Home About us Contact
|
Home About us Contact | ||
|
|
||
Large Values (large + value)
Selected AbstractsModelling interactions between fold,thrust belt deformation, foreland flexure and surface mass transportBASIN RESEARCH, Issue 2 2006Guy D. H. Simpson ABSTRACT Interactions between fold and thrust belt deformation, foreland flexure and surface mass transport are investigated using a newly developed mathematical model incorporating fully dynamic coupling between mechanics and surface processes. The mechanical model is two dimensional (plane strain) and includes an elasto-visco-plastic rheology. The evolving model is flexurally compensated using an elastic beam formulation. Erosion and deposition at the surface are treated in a simple manner using a linear diffusion equation. The model is solved with the finite element method using a Lagrangian scheme with marker particles. Because the model is particle based, it enables straightforward tracking of stratigraphy and exhumation paths and it can sustain very large strain. It is thus ideally suited to study deformation, erosion and sedimentation in fold,thrust belts and foreland basins. The model is used to investigate how fold,thrust deformation and foreland basin development is influenced by the non-dimensional parameter , which can be interpreted as the ratio of the deformation time scale to the time scale for surface processes. Large values of imply that the rate of surface mass transport is significantly greater than the rate of deformation. When , the rates of surface processes are so slow that one observes a classic propagating fold,thrust belt with well-developed wedge top basins and a largely underfilled foreland flexural depression. Increasing causes (1) deposition to shift progressively from the wedge top into the foredeep, which deepens and may eventually become filled, (2) widespread exhumation of the fold,thrust belt, (3) reduced rates of frontal thrust propagation and possible attainment of a steady-state orogen width and (4) change in the style and dynamics of deformation. Together, these effects indicate that erosion and sedimentation, rather than passively responding to tectonics, play an active and dynamic role in the development of fold,thrust belts and foreland basins. Results demonstrate that regional differences in the relative rates of surface processes (e.g. because of different climatic settings) may lead to fold,thrust belts and foreland basins with markedly different characteristics. Results also imply that variations in the efficiency of surface processes through time (e.g., because of climate change or the emergence of orogens above sea level) may cause major temporal changes in orogen and basin dynamics. [source] Laminar diffusion of suspended particulate matter using a two phase flow modelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2002T. C. Panda Abstract The present paper envisages laminar mixing of a two-dimensional jet of particulate suspension in an incompressible carrier fluid with a free stream in direction of the jet axis. Finite difference technique has been employed for finding out solution of governing equations. It is found that the diffusion parameter ,, the ratio of particle diffusion coefficient and kinematic viscosity of the carrier fluid, have significant influence on the concentration of particles. A large value of , has the effect in increasing the perturbation velocity up and perturbation density ,p. It is observed that the volume fraction ,, has no significant effect on perturbation velocity u and up but has profound effect on perturbation velocity v and vp. It is also found that the particle phase as well as the carrier fluid velocity attain free stream value for the large ,, the modified x -co-ordinate. Further the magnitude of the perturbation quantities u, up, v, vp decreases as , increases i.e. at far away from the nozzle exit. Copyright © 2002 John Wiley & Sons, Ltd. [source] Kinetics and mechanism of the pyridinolysis of aryl dithiobenzoates in acetonitrileINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2004Hyuck Keun Oh The kinetics and mechanism of the aminolysis of aryl dithiobenzoates (RC(S)SC6H4Z; R = Ph) with pyridines (XC5H4N) in acetonitrile at 60.0°C have been studied. A biphasic Brönsted plot is obtained with a change in slope from a large value (,X , 0.7,0.8) to a small value (,X , 0.2) at pKa° = 5.2, which is interpreted to indicate a change of the rate-determining step from breakdown to formation of the zwitterionic tetrahedral intermediate, T±, at pKa = 5.2 as the pyridine basicity is increased. Rates are compared with those corresponding values for aryl dithioacetates (R = Me). The faster rates for R = Me than for R = Ph, for the rate-limiting formation of T±, can be attributed to the hyperconjugative charge transfer effect of the Me group, pseudo-,Me , ,*C=S. Clear-cut change in the cross-interaction constants, ,XZ, from +1.47 to ,0.20 supports the proposed mechanistic change. The breakpoint at pKa° = 5.2 for R = Ph in the present work is in agreement with those for the pyridinolysis of R = Me and 2-furyl, and attests to the insignificant effect of acyl group, R, on the breakpoint. