Roads, lands, markets, and deforestation : a spatial model of land use in Belize

Rural roads promote economic development but also facilitate deforestation. To explore the tradeoffs between development and environmental damage posed by road building, the authors develop and estimate a spatially explicit model of land use. This model takes into account location and land characteristics and predicts land use at each point on the landscape. They find that: (a) market access and distance to roads strongly affect the probability of agricultural use, especially for commercial agriculture; (b) high slopes, poor drainage, and low soil fertility discourage both commercial and semi subsistence agriculture; and (c) semi-subsistence agriculture is especially sensitive to soil acidity and lack nitrogen (confirming anthropological findings that subsistence farmers are shrewd judges of soil). Spatially explicit models are analytically powerful because they exploit rich spatial variation in causal variables, including the precise siting of roads. They are useful for policy because they can pinpoint threats to particular critical habitats and watersheds. This model is a descendant of the venerable von Thunen model. It assumes that land will tend to be devoted to its highest-value use, taking into account tenure and other constraints. The value of a plot for a particular use depends on the land's physical productivity for that use and the farmgate prices of relevant inputs and outputs. A reduced-form, multinomial logit specification of this model calculates implicit values of land in alternative uses as a function of land location and characteristics. The resulting equations can then be used for prediction or analysis. The model was applied to cross-sectional data for 1989-92 for Belize, a forested country currently experiencing rapid expansion of both subsistence and commercial agriculture. A geographic information system was used to manage the spatial data and extract variables based on the three kilometer sample grid. Three land uses were distinguished:"natural"vegetation, comprising forests, woodlands, wetlands, and savanna; semi-subsistence agriculture, comprising traditional milpa (slash-and-burn) cultivation and other nonmechanized cultivation of annual crops; and commercial agriculture, consisting mainly of sugarcane, pasture, citrus, and mechanized production of corn and kidney beans. Two dimensions of distance to market were distinguished: the distance from each sample point to the road, and on-road travel time to the nearest town. Data on a wide variety of land and soil characteristics were also used.Wetlands,Water Conservation,Environmental Economics&Policies,Climate Change,Land Use and Policies,Forestry,Environmental Economics&Policies,Climate Change,Energy and Environment,Wetlands

An investigation of pre-service teachers\u27 and professional mathematicians\u27 perceptions of mathematical proof at the secondary school level

The National Council of Teachers of Mathematics [NCTM] states that by the time students graduate high school, they should learn to present arguments consisting of logically rigorous deductions of conclusions from hypotheses (NCTM, 2000, p. 56) in written forms that would be acceptable to professional mathematicians (NCTM, 2000, p. 58). Research studies indicate, however, that students and teachers have difficulty with many aspects of mathematical proof, including its nature and meaning. In addition, there appears to be a disconnect between school teachers\u27 and university mathematicians\u27 expectations for their respective students regarding mathematical proof. This study examined the perceptions of thirteen pre-service teachers and eight professional mathematicians with regard to mathematical proof in both the discipline of mathematics and proof in high school mathematics. Participants were asked to complete a questionnaire composed of open-ended questions related to their perceptions of mathematical proof on these two dimensions, for example, their perceptions about the purpose and importance of mathematical proof, their perceptions about what is acceptable and valid as mathematical proof, and their expectations for students. The participants\u27 responses to the questions on the questionnaire served as the primary data source. Analysis of the data occurred in three phases: (1) a coarse reading through all the data to get a feel for the responses; (2) a line-by-line microanalysis and coding of the data; (3) a global analysis whereby responses were collected and chunked into episodes pertaining to a single concept or topic. As the data were being analyzed, hypotheses were formed. The hypotheses were then compared with the data to help bolster the plausibility of the hypotheses or to provide direction or modification of the hypotheses. Results indicate that there are important differences in the perceptions between pre-service teachers and professional mathematicians regarding the nature and meaning of mathematical proof and its place in the high school curriculum. Some of the observed differences include: (1) professional mathematicians value the content of an argument over its form, while pre-service teachers place more importance on the details of the form of an argument, in some cases, to the exclusion of its content; (2) professional mathematicians\u27 view of what constitutes proof is flexible and context dependent, while pre-service teachers\u27 perceptions about what is acceptable is much less context dependent, and in some cases, rigid and unyielding; (3) pre-service teachers expect high school students to know specific derivations of formulas and particular formats for arguments, while professional mathematicians expressed the desire that high school students know about the nature of proof in mathematics

