Children's understanding of the commutative law of addition

This study analyzes the bases that support the comprehension ami development of the commutative law of addition. We presented two commutative tasks (compare sums and find the unknown addend) to 72 children from 5 to 8 years of age. Within each task we varied the presence/absence of the result, and the kind-of-addend. In general, our results show that children focus mainly on the addends in judging the equivalence between two commuted pairs. Equally, the errors and strategies analysis offers evidence of the children's inclination to focus on the addends, although older children also center on the results, especially in sorne conditions. Finally, we tentatively suggest a five level model of understanding of commutativity

Tartessian stelae: funerary slabs, ethnic markers or representations of warrior deities?

Este trabajo pretende revisar un aspecto arqueológico singular y controvertido del Bronce Final del SW peninsular: las estelas decoradas o estelas de guerreros. Se analizan distintas hipótesis vertidas sobre estos monumentos, ofreciendo nuevas interpretaciones sobre su contexto arqueológico e iconografía, proponiendo una lectura en clave mítica y religiosa de las imágenes antropomorfas y los símbolos que las acompañan.This work claim to revise one archaeological and singular aspect of the Final Bronze in the South West of the Iberian Peninsula: the decorated steles or steles of warriors. We analyse different hypotheses about these monuments, offering new interpretations about its archaeological contexts and also about its iconography, making one lecture in mythical and religious key about the human images and its symbols

Problemas verbales de comparación y comprensión de la relación comparativa

La dificultad de los niños para resolver problemas verbales de comparación ha suscitado un gran número de trabajos, que se aglutinan en torno a dos líneas principales de investigación: la búsqueda de formulaciones alternativas y el entrenamiento en las habilidades de representación. Sin embargo, no se han realizado estudios en los que se analice la capacidad cognitiva requerida para comprender las relaciones numéricas de comparación implicadas en este tipo de problemas verbales. Con este objetivo, examinamos el ccomportamiento de niños de 1.º, 2.º y 3.º en cuatro tareas (i.e., CMA, CMC, PVCM y PVCA) a sujetos de l.º, 2.º y 3.º de EP. Los resultados obtenidos a partir de las ejecuciones correctas de los niños en las distintas tareas revelan que no todas comportan la misma dificultad, siendo posible detectar la existencia de una secuencia evolutiva de adquisición de las mismas. En concreto, los niños se muestran capaces de resolver estas tareas en el siguiente orden: CM A -> CM C -> PVCA -> PVCM, excepto cuando se analiza solamente el comportamiento de l.º y 2.º de EP, que invierten el orden de las dos últimas tareas, apareciendo entonces el orden previsto. El examen de las estrategias y errores cometidos por los niños permiten suponer la existencia de diferentes factores que explicarían el comportamiento infantil en las distintas tareas, como la familiaridad, los vínculos de algunas tareas con habilidades más tempranas, la asimilación de los PVCM a categorías semánticas más sencillas, la interferencia con nuevos aprendizajes y la formulación de enunciados consistentes con la operación que permite solucionar el problema

Phosphate sorption and desorption by two contrasting volcanic soils of equatorial Africa

