day cosmic ray composition, as shown in Meyer & Ellison, this volume.
Pairing correlations. Part 2: Microscopic analysis of odd-even mass staggering in nuclei
T. Duguet,P. Bonche,P. -H. Heenen,J. Meyer
Physics , 2001, DOI: 10.1103/PhysRevC.65.014311
Abstract: The odd-even mass staggering in nuclei is analyzed in the context of self-consistent mean-field calculations, for spherical as well as for deformed nuclei. For these nuclei, the respective merits of the energy differences $\Delta^{(3)}$ and $\Delta^{(5)}$ to extract both the pairing gap and the time-reversal symmetry breaking effect at the same time are extensively discussed. The usual mass formula $\Delta^{(3)}$, is shown to contain additional mean-field contributions when realistic pairing is used in the calculation. A simple tool is proposed in order to remove time-reversal symmetry breaking effects from $\Delta^{(5)}$. Extended comparisons with the odd-even mass staggering obtained in the zero pairing limit (schematic model and self-consistent calculations) show the non-perturbative contribution of pairing correlations on this observable. PACS: 21.10Dr; 21.10.Hw; 21.30.-x. Keywords: Mean-field theories; Pairing correlations; Odd-even mass staggering;
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