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 434,440, 2004 [source] Rheological characterization of schizophyllan aqueous solutions after denaturation,renaturation treatmentBIOPOLYMERS, Issue 4 2004Yapeng Fang Abstract Schizophyllan (SPG) with a molecular weight of 2.6×106, designated SPG-1, is denatured and then renatured at a concentration of 1.8 wt % by alkalization,neutralization. The prepared denatured,renatured samples (DRSPG-1) are diluted to various concentrations and equilibrated for 10 days before rheological and intrinsic viscosity measurements. When concentration (Cp) is above 0.75 wt %, DRSPG-1 aqueous systems have weak gel-type rheological properties. However, for 0.28 wt % , Cp , 0.65 wt % and Cp , 0.19 wt %, DRSPG-1 aqueous systems behave as power law fluids and Newtonian fluids, respectively, which are attributed to the moderate isotropy degree of DRSPG-1 chains. Furthermore, a critical overlap parameter of c*[,] = 1.2 is determined for DRSPG-1 in aqueous solutions, which is close to that of 1 for intact SPG in water while far smaller than that of 4.3 for SPG in DMSO. This is considered to be due to the strong interactions of DRSPG-1 chains in water, further confirmed by the intrinsic viscosity measurements in which the DRSPG-1 aqueous solution shows an abnormally large value of Huggins constant. Regarding the structure of DRSPG-1 weak gels, multiruns of dynamic strain sweep measurements suggest that the dominant structures are aggregates formed by hydrogen-bonding associations of DRSPG-1 chains rather than the permanent three-dimensional network. In addition, the step-shear rate tests are performed to study the thixotropic properties of DRSPG-1 aqueous systems. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004 [source] Asymmetric Hydrogenation with Highly Active IndolPhos,Rh Catalysts: Kinetics and Reaction MechanismCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2010Jeroen Wassenaar Abstract The mechanism of the IndolPhos,Rh-catalyzed asymmetric hydrogenation of prochiral olefins has been investigated by means of X-ray crystal structure determination, kinetic measurements, high-pressure NMR spectroscopy, and DFT calculations. The mechanistic study indicates that the reaction follows an unsaturate/dihydride mechanism according to Michaelis,Menten kinetics. A large value of KM (KM=5.01±0.16,M) is obtained, which indicates that the Rh,solvate complex is the catalyst resting state, which has been observed by high-pressure NMR spectroscopy. DFT calculations on the substrate,catalyst complexes, which are undetectable by experimental means, suggest that the major substrate,catalyst complex leads to the product. Such a mechanism is in accordance with previous studies on the mechanism of asymmetric hydrogenation reactions with C1 -symmetric heteroditopic and monodentate ligands. [source] Power laws without complexityECOLOGY LETTERS, Issue 4 2005Andrew R. Solow Abstract Power laws have been invoked in describing the sizes of a wide variety of objects in evolution and ecology. The apparent ubiquity of power laws is commonly attributed to a form of complex behaviour called self-organizing criticality. It is shown that power law behaviour inevitably arises from the statistics of large values from heavy-tailed distributions and that such distributions can be generated by processes that do not involve self-organized criticality. It follows that power law behaviour cannot be taken as prima facie evidence of self-organizing criticality. [source] Network bias in air quality monitoring designENVIRONMETRICS, Issue 7 2008Nicola Loperfido Abstract We develop a statistical model for the bias resulting from designing an air quality monitoring network with the aim of finding large values, and then using the data obtained in studies of health effects of air quality. Appropriate conditional distributions are shown to be well-known generalizations of the normal one. Theoretical results are applied to an ozone monitoring network in the state of Washington, USA. Copyright © 2008 John Wiley & Sons, Ltd. [source] Assessing extremal dependence of environmental spatial fieldsENVIRONMETRICS, Issue 2 2008L. Bel Abstract Spatial environmental processes often exhibit dependence in their large values. In order to model such processes, their dependence properties must be characterized and quantified. In this paper, the performances of several estimators of the extremal coefficient are tested and compared using simulations of various spatial