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Search Results: 1 - 10 of 224202 matches for " R. Villalba "
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Energy Spectrum of a Relativistic Two-dimensional Hydrogen-like Atom in a Constant Magnetic Field of arbitrary strength
V. M. Villalba,R. Pino
Physics , 2001, DOI: 10.1016/S1386-9477(01)00037-6
Abstract: We compute, via a variational mixed-base method, the energy spectrum of a two dimensional relativistic atom in the presence of a constant magnetic field of arbitrary strength. The results are compared to those obtained in the non-relativistic and spinless case. We find that the relativistic spectrum does not present $s$ states.
Fluctuations of Glaciar Esperanza Norte in the north Patagonian Andes of Argentina during the past 400 yr
L. Ruiz, M. H. Masiokas,R. Villalba
Climate of the Past (CP) & Discussions (CPD) , 2012,
Abstract: The number of studies of Little Ice Age (LIA) glacier fluctuations in southern South America has increased in recent years but is largely biased towards sites in the south Patagonian Andes. In this paper we present a detailed record of length and areal fluctuations of Glaciar Esperanza Norte (GEN) in the north Patagonian Andes of Argentina during the past four centuries. The GEN record was reconstructed through the dendro-geomorphological dating of moraines and the analysis of satellite imagery, aerial photographs and documentary material complemented with extensive field surveys. The maximum LIA extent at GEN was associated with an outer moraine dated to the mid 17th century. At least 19 subsequent readvances or standstills evidenced by morainic ridges were identified inside the most extensive LIA moraine. The dating and spacing of these moraines and the additional information available indicate that the ice front retreated much more rapidly during the 20th century than during earlier centuries. Comparison with the record of LIA fluctuations of Glaciar Frías, an ice mass of similar characteristics located 110 km to the north of GEN, shows a similar pattern of recession over the past 400 yr. Both glacier records have the peak LIA event occurring roughly during the same interval (early-mid 17th century) and show a minor readvance during the 1970s, but there are still a few discrepancies in the dating of some inner moraines. These differences may be due to local, specific factors or associated with the inherent uncertainties in the dating of the moraines. The chronologies of GEN and Frías are among the most detailed currently available in Patagonia, but a larger number of study sites is needed to develop robust, regionally representative glacier chronologies. Detailed glaciological, geomorphological and meteorological data are also needed to understand the glacier-climate relationships in this region and develop reliable paleoclimatic reconstructions.
Fluctuations of Glaciar Esperanza Norte in the North Patagonian Andes of Argentina during the past 400 yr
L. Ruiz,M. H. Masiokas,R. Villalba
Climate of the Past Discussions , 2011, DOI: 10.5194/cpd-7-4073-2011
Abstract: The number of studies of Little Ice Age (LIA) glacier fluctuations in Southern South America has increased in recent years but is largely biased towards sites in the South Patagonian Andes. In this paper we present a detailed record of length and areal fluctuations of Glaciar Esperanza Norte (GEN), in the North Patagonian Andes of Argentina, during the past four centuries. The GEN record was reconstructed through the dendro-geomorphological dating of moraines and the analysis of satellite imagery, aerial photographs and documentary material complemented with extensive field surveys. The maximum LIA extent at GEN was associated with an outer moraine dated to the mid 17th century. At least 19 subsequent readvances or standstills evidenced by morainic ridges were identified inside the most extensive LIA moraine. The dating and spacing of these moraines and the additional information available indicate that the ice front retreated much