Risk - An Opportunity or Threat for Entrepreneurial Farmers in the Global Food Market?

uncertainty, farm management, opportunities, threats, Agribusiness, Q12,

What is the object of the encapsulation of a process?

Several theories have been proposed to describe the transition from process to object in mathematical thinking. Yet, what is the nature of this ''object'' produced by the ''encapsulation'' of a process? Here, we outline the development of some of the theories (including Piaget, Dienes, Davis, Greeno, Dubinsky, Sfard, Gray, and Tall) and consider the nature of the mental objects (apparently) produced through encapsulation and their role in the wider development of mathematical thinking. Does the same developmental route occur in geometry as in arithmetic and algebra? Is the same development used in axiomatic mathematics? What is the role played by imagery

EPR spectroscopy of iron- and nickel-doped [ZnAl]-layered double hydroxides: modeling active sites in heterogeneous water oxidation catalysts

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to inter-spin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into non-magnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites is solvent-exposed (i.e. at an edge, corner, or defect site). These solvent-exposed sites feature an axial ZFS of 0.21 cm⁻¹ when hydrated. The ZFS increases dramatically upon dehydration (to -1.5 cm⁻¹), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other (“inert”) Fe(III) site maintains an axial ZFS of 0.19-0.20 cm⁻¹ under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (±0.30 cm⁻¹) in hydrated material, whereas two distinct Ni(II) ZFS values (±0.30 and ±1.1 cm⁻¹) were observed in the dehydrated samples. Although the magnetically-dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g. by simple vacuum drying) is an important step towards identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials

Fluid-Induced Propulsion of Rigid Particles in Wormlike Micellar Solutions

In the absence of inertia, a reciprocal swimmer achieves no net motion in a viscous Newtonian fluid. Here, we investigate the ability of a reciprocally actuated particle to translate through a complex fluid that possesses a network using tracking methods and birefringence imaging. A geometrically polar particle, a rod with a bead on one end, is reciprocally rotated using magnetic fields. The particle is immersed in a wormlike micellar (WLM) solution that is known to be susceptible to the formation of shear bands and other localized structures due to shear-induced remodeling of its microstructure. Results show that the nonlinearities present in this WLM solution break time-reversal symmetry under certain conditions, and enable propulsion of an artificial "swimmer." We find three regimes dependent on the Deborah number (De): net motion towards the bead-end of the particle at low De, net motion towards the rod-end of the particle at intermediate De, and no appreciable propulsion at high De. At low De, where the particle time-scale is longer then the fluid relaxation time, we believe that propulsion is caused by an imbalance in the fluid first normal stress differences between the two ends of the particle (bead and rod). At De~1, however, we observe the emergence of a region of network anisotropy near the rod using birefringence imaging. This anisotropy suggests alignment of the micellar network, which is "locked in" due to the shorter time-scale of the particle relative to the fluid

Second cohomology groups and finite covers

For D an infinite set, k>1 and W the set of k-sets from D, there is a natural closed permutation group G_k which is a non-split extension of \mathbb{Z}_2^W by \Sym(D). We classify the closed subgroups of G_k which project onto \Sym(D)$. The question arises in model theory as a problem about finite covers, but here we formulate and solve it in algebraic terms.Comment: Typos corrected; change of title to 'Second cohomology groups and finite covers of infinite symmetric groups' in published versio

Hurricane spawned tornadoes

May 1973.Includes bibliographical references.Partially sponsored by NOAA N22-65-73(G).Partially sponsored by NSF GA-32589X1