Modelling a Dune Field

We present a model to describe the collective motion of barchan dunes in a field. Our model is able to reproduce the observation that a typical dune stays confined within a stripe. We also obtain some of the pattern structures which ressemble those observed from aerial photos which we do analyse and compare with the specific field of La\^ayounne.Comment: 15 pages, 13 figure

Les déficiences minérales du palmier à huile (E. guineensis Jacq.) dans la région de Belém, Parà (Brésil)

Dans la région de Belém (Para, Brésil), après 15 ans d'expérimentation au champ des fumures N, P, K, Mg sur palmier à huile, on peut tirer les enseignements suivants : la nutrition azotée est naturellement suffisante, sauf dans le très jeune âge et la déficience en phosphore, très accentuée entraîne une faible croissance, une surface foliaire réduite et une production très médiocre. Sa correction permet de doubler les rendements au jeune âge et de les quadrupler à 12-15 ans. A l'unité fertilisante, le phosphate naturel bien meilleur marché aurait une efficacité supérieure à celle du superphosphate triple sur ces latosols sableux; par suite du faible potassium échangeable du sol et de l'effet dépressif des phosphates sur la nutrition en K, il est nécessaire de corriger la déficience en cet élément dès la plantation; pour les mêmes raisons, avec en plus l'antagonisme K-Mg, de la fumure magnésienne doit aussi être apportée à l'âge adulte à partir de 8-10 ans. Les niveaux critiques de la nutrition minérale dans les feuilles sont précisés et comparés à d'autres résultats obtenus ailleurs dans le monde. A partir de ces données, des barèmes de fumure, fonction de l'âge des cultures et du contrôle de la nutrition par le diagnostic foliaire, sont proposés pour cette régio

Strategic management accounting: definitions and dimensions

The purposes of this paper are twofold: firstly it aims to discuss prior definitions of strategic management accounting (SMA) in order to understand the main purposes and the scope of SMA; secondly the management accountant’s participation in the strategic decision-making and SMA practices are discussed. The SMA literature review shows us that there is no consensus in the SMA definition. However, there is some common elements in the SMA definitions, such as: i) a development of management accounting with a strategic orientation; ii) an external orientation (outward-looking orientation); and, iii) an orientation for future (forward-looking orientation). In addition, SMA adopts both financial and non-financial measurement typologies and an orientation to internal resources and organizational capabilities (intellectual capital). There is also some consensus on the main purpose. SMA must cooperate and provide strategic information for the strategic management, marketing, and other managerial functions. The purpose is to create and achieve competitive advantages and enhance organizational performance. To accomplish this goal SMA considers two dimensions related with the management accountant’s participation in the strategic decisionmaking process and a set of SMA practices. Therefore, this paper contributes to a better understanding of the concept and scope of SMA and its two dimensions. In this sense, it assists practitioners and researchers to understand, adopt, use, and research the SMA.info:eu-repo/semantics/publishedVersio

Using Entropy-Based Methods to Study General Constrained Parameter Optimization Problems

In this letter we propose the use of physics techniques for entropy determination on constrained parameter optimization problems. The main feature of such techniques, the construction of an unbiased walk on energy space, suggests their use on the quest for optimal solutions of an optimization problem. Moreover, the entropy, and its associated density of states, give us information concerning the feasibility of solutions.Comment: 10 pages, 3 figures, references correcte

Cosmological particle production, causal thermodynamics, and inflationary expansion

Combining the equivalence between cosmological particle creation and an effective viscous fluid pressure with the fact that the latter represents a dynamical degree of freedom within the second-order Israel-Stewart theory for imperfect fluids, we reconsider the possibility of accelerated expansion in fluid cosmology. We find an inherent self-limitation for the magnitude of an effective bulk pressure which is due to adiabatic (isentropic) particle production. For a production rate which depends quadratically on the Hubble rate we confirm the existence of solutions which describe a smooth transition from inflationary to noninflationary behavior and discuss their interpretation within the model of a decaying vacuum energy density. An alternative formulation of the effective imperfect fluid dynamics in terms of a minimally coupled scalar field is given. The corresponding potential is discussed and an entropy equivalent for the scalar field is found.Comment: 16 pages, revtex file, submitted to Phys. Rev.

