1,250 research outputs found

    Analysis of marketing systems on traditional bananas and plantains in Peru

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    Poster presented at Tropentag 2011 - Development on the Margin. Bonn (Germany), 3-7 Oct 2011

    Dynamic Problems of Evolution

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    Evolution and growth of natural and man-made processes have impressed human beings from the very beginning. What is evolution? Is it the passage from an initial to a higher stage? What does "higher" mean in a world of many objectives? Is "higher" bound to the existence of monotonous indicators like entropy, or is it "gambling" within a predetermined combinatoric multifold of possibilities? Questions of this kind arise from the phenomena in our environment, from the spring-off of new species, but also from processes in our man-made technological world. How is the transition of basic innovation to technology and use of the corresponding products by society, what forecast can be made from increasing CO2, in the atmosphere on the impact on climate, from features of seismologic waves on future events etc. That means there is a strong connection between evolution processes and the emphasis of systems analysis as a help for strategic actions. This paper deals with general considerations about possible growth mechanisms as a base for creating valid growth models. But the main goal is to show how the parameters in growth models can be estimated using on one hand a fuzzy approach together with vector optimization and on the other hand a Bayesian approach. It can be seen that both approaches are useful and applicable and we get informations from one approach which the other one cannot give us. We studied already the growth of cracks in materials, processes well described in [10]. Preliminary results are contained in [13]. Research will be continued to identify the superposition of driving forces and of coupled systems in which oscillations can arise because of time delays between their driving-force pulses

    Renormalization approach to many-particle systems

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    This paper presents a renormalization approach to many-particle systems. By starting from a bare Hamiltonian H=H0+H1{\cal H}= {\cal H}_0 +{\cal H}_1 with an unperturbed part H0{\cal H}_0 and a perturbation H1{\cal H}_1,we define an effective Hamiltonian which has a band-diagonal shape with respect to the eigenbasis of H0{\cal H}_0. This means that all transition matrix elements are suppressed which have energy differences larger than a given cutoff λ\lambda that is smaller than the cutoff Λ\Lambda of the original Hamiltonian. This property resembles a recent flow equation approach on the basis of continuous unitary transformations. For demonstration of the method we discuss an exact solvable model, as well as the Anderson-lattice model where the well-known quasiparticle behavior of heavy fermions is derived.Comment: 11 pages, final version, to be published in Phys. Rev.

    Analytical and numerical investigation of escape rate for a noise driven bath

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    We consider a system-reservoir model where the reservoir is modulated by an external noise. Both the internal noise of the reservoir and the external noise are stationary, Gaussian and are characterized by arbitrary decaying correlation functions. Based on a relation between the dissipation of the system and the response function of the reservoir driven by external noise we numerically examine the model using a full bistable potential to show that one can recover the turn-over features of the usual Kramers' dynamics when the external noise modulates the reservoir rather than the system directly. We derive the generalized Kramers' rate for this nonequilibrium open system. The theoretical results are verified by numerical simulation.Comment: Revtex, 25 pages, 5 figures. To appear in Phys. Rev.

    Landscape equivalent of the shoving model

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    It is shown that the shoving model expression for the average relaxation time of viscous liquids follows largely from a classical "landscape" estimation of barrier heights from curvature at energy minima. The activation energy involves both instantaneous bulk and shear moduli, but the bulk modulus contributes less than 8% to the temperature dependence of the activation energy. This reflects the fact that the physics of the two models are closely related.Comment: 4 page

    BMI and recommended levels of physical activity in school children

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    Background: Physical activity (PA) and its health benefits are a continuous point of discussion. Recommendations for children's daily PA vary between guidelines. To better define the amount of PA necessary to prevent overweight and obesity in children, further research is needed. The present study investigates children's compliance to physical activity guidelines (PAGs) and the association between objectively measured PA and body mass index (BMI). Methods: Participating children were 11 years old (n = 419) and part of the European CHOP trial, which was conducted in Germany, Belgium, Poland, Spain, Italy. At least 2 days of PA measurements were collected from each child using a SenseWear\u2122 armband. BMI was calculated from children's height and weight. Thresholds of min\ub7day-1 in PA needed to differentiate between normal and excess weight (overweight/obesity) were determined with Receiver Operator Characteristics (ROC) analysis. Additionally, adjusted linear and logistic regressions models were calculated for group differences and effects of a 5, 15 and 60 min\ub7day-1 increases in PA on BMI. Results: Median time spent in total PA was 462 min\ub7day-1 (25th percentile; 75th percentile: 389; 534) and 75 min\ub7day-1 (41; 115) in moderate to vigorous PA (MVPA). Girls spent 36 min\ub7day-1 less in MVPA than boys and overweight/obese children 24 min\ub7day-1 less than normal weight children (linear regression, p < 0.001). 63.2% of the children met PAGs of 60 min\ub7day-1 in MVPA. The optimal threshold for min\ub7day-1 in MVPA determined with ROC analysis was 46 min\ub7day-1. Comparing 5, 15 and 60 min\ub7day-1 increases in PA revealed that an additional 15 min\ub7day-1 of vigorous PA had the same effect as 60 min\ub7day-1 of MVPA. Sedentary time and light PA showed contrary associations to one another, with light PA being negatively and sedentary time being positively associated with excessive weight. Conclusions: Current PAGs are met by 2/3 of children and seem appropriate to prevent excess weight in children. An official recommendation of daily 15-20 min of vigorous PA and further reduction of sedentary time could help to fight youth overweight and thus be of potential public health importance

    Some Unusual Properties of Turbulent Convection and Dynamos in Rotating Spherical Shells

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    The dynamics of convecting fluids in rotating spherical shells is governed at Prandtl numbers of the order unity by the interaction between differential rotation and roll-like convection eddies. While the differential rotation is driven by the Reynolds stresses of the eddies, its shearing action inhibits convection and causes phenomena such as localized convection and turbulent relaxation oscillations. The response of the system is enriched in the case of dynamo action. Lorentz forces may brake either entirely or partially the geostrophic differential rotation and give rise to two rather different dynamo states. Bistability of turbulent dynamos exists for magnetic Prandtl numbers of the order unity. While the ratios between mean magnetic and kinetic energies differ by a factor of 5 or more for the two dynamo states, the mean convective heat transports are nearly the same. They are much larger than in the absence of a magnetic field.Comment: To appear in Procs. IUTAM Symposium on Turbulence in the Atmosphere and Oceans, 08-7 = GA.06-0
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