Volcanic soils cover 1% of the Earth's surface but support 10% of the world's population. They are among the most fertile soils in the world, due to their excellent physical properties and richness in available nutrients. The major limiting factor for plant growth in volcanic soils is phosphate fixation, which is mainly attributable to active species of aluminium and iron. The sorption and desorption of phosphate is studied on the surface horizons of two African agricultural soils, a silandic Andosol (Rwanda) and a vitric Andosol (São Tomé and Principe). Both soils are slightly acid. The silandic Andosol is rich in active aluminium forms, while the vitric Andosol has high amounts of crystalline iron and aluminium oxides. Sorption isotherms were determined by equilibrating at 293K soil samples with phosphate solutions of concentrations between 0 and 100 mg P L-1 in NaNO3; phosphate was determined by visible spectrophotometry in the equilibrium solution. To study desorption, the soil samples from the sorption experiment were equilibrated with 0.02 M NaNO3. The isotherms were adjusted to mathematical models. In almost all the concentration range, the adsorption of phosphate by the silandic Andosol was greater than 90% of the amount added, being lower in the vitric Andosol but always higher than 65%. The high sorption by the silandic Andosol is attributed to its richness in non-crystalline Fe and Al, while in the vitric Andosol crystalline iron species seem to play a relevant role in the adsorption. The sorption isotherms of both soils fitted to the Temkin model, the adjustment to the Langmuir or Freundlich models being unsatisfactory; throughout the range studied, the sorption increases with increasing phosphorus concentration, a maximum sorption is not predictable (as occurs when the sorption is adjusted to the Langmuir model). For an added P concentration of 100 mg L-1 (3.2 mmol L-1), the sorption is 47.7 mmol P g-1 in the silandic Andosol and 41.6 mmol P g-1 in the vitric Andosol. The desorption is low and the comparison of the sorption and desorption isotherms reveals a pronounced hysteresis, that is, the irreversibility of the sorption. The high phosphate sorption and its irreversibility are comparable to those published for other volcanic soils with high contents of allophane, active aluminium and free iron. The strong phosphate adsorption is a serious limiting factor for plant growth, which requires a careful management of phosphorus fertilization.S

Sorption and Desorption of Vanadate, Arsenate and Chromate by Two Volcanic Soils of Equatorial Africa

Sorption of oxyanions by soils and mineral surfaces is of interest due to their role as nutrients or pollutants. Volcanic soils are variable charge soils, rich in active forms of aluminum and iron, and capable of sorbing anions. Sorption and desorption of vanadate, arsenate, and chromate by two African andosols was studied in laboratory experiments. Sorption isotherms were determined by equilibrating at 293 K soil samples with oxyanion solutions of concentrations between 0 and 100 mg L−1 V, As, or Cr, equivalent to 0−2.0 mmol V L−1, 0−1.3 mmol As L−1, and 0−1.9 mmol Cr L−1, in NaNO3; V, As, or Cr were determined by ICP-mass spectrometry in the equilibrium solution. After sorption, the soil samples were equilibrated with 0.02 M NaNO3 to study desorption. The isotherms were adjusted to mathematical models. After desorption with NaNO3, desorption experiments were carried out with a 1 mM phosphate. The sorption of vanadate and arsenate was greater than 90% of the amount added, while the chromate sorption was much lower (19–97%). The sorption by the Silandic Andosol is attributed to non-crystalline Fe and Al, while in the Vitric Andosol, crystalline iron species play a relevant role. The V and Cr sorption isotherms fitted to the Freundlich model, while the As sorption isotherms conformed to the Temkin model. For the highest concentrations of oxyanions in the equilibrating solution, the sorbed concentrations were 37–38 mmol V kg−1, 25 mmol As kg−1, and 7.2–8.8 mmol Cr kg−1. The desorption was low for V and As and high for Cr. The comparison of the sorption and desorption isotherms reveals a pronounced hysteresis for V in both andosols and for Cr in the Silandic Andosol. Phosphate induced almost no V desorption, moderate As desorption, and considerable Cr desorptionS

Estudio comparado de la diversidad florística de los prados de siega y diente en uso y abandonados en el Valle de Fornela, León = Comparative study of floristic diversity in both abandoned and currently in use meadows for mowing and grazing in Fornela Valley, León