Sliding blocks with random friction and absorbing random walks

With the purpose of explaining recent experimental findings, we study the distribution $A(\lambda)$ of distances $\lambda$ traversed by a block that slides on an inclined plane and stops due to friction. A simple model in which the friction coefficient $\mu$ is a random function of position is considered. The problem of finding $A(\lambda)$ is equivalent to a First-Passage-Time problem for a one-dimensional random walk with nonzero drift, whose exact solution is well-known. From the exact solution of this problem we conclude that: a) for inclination angles $\theta$ less than \theta_c=\tan(\av{\mu}) the average traversed distance \av{\lambda} is finite, and diverges when $\theta \to \theta_c^{-}$ as \av{\lambda} \sim (\theta_c-\theta)^{-1}; b) at the critical angle a power-law distribution of slidings is obtained: $A(\lambda) \sim \lambda^{-3/2}$. Our analytical results are confirmed by numerical simulation, and are in partial agreement with the reported experimental results. We discuss the possible reasons for the remaining discrepancies.Comment: 8 pages, 8 figures, submitted to Phys. Rev.

Direct and Inverse Magnetocaloric effects in A-site ordered PrBaMn2O6 manganite in low magnetic fields

The magnetocaloric effect (MCE) of A-site ordered PrBaMn2O6 manganite has been studied by direct methods and by the specific heat measurements. Direct measurements of the MCE in low magnetic fields were performed using recently proposed modulation technique and by classic direct method in high fields. Direct and inverse MCE are observed at Curie and Neel points correspondingly. A value of the inverse MCE in the heating run is less than in the cooling regime. This effect can be attributing to competition between ferromagnetic and antiferromagnetic interactions. Indirectly estimated and direct MCE values considerably differ in around first order AF transition.Comment: 8 pages, 5 figure

On the Cut-Off Prescriptions Associated with Power-Law Generalized Thermostatistics

We revisit the cut-off prescriptions which are needed in order to specify completely the form of Tsallis' maximum entropy distributions. For values of the Tsallis entropic parameter $q>1$ we advance an alternative cut-off prescription and discuss some of its basic mathematical properties. As an illustration of the new cut-off prescription we consider in some detail the $q$-generalized quantum distributions which have recently been shown to reproduce various experimental results related to high $T_c$ superconductors

Minimal model for aeolian sand dunes

We present a minimal model for the formation and migration of aeolian sand dunes. It combines a perturbative description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The latter are shown to provide the characteristic length scale. The model can explain the origin of important features of dunes, such as the formation of a slip face, the broken scale invariance, and the existence of a minimum dune size. It also predicts the longitudinal shape and aspect ratio of dunes and heaps, their migration velocity and shape relaxation dynamics. Although the minimal model employs non-local expressions for the wind shear stress as well as for the sand flux, it is simple enough to serve as a very efficient tool for analytical and numerical investigations and to open up the way to simulations of large scale desert topographies.Comment: 19 pages, 22 figure

Entropic Upper Bound on Gravitational Binding Energy

We prove that the gravitational binding energy {\Omega} of a self gravitating system described by a mass density distribution {\rho}(x) admits an upper bound B[{\rho}(x)] given by a simple function of an appropriate, non-additive Tsallis' power-law entropic functional Sq evaluated on the density {\rho}. The density distributions that saturate the entropic bound have the form of isotropic q-Gaussian distributions. These maximizer distributions correspond to the Plummer density profile, well known in astrophysics. A heuristic scaling argument is advanced suggesting that the entropic bound B[{\rho}(x)] is unique, in the sense that it is unlikely that exhaustive entropic upper bounds not based on the alluded Sq entropic measure exit. The present findings provide a new link between the physics of self gravitating systems, on the one hand, and the statistical formalism associated with non-additive, power-law entropic measures, on the other hand