Anisotropic Random Networks of Semiflexible Polymers

Motivated by the organization of crosslinked cytoskeletal biopolymers, we present a semimicroscopic replica field theory for the formation of anisotropic random networks of semiflexible polymers. The networks are formed by introducing random permanent crosslinks which fix the orientations of the corresponding polymer segments to align with one another. Upon increasing the crosslink density, we obtain a continuous gelation transition from a fluid phase to a gel where a finite fraction of the system gets localized at random positions. For sufficiently stiff polymers, this positional localization is accompanied by a {\em continuous} isotropic-to-nematic (IN) transition occuring at the same crosslink density. As the polymer stiffness decreases, the IN transition becomes first order, shifts to a higher crosslink density, and is preceeded by an orientational glass (statistically isotropic amorphous solid) where the average polymer orientations freeze in random directions.Comment: 5 pages, 2 figures; final version with expanded discussion to appear in PR

Is there a commercial case for tropical timber certification?

The authors estimate the potential commercial benefits that tropical timber producing countries could enjoy by adopting timber certification schemes. Such benefits are crucial for encouraging the supply of certified timber. Timber certification is a reality: various countries and organizations have launched initiatives for it. The initial response among producing countries was less than positive, but some have come to realize its potential benefits and have begun to adopt timber certification schemes. Tropical timber trade accounts for only a small fraction of tropical timber production, and most of that trade is concentrated among developing countries in Asia and Japan - markets where demand for certified timber is currently weak. Only a small part of the trade reaches the eco-sensitive markets of Europe and the United States, where there is demand for certified timber. Developing countries can benefit commercially from timber certification in two ways: through the"green premium"(consumers'willingness to pay a premium for certified timber"and by averting losses of market share in the tropical timber market from not having timber certified. Based on surveys, on discussions with nongovernmental organizations, on market participants and analysts, and on estimates of price elasticity, the authors develop a scenario for estimating the potential commercial benefits from adopting timber certification. Under this scenario, benefits would not exceed US$500 million a year (roughly 4 percent of all tropical-timber-related revenues earned by developing countries). Timber certification is not expected to provide significant commercial benefits to developing countries in the near future. But timber certification could provide significant rents to individual firms that develop market niche strategies. And producing countries that pursue certification may enjoy longer-term social, economic, and environmental benefits by adopting the better forest management practices required for timber certification.Environmental Economics&Policies,Silviculture,Agribusiness&Markets,Roads&Highways,Forests and Forestry,Environmental Economics&Policies,Forestry,Silviculture,Agribusiness&Markets,Roads&Highways

Tropical timber trade policies : what impact will eco-labeling have?

About 20 percent of the total production of tropical timber is traded internationally. But for Indonesia, Malaysia, Papua New Guinea, and some countries in West-Central Africa, tropical timber trade accounts for more than 50 percent of production. Although the tropical timber trade has often been blamed for deforestation, the authors find that it contributes much less to deforestation than do poor policies for the production of tropical timber. Lack of tenure rights, short and uncertain logging concessions, low stumpage values, and inadequate monitoring of logging activities are among the major policy failures that help deplete the tropical forests. Trade policies, often identified as an instrument for enforcing environmental objectives internationally, are inefficient instruments for correcting domestic distortions, and in the case of tropical timber trade, may affect the environment perversely. Export and import restrictions ultimately depress the value of an already underpriced resource - the forest. Restrictions on log exports, for example, encourage wasteful processing of logs. Unless sound forest management policies are enforced domestically, the net effect could even be an increase in the rate of deforestation. Import restrictions may have a marginal impact, since trade accounts for less than 20 percent of production and most of the tropical timber is imported in Asia, where such restrictions currently do not exist. Even if import restrictions had a significant impact, it would be in a reduction in value of tropical logs that would make alternative uses of the forest lands more profitable - so the rate of deforestation might not be reduced. Eco-labeling's main strength is its capacity to discriminate (through market signals) in favor of timber produced under sound environmental practices. By contrast, bans and boycotts have an indiscriminate, perverse impact. But if eco-labeling is imposed unilaterally by a subset of countries, its effectiveness will be doubtful. It will lead to trade diversion and potentially perverse environmental results, not to mention an increase in GATT trade disputes. Even if eco-labeling is adopted by all importing countries, there could still be trade diversion in tropical timber products because some consumers may not prefer certified timber, given its higher price. Eco-labeling programs should be designed so that producers see them not as a nontariff barrier but as an instrument for capturing the rents associated with prevailing environmental concerns in the developed world. Consumer education is important to the success of such programs, and eco-labeling programs should be designed accordingly.Environmental Economics&Policies,Forestry,Silviculture,Forests and Forestry,Economic Theory&Research

Linear response of a grafted semiflexible polymer to a uniform force field

We use the worm-like chain model to analytically calculate the linear response of a grafted semiflexible polymer to a uniform force field. The result is a function of the bending stiffness, the temperature, the total contour length, and the orientation of the field with respect to that of the grafted end. We also study the linear response of a worm-like chain with a periodic alternating sequence of positive and negative charges. This can be considered as a model for a polyampholyte with intrinsic bending siffness and negligible intramolecular interactions. We show how the finite intrinsic persistence length affects the linear response to the external field.Comment: 6 pages, 3 figure