models. Two case studies (annual temperatures in France and precipitations in the French province Burgundy) are also presented. Copyright © 2007 John Wiley & Sons, Ltd. [source] Robust principal component analysis and outlier detection with ecological dataENVIRONMETRICS, Issue 2 2004Donald A. Jackson Abstract Ecological studies frequently involve large numbers of variables and observations, and these are often subject to various errors. If some data are not representative of the study population, they tend to bias the interpretation and conclusion of an ecological study. Because of the multivariate nature of ecological data, it is very difficult to identify atypical observations using approaches such as univariate or bivariate plots. This difficulty calls for the application of robust statistical methods in identifying atypical observations. Our study provides a comparison of a standard method, based on the Mahalanobis distance, used in multivariate approaches to a robust method based on the minimum volume ellipsoid as a means of determining whether data sets contain outliers or not. We evaluate both methods using simulations varying conditions of the data, and show that the minimum volume ellipsoid approach is superior in detecting outliers where present. We show that, as the sample size parameter, h, used in the robust approach increases in value, there is a decrease in the accuracy and precision of the associated estimate of the number of outliers present, in particular as the number of outliers increases. Conversely, where no outliers are present, large values for the parameter provide the most accurate results. In addition to the simulation results, we demonstrate the use of the robust principal component analysis with a data set of lake-water chemistry variables to illustrate the additional insight available. We suggest that ecologists consider that their data may contain atypical points. Following checks associated with normality, bivariate linearity and other traditional aspects, we advocate that ecologists examine their data sets using robust multivariate methods. Points identified as being atypical should be carefully evaluated based on background information to determine their suitability for inclusion in further multivariate analyses and whether additional factors explain their unusual characteristics. Copyright © 2004 John Wiley & Sons, Ltd. [source] QUANTITATIVE GENETICS OF PLASTRON SHAPE IN SLIDER TURTLES (TRACHEMYS SCRIPTA)EVOLUTION, Issue 3 2006Erin M. Myers Abstract Shape variation is widespread in nature and embodies both a response to and a source for evolution and natural selection. To detect patterns of shape evolution, one must assess the quantitative genetic underpinnings of shape variation as well as the selective environment that the organisms have experienced. Here we used geometric morphometrics to assess variation in plastron shell shape in 1314 neonatal slider turtles (Trachemys scripta) from 162 clutches of laboratory-incubated eggs from two nesting areas. Multivariate analysis of variance indicated that nesting area has a limited role in describing plastron shape variation among clutches, whereas differences between individual clutches were highly significant, suggesting a prominent clutch effect. The covariation between plastron shape and several possible maternal effect variables (yolk hormone levels and egg dimensions) was assessed for a subset of clutches and found to be negligible. We subsequently employed several recently proposed methods for estimating heritability from shape variables, and generalized a univariate approach to accommodate unequal sample sizes. Univariate estimates of shape heritability based on Procrustes distances yielded large values for both nesting populations (h2, 0.86), and multivariate estimates of maximal additive heritability were also large for both nesting populations (h2max, 0.57). We also estimated the dominant trend in heritable shape change for each nesting population and found that the direction of shape evolution was not the same for the two sites. Therefore, although the magnitude of shape evolution was similar between nesting populations, the manner in which plastron shape is evolving is not. We conclude that the univariate approach for assessing quantitative genetic parameters from geometric morphometric data has limited utility, because it is unable to accurately describe how shape is evolving. [source] Nanocrystalline Electroplated Cu,Ni: Metallic Thin Films with Enhanced Mechanical Properties and Tunable Magnetic BehaviorADVANCED FUNCTIONAL MATERIALS, Issue 6 2010Eva Pellicer Abstract Nanocrystalline 3,µm thick Cu1,xNix (0.45,,,x,,,0.87) films are electrodeposited galvanostatically onto Cu/Ti/Si (100) substrates, from a citrate- and sulphate-based bath containing sodium lauryl sulphate and saccharine