Los pastos naturales españoles guardan una estrecha relación con la gestión pastoral y esta a su vez depende de la situación económica, social y cultural de las comunidades antrópicas (San Miguel Ayanz, 2001). El pastoreo ha sido tradicionalmente y es una herramienta fundamental para la conservación de pastos, con los beneficios económicos, sociales y ambientales que ello conlleva. El estudio que se pretende realizar se desarrolla en diferentes zonas del Valle de Fornela (León), el cual se caracteriza por albergar paisajes de diferente índole, alternando zonas de matorral degradadas por el fuego, matas de bosques bien conservados, zonas de prados segados y pastoreados en el fondo de valle que se mezclan con prados abandonados en cuanto a gestión. Es una zona de suelos pizarrosos, con un clima propio de la montaña leonesa con inviernos prolongados y nevadas frecuentes, y con poca sequía estival. El aprovechamiento ganadero en la zona ha sido tradicionalmente ganadería de vacuno en extensivo y lo sigue siendo a día de hoy, aunque en mucha menor medida en las últimas décadas. El estudio se centra en el análisis y comparación de la diversidad florística en pastos que se han gestionado tradicionalmente mediante el pastoreo a diente del ganado y la siega anual, estableciendo una parcela de siega y otra de pastizal de diente tradicional , y en pastos en los que la gestión ha ido a menos hasta llegar al abandono total de estos, diferenciando en este grupo pastos abandonados en su totalidad y pastos que han sufrido abandono pero se han vuelto a pastar a diente recientemente. Para ello se ha realizado inventarios en verano, identificando las especies presentes en cada ecosistema, y sus correspondientes coberturas, con el fin de calcular los parámetros que determinan la diversidad, la calidad y el estado de los ecosistemas establecidos. Con los resultados se han sacado conclusiones de la influencia y la ausencia que tiene el ganado vacuno y la gestión que se deriva de su presencia, en relación a la diversidad florística de los pastos de la zona objeto de estudio

A hydrated ion model of [UO2]2+ in water: Structure, dynamics, and spectroscopy from classical molecular dynamics

A new ab initio interaction potential based on the hydrated ion concept has been developed to obtain the structure, energetics, and dynamics of the hydration of uranyl in aqueous solution. It is the first force field that explicitly parameterizes the interaction of the uranyl hydrate with bulk water molecules to accurately define the second-shell behavior. The [UO2(H2O)5 ] 2+ presents a first hydration shell U–O average distance of 2.46 Å and a second hydration shell peak at 4.61 Å corresponding to 22 molecules using a coordination number definition based on a multisite solute cavity. The second shell solvent molecules have longer mean residence times than those corresponding to the divalent monatomic cations. The axial regions are relatively de-populated, lacking direct hydrogen bonding to apical oxygens. Angle-solved radial distribution functions as well as the spatial distribution functions show a strong anisotropy in the ion hydration. The [UO2(H2O)5 ] 2+ solvent structure may be regarded as a combination of a conventional second hydration shell in the equatorial and bridge regions, and a clathrate-like low density region in the axial region. Translational diffusion coefficient, hydration enthalpy, power spectra of the main vibrational modes, and the EXAFS spectrum simulated from molecular dynamics trajectories agree fairly well with the experiment.Junta de Andalucía de España, Proyecto de Excelencia-P11-FQM 760

A general study of actinyl hydration by molecular dynamics simulations using ab initio force fields

A set of new ab initio force fields for aqueous [AnO2] 2+/+ (An = Np(VI,V), Pu(VI), Am(VI)) has been developed using the Hydrated Ion (HI) model methodology previously used for [UO2] 2+. Except for the non-electrostatic contribution of the HI-bulk water interaction, the interaction potentials are individually parameterized. Translational diffusion coefficients, hydration enthalpies, and vibrational normal mode frequencies were calculated from the MD simulations. Physico-chemical properties satisfactorily agree with experiments validating the robustness of the force field strategy. The solvation dynamics and structure for all hexavalent actinoids are extremely similar and resemble our previous analysis of the uranyl cation. This supports the idea of using the uranyl cation as a reference for the study of other minor actinyls. The comparison between the NpO2+ 2 and NpO+ 2 hydration only provides significant differences in first and second shell distances and second-shell mean residence times. We propose a single general view of the [AnO2] 2+/+ hydration structure: aqueous actinyls are amphiphilic anisotropic solutes which are equatorially conventional spherically symmetric cations capped at the poles by clathrate-like water structures.Junta de Andalucía de España, Plan Andaluz de Investigación-FQM-28

Computer vision for Pedestrian detection using Histograms of Oriented Gradients

Proyecto de visión por computador para la detección de peatones, entendidas como personas en posición vertical, en imágenes estáticas. La detección se basa en el descriptor propuesto por Dalal & Triggs; Histograma de gradientes orientados (HOG) más un clasificador SVM