Stretching semiflexible filaments with quenched disorder

We study the effect of quenched randomness in the arc-length dependent spontaneous curvature of a wormlike chain under tension. In the weakly bending approximation in two dimensions, we obtain analytic results for the force-elongation curve and the width of transverse fluctuations. We compare quenched and annealed disorder and conclude that the former cannot always be reduced to a simple change in the stiffness of the pure system. We also discuss the effect of a random transverse force on the stretching response of a wormlike chain without spontaneous curvature.Comment: 5 pages, minor changes, added references, final version as published in PR

Debt dynamics in Europe: a network general equilibrium GVAR approach

In this work, we investigate the dynamic interdependencies among the EU12 economies using a competitive general equilibrium network system representation. Additionally, using Bayesian techniques, we estimate the autoregressive scheme that characterizes the equilibrium price system of the network, while characterizing each economy/node in the universe of our network in terms of its degree of pervasiveness. In this context, we unveil the dominant(s) unit(s) in our model and estimate the dynamic linkages between the economies/nodes. Lastly, in terms of robustness analysis, we compare the findings of the degree pervasiveness of each economy against other popular quantitative methods in the literature. According to our findings, the economy of Germany acts as weakly dominant entity in the EU12 economy. Meanwhile, all shocks die out in the short run, without any long lasting effect

System estimation of GVAR with two dominants and network theory:evidence for BRICs

The dynamics of traditional economic structures changed dramatically in the US and globally after 2006. In this context, the need for modeling complex macroeconomic interactions, has led us to develop an upgraded compact global (macro) econometric GVAR model, which is capable of incorporating both the complex interdependencies that exist between the various economic entities and the fact that in the global economy more than one of these entities could have a predominant role, without neglecting the channels of trade and finance. Additionally, based on the trade weight matrix that lies in the core of the GVAR framework, we provide both an analytical procedure and an ex-post econometric criterion for the selection of dominant entities. We demonstrate the dynamics of our model by focusing on the impact of a potential slowdown in the BRICs on the US and EU17 economies. According to our findings, the dominant economies are those of the USA and EU17, while the results suggest that EU17 is more vulnerable than the USA to shocks from the BRICs, implying that a potential slowdown in the BRICs will primarily affect the EU17 economy. Clearly, the proposed model can be easily used for analyzing a number of transmission mechanisms, contagion effects and network interdependencies in various settings

Two-Component 3D Atomic Bose-Einstein Condensates Support Complex Stable Patterns

We report the computational discovery of complex, topologically charged, and spectrally stable states in three-dimensional multi-component nonlinear wave systems of nonlinear Schrödinger type. While our computations relate to two-component atomic Bose-Einstein condensates in parabolic traps, our methods can be broadly applied to high-dimensional, nonlinear systems of partial differential equations. The combination of the so-called deflation technique with a careful selection of initial guesses enables the computation of an unprecedented breadth of patterns, including ones combining vortex lines, rings, stars, and “vortex labyrinths”. Despite their complexity, they may be dynamically robust and amenable to experimental observation, as confirmed by Bogolyubov-de Gennes spectral analysis and numerical evolution simulations

Elastic intramedullary nailing and DBM-Bone marrow injection for the treatment of simple bone cysts

<p>Abstract</p> <p>Background</p> <p>Simple or unicameral bone cysts are common benign fluid-filled lesions usually located at the long bones of children before skeletal maturity.</p> <p>Methods</p> <p>We performed demineralized bone matrix and iliac crest bone marrow injection combined with elastic intramedullary nailing for the treatment of simple bone cysts in long bones of 9 children with a mean age of 12.6 years (range, 4 to 15 years).</p> <p>Results</p> <p>Two of the 9 patients presented with a pathological fracture. Three patients had been referred after the failure of previous treatments. Four patients had large lesions with impending pathological fractures that interfered with daily living activities. We employed a ratio to ascertain the severity of the lesion. The extent of the lesion on the longitudinal axis was divided with the normal expected diameter of the long bone at the site of the lesion. The mean follow-up was 77 months (range, 5 to 8 years). All patients were pain free and had full range of motion of the adjacent joints at 6 weeks postoperatively. Review radiographs showed that all 7 cysts had consolidated completely (Neer stage I) and 2 cysts had consolidated partially (Neer stage II). Until the latest examination there was no evidence of fracture or re-fracture.</p> <p>Conclusion</p> <p>Elastic intramedullary nailing has the twofold benefits of continuous cyst decompression, and early immediate stability to the involved bone segment, which permits early mobilization and return to the normal activities of the pre-teen patients.</p

Compensatory mechanisms in anterior cruciate ligament deficiency

The literature cites numerous studies involving the analysis of movement patterns in anterior cruciate ligament deficient (ACLD) patients. Although several in vivo biomechanical studies have shown that ACLD patients develop protective mechanisms against degenerative diseases, it seems that these adaptations fail to protect the knee from future pathology. Some authors state that ACLD patients adapt to the injury by avoiding quadriceps contraction during gait when the knee is near full extension. However, others have found increased hamstrings and decreased gastrocnemius activity, which normally contribute to the stability of the knee. It seems that further in vivo biomechanical investigation is required to understand the mechanisms of pathological knee joint motions and develop rehabilitation programs, which would delay the progress of developing long-term degenerative diseases