as additives. The films exhibit large values of reduced Young's modulus (173,<,Er,<,192,GPa) and hardness (6.4,<,H,<,8.2,GPa), both of which can be tailored by varying the alloy composition. The outstanding mechanical properties of these metallic films can be ascribed to their nanocrystalline nature,as evidenced by X-ray diffraction, transmission electron microscopy, and atomic force microscopy,along with the occurrence of stacking faults and the concomitant formation of intragranular nanotwins during film growth. Due to their nanocrystalline character, these films also show very low surface roughness (root mean square deviation of around 2,nm). Furthermore, tunable magnetic properties, including a transition from paramagnetic to ferromagnetic behavior, are observed when the Ni percentage is increased. This combination of properties, together with the simplicity of the fabrication method, makes this system attractive for widespread technological applications, including hard metallic coatings or magnetic micro/nano-electromechanical devices. [source] Analytical power series solutions to the two-dimensional advection,dispersion equation with distance-dependent dispersivitiesHYDROLOGICAL PROCESSES, Issue 24 2008Jui-Sheng Chen Abstract As is frequently cited, dispersivity increases with solute travel distance in the subsurface. This behaviour has been attributed to the inherent spatial variation of the pore water velocity in geological porous media. Analytically solving the advection,dispersion equation with distance-dependent dispersivity is extremely difficult because the governing equation coefficients are dependent upon the distance variable. This study presents an analytical technique to solve a two-dimensional (2D) advection,dispersion equation with linear distance-dependent longitudinal and transverse dispersivities for describing solute transport in a uniform flow field. The analytical approach is developed by applying the extended power series method coupled with the Laplace and finite Fourier cosine transforms. The developed solution is then compared to the corresponding numerical solution to assess its accuracy and robustness. The results demonstrate that the breakthrough curves at different spatial locations obtained from the power series solution show good agreement with those obtained from the numerical solution. However, owing to the limited numerical operation for large values of the power series functions, the developed analytical solution can only be numerically evaluated when the values of longitudinal dispersivity/distance ratio eL exceed 0·075. Moreover, breakthrough curves obtained from the distance-dependent solution are compared with those from the constant dispersivity solution to investigate the relationship between the transport parameters. Our numerical experiments demonstrate that a previously derived relationship is invalid for large eL values. The analytical power series solution derived in this study is efficient and can be a useful tool for future studies in the field of 2D and distance-dependent dispersive transport. Copyright © 2008 John Wiley & Sons, Ltd. [source] A second order discontinuous Galerkin method for advection on unstructured triangular meshesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 4 2003H. J. M. Geijselaers Abstract In this paper the advection of element data which are linearly distributed inside the elements is addressed. Across element boundaries the data are assumed discontinuous. The equations are discretized by the Discontinuous Galerkin method. For stability and accuracy at large step sizes (large values of the Courant number), the method is extended to second order. Furthermore the equations are enriched with selective implicit terms. This results in an explicit and local advection scheme, which is stable and accurate for Courant numbers less than .95 on unstructured triangle meshes. Results are shown of some pure advection test problems. Copyright © 2003 John Wiley & Sons, Ltd. [source] Quasi optimal finite difference method for Helmholtz problem on unstructured gridsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 10 2010Daniel T. Fernandes Abstract A quasi optimal finite difference method (QOFD) is proposed for the Helmholtz problem. The stencils' coefficients are obtained numerically by minimizing a least-squares functional of the local truncation error for plane wave solutions in any direction. In one dimension this approach leads to a nodally exact scheme, with no truncation error, for uniform or non-uniform meshes. In two dimensions, when applied to a uniform cartesian grid, a 9-point sixth-order scheme is derived with the same truncation error of the quasi-stabilized finite element method (QSFEM) introduced by Babu,ka et al. (Comp. Meth. Appl. Mech. Eng. 1995; 128:325,359). Similarly, a 27-point sixth-order stencil is derived in three dimensions. The QOFD formulation, proposed here, is naturally applied on uniform, non-uniform and unstructured meshes in any dimension. Numerical results are presented showing optimal rates of convergence and reduced pollution effects for large values of the wave number. Copyright © 2009 John Wiley & Sons, Ltd. [source] A general methodology for investigating flow instabilities in complex geometries: application to natural convection in enclosuresINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2001E. Gadoin Abstract This paper presents a general methodology for studying instabilities of natural convection flows enclosed in cavities of complex geometry. Different tools have been developed, consisting of time integration of the unsteady equations, steady state solving, and computation of the most unstable eigenmodes of the Jacobian and its adjoint. The methodology is validated in the classical differentially heated cavity, where the steady solution branch is followed for vary large values of the Rayleigh number and most unstable eigenmodes are computed at selected Rayleigh values. Its effectiveness for complex geometries is illustrated on a configuration consisting of a cavity with internal heated partitions. We finally propose to reduce the Navier,Stokes equations to a differential system by expanding the unsteady solution as the sum of the steady state solution and of a linear combination of the leading eigenmodes. The principle of the method is exposed and preliminary results are presented. Copyright © 2001 John Wiley & Sons, Ltd. [source] On the efficient computation of closed-form Green's functions in planar stratified mediaINTERNATIONAL JOURNAL OF RF AND MICROWAVE COMPUTER-AIDED ENGINEERING, Issue 2 2008A. G. Polimeridis Abstract The spatial-domain Green's functions for the vector and scalar potentials in planar stratified media are cast into closed forms via a two-level approximation of their spectral-domain counterparts. The proposed methodology begins with the approximation of the spectral-domain Green's functions over large values of the spectral variable by complex exponentials, and continues with the approximation of the remainder by rational functions. Finally, the closed-form Green's functions in terms of spherical and cylindrical waves are derived, making use of some well-known integral identities. A key-feature of the proposed approach is that although it does not call for an analytical extraction of the quasistatic terms and the surface wave poles, it provides the means for the accurate description of both the near-field and far-field physics. Moreover, the rational function spectrum fitting proposed here overcomes the problem of the spurious singular behavior of the spatial-domain Green's functions because of the use of Hankel functions. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008. [source] Fragmentation model of meteoroid motion, mass loss, and radiation in the atmosphereMETEORITICS & PLANETARY SCIENCE, Issue 1 2005k CEPLECHA We solve them numerically including two possible types of fragmentation: into large pieces and into a cluster of small fragments. We have written a Fortran code that computes the motion, ablation and light intensity of a meteoroid at chosen heights, and allows for the ablation and shape density coefficients , and K, as well as the luminous efficiency ,, to be variable with height/time. We calibrated our fragmentation model (FM) by the best fit to observational values for the motion, ablation, radiation, fragmentation and the terminal masses (recovered meteorites) for the Lost City bolide. The FM can also handle multiple and overlapping meteor flares. We separately define both the apparent and intrinsic values of ,, K, and ,. We present in this paper values of the intrinsic luminous efficiency as function of velocity, mass, and normalized air density. Detailed results from the successful application of the FM to the Lost City, Innisfree, and Benesov bolides are also presented. Results of applying the FM to 15 bolides with very precise observational data are presented in a survey mode (Table 7). Standard deviations of applying our FM to all these events correspond to the precision of the observed values. Typical values of the intrinsic ablation coefficient are low, mostly in the range from 0.004 to 0.008 s2 km,2, and do not depend on the bolide type. The apparent ablation coefficients reflect the process of fragmentation. The bolide types indicate severity of the fragmentation process. The large differences of the "dynamic" and "photometric" mass from numerous earlier studies are completely explained by our FM. The fragmentation processes cannot be modeled simply by large values of the apparent ablation coefficient and of the apparent luminous efficiency. Moreover, our new FM can also well explain the radiation and full dynamics of very fast meteoroids at heights from 200 km to 130 km. [source] Theoretical Prediction of the Phenoxyl Radical Formation Capacity and Cyclooxygenase Inhibition Relationships by Phenolic CompoundsMOLECULAR INFORMATICS, Issue 6 2002Juan Ruiz Abstract Due to the importance of the O-H bond dissociation in the antioxidant mechanism of anti-inflammatory phenols, we studied the biradical process Ph-OH,PhO.+H. for 25 phenolic compounds using ab initio calculations. Enthalpies of reaction (,Hr), changes in the electron density at the O-H bond critical point (,OH) and total atomic charges of ortho and para carbon atoms strongly correlate with the in vitro inhibition of cyclooxygenase activity by phenols. The most active compounds have large values of the electron density at the O-H bond (,OH), thus favouring the O-H bond dissociation. In contrast, inactive compounds have small values of the electron density at the O-H bond (,OH), thus reducing the hydrogen donation ability. These results are also supported by the representation of the molecular electrostatic potentials maps. The prediction of the cyclooxygenase inhibitory activity of the proposed QSAR equations is analysed using the multilineal (MLR) method. Finally, the differences in biological activity are examined by analysing the binding interactions of active compounds in the pocket site of human COX-2 enzyme structure derived from crystallographic X -ray data. [source] Phase diagram of a thin Heisenberg antiferromagnetic filmPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2007J. Cabral Neto Abstract We investigate the thickness dependence of the Néel temperature of a thin quantum spin-1/2 Heisenberg antiferromagnetic film as a function of the magnetic field on a simple cubic lattice. The Néel temperature TN(H, ,) is obtained by using the framework of the effective-field theory for films consisting of , = 2, 3, 5, 10 and , (bulk) interacting layers. We present the phase diagram of T versus H in the whole range of the magnetic field for several values of ,. A continuous phase-transition line separating the antiferromagnetic and ferromagnetic phases is observed. The critical temperature TN(H, ,) of the film is smaller than the corresponding bulk critical temperature (H) , TN(H, ,) of the infinite system, which has been analyzed recently by Bublitz Filho and de Sousa [Phys. Lett. A 323, 9 (2004)]; as , is increased, TN(H, ,) also increases and approaches (H) for large values of ,. We have, also, studied the quantum phase transition where three critical fields were found: Hc(,) = 6.224 for , , 3 (three-dimensional regime), Hc(, = 2) = 5.210 (intermediate regime) for , = 2 and, finally, the two-dimensional regime at , = 1 with Hc(, = 1) = 4.194. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effects of hydrostatic stress on the density of impurity states and donor-related optical absorption spectra in GaAs,(Ga,Al)As quantum wellsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2003S. Y. López Abstract The effects of hydrostatic stress on the density of donor impurity states and donor-related optical absorption spectra in a GaAs,(Ga,Al)As quantum well are investigated. The shallow-donor binding energy for different well widths and different values of the hydrostatic stress has been calculated. It has been found that for wider well widths the binding energy increases slowly with hydrostatic stress contrary to the behavior of the binding energy for wells with smaller widths. In particular, it has been found that the binding energy does not change appreciably with the impurity position when the width of the well is small and for large values of hydrostatic stress. Two structures in both the density of states and the optical absorption spectra, associated with impurities located close to the center and to the edges of the structure, are obtained. Also, it has been observed that the density of states and the optical absorption spectra depend strongly on the applied hydrostatic stress. [source] Phase modulation effects in X-ray diffraction from a highly deformed crystal with variable strain gradientACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2009M. Shevchenko The X-ray interbranch scattering by lattice distortions is studied for a thin crystal whose thickness is appreciably less than the conventional X-ray extinction length. The concept of interbranch phase modulation of the X-ray wavefield is extended to the case of a large gradient which depends on depth inside the crystal. The prominent interbranch features of the diffracted intensity are also established within this concept. Numerical calculations of the diffracted intensity are presented for an exponential strain gradient model to illustrate this. Diffraction (extinction) contrast is discussed for a strongly deformed specimen containing a single dislocation. It is predicted that for large values of the X-ray extinction length the extinction contrast may arise even in the case of a very thin crystal. This effect, owing to the interbranch phase changes of the waves scattered in the deformed matrix, is observed in experiments with protein crystals. [source] On ,-biased generators in NC0RANDOM STRUCTURES AND ALGORITHMS, Issue 1 2006Elchanan Mossel Cryan and Miltersen (Proceedings of the 26th Mathematical Foundations of Computer Science, 2001, pp. 272,284) recently considered the question of whether there can be a pseudorandom generator in NC0, that is, a pseudorandom generator that maps n -bit strings to m -bit strings such that every bit of the output depends on a constant number k of bits of the seed. They show that for k = 3, if m , 4n + 1, there is a distinguisher; in fact, they show that in this case it is possible to break the generator with a linear test, that is, there is a subset of bits of the output whose XOR has a noticeable bias. They leave the question open for k , 4. In fact, they ask whether every NC0 generator can be broken by a statistical test that simply XORs some bits of the input. Equivalently, is it the case that no NC0 generator can sample an ,-biased space with negligible ,? We give a generator for k = 5 that maps n bits into cn bits, so that every bit of the output depends on 5 bits of the seed, and the XOR of every subset of the bits of the output has bias 2. For large values of k, we construct generators that map n bits to bits such that every XOR of outputs has bias . We also present a polynomial-time distinguisher for k = 4,m , 24n having constant distinguishing probability. For large values of k we show that a linear distinguisher with a constant distinguishing probability exists once m , ,(2kn,k/2,). Finally, we consider a variant of the problem where each of the output bits is a degree k polynomial in the inputs. We show there exists a degree k = 2 pseudorandom generator for which the XOR of every subset of the outputs has bias 2,,(n) and which maps n bits to ,(n2) bits. © 2005 Wiley Periodicals, Inc. Random Struct. Alg., 2006 [source] Coin flipping from a cosmic source: On error correction of truly random bitsRANDOM STRUCTURES AND ALGORITHMS, Issue 4 2005Elchanan Mossel We study a problem related to coin flipping, coding theory, and noise sensitivity. Consider a source of truly random bits x , {0, 1}n, and k parties, who have noisy version of the source bits yi , {0, 1}n, when for all i and j, it holds that P[y = xj] = 1 , ,, independently for all i and j. That is, each party sees each bit correctly with probability 1 , ,, and incorrectly (flipped) with probability ,, independently for all bits and all parties. The parties, who cannot communicate, wish to agree beforehand on balanced functions fi: {0, 1}n , {0, 1} such that P[f1(y1) = , = fk(yk)] is maximized. In other words, each party wants to toss a fair coin so that the probability that all parties have the same coin is maximized. The function fi may be thought of as an error correcting procedure for the source x. When k = 2,3, no error correction is possible, as the optimal protocol is given by fi(yi) = y. On the other hand, for large values of k, better protocols exist. We study general properties of the optimal protocols and the asymptotic behavior of the problem with respect to k, n, and ,. Our analysis uses tools from probability, discrete Fourier analysis, convexity, and discrete symmetrization. © 2005 Wiley Periodicals, Inc. Random Struct. Alg., 2005 [source] Proof of a tiling conjecture of KomlósRANDOM STRUCTURES AND ALGORITHMS, Issue 2 2003Ali Shokoufandeh Abstract A conjecture of Komlós states that for every graph H, there is a constant K such that if G is any n -vertex graph of minimum degree at least (1 , (1/,cr(H)))n, where ,cr(H) denotes the critical chromatic number of H, then G contains an H -matching that covers all but at most K vertices of G. In this paper we prove that the conjecture holds for all sufficiently large values of n. © 2003 Wiley Periodicals, Inc. Random Struct. Alg., 23: 180,205, 2003 [source] The diurnal cycle of shallow cumulus clouds over land: A single-column model intercomparison studyTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004Geert Lenderink Abstract An intercomparison study for single-column models (SCMs) of the diurnal cycle of shallow cumulus convection is reported. The case, based on measurements at the Atmospheric Radiation Measurement program Southern Great Plains site on 21 June 1997, has been used in a large-eddy simulation intercomparison study before. Results of the SCMs reveal the following general deficiencies: too large values of cloud cover and cloud liquid water, unrealistic thermodynamic profiles, and high amounts of numerical noise. Results are also strongly dependent on vertical resolution. These results are analysed in terms of the behaviour of the different parametrization schemes involved: the convection scheme, the turbulence scheme, and the cloud scheme. In general the behaviour of the SCMs can be grouped in two different classes: one class with too strong mixing by the turbulence scheme, the other class with too strong activity by the convection scheme. The coupling between (subcloud) turbulence and the convection scheme plays a crucial role. Finally, (in part) motivated by these results several models have been successfully updated with new parametrization schemes and/or their present schemes have been successfully modified. © Royal Meteorological Society, 2004. S. Irons's contribution is Crown copyright [source] A planetary system with an escaping MarsASTRONOMISCHE NACHRICHTEN, Issue 1 2007Á. Süli Abstract The chaotic behaviour of the motion of the planets in our Solar System is well established. In this work to model a hypothetical extrasolar planetary system our Solar System was modified in such a way that we replaced the Earth by a more massive planet and let the other planets and all the orbital elements unchanged. The major result of former numerical experiments with a modified Solar System was the appearance of a chaotic window at ,E , (4, 6), where the dynamical state of the system was highly chaotic and even the body with the smallest mass escaped in some cases. On the contrary for very large values of the mass of the Earth, even greater than that of Jupiter regular dynamical behaviour was observed. In this paper the investigations are extended to the complete Solar System and showed, that this chaotic window does still exist. Tests in different ,Solar Systems' clarified that including only Jupiter and Saturn with their actual masses together with a more ,massive' Earth (4 < ,E < 6) perturbs the orbit of Mars so that it can even be ejected from the system. Using the results of the Laplace-Lagrange secular theory we found secular resonances acting between the motions of the nodes of Mars, Jupiter and Saturn. These secular resonances give rise to strong chaos, which is the cause of the appearance of the instability window. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Modeling Longitudinal Spatial Periodontal Data: A Spatially Adaptive Model with Tools for Specifying Priors and Checking FitBIOMETRICS, Issue 3 2008Brian J. Reich Summary Attachment loss (AL), the distance down a tooth's root that is no longer attached to surrounding bone by periodontal ligament, is a common measure of periodontal disease. In this article, we develop a spatiotemporal model to monitor the progression of AL. Our model is an extension of the conditionally autoregressive (CAR) prior, which spatially smooths estimates toward their neighbors. However, because AL often exhibits a burst of large values in space and time, we develop a nonstationary spatiotemporal CAR model that allows the degree of spatial and temporal smoothing to vary in different regions of the mouth. To do this, we assign each AL measurement site its own set of variance parameters and spatially smooth the variances with spatial priors. We propose a heuristic to measure the complexity of the site-specific variances, and use it to select priors that ensure parameters in the model are well identified. In data from a clinical trial, this model improves the fit compared to the usual dynamic CAR model for 90 of 99 patients' AL measurements. [source] Semiclassical determination of exponentially small intermode transitions for 1 + 1 spacetime scattering systems,COMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 8 2007Alain Joye We consider the semiclassical limit of systems of autonomous PDEs in 1 + 1 spacetime dimensions in a scattering regime. We assume the matrix-valued coefficients are analytic in the space variable, and we further suppose that the corresponding dispersion relation admits real-valued modes only with one-dimensional polarization subspaces. Hence a BKW-type analysis of the solutions is possible. We typically consider time-dependent solutions to the PDE that are carried asymptotically in the past and as x , ,, along one mode only and determine the piece of the solution that is carried for x , +, along some other mode in the future. Because of the assumed nondegeneracy of the modes, such transitions between modes are exponentially small in the semiclassical parameter; this is an expression of the Landau-Zener mechanism. We completely elucidate the spacetime properties of the leading term of this exponentially small wave, when the semiclassical parameter is small, for large values of x and t, when some avoided crossing of finite width takes place between the involved modes. © 2006 Wiley Periodicals, Inc. [source] | |||||||||||||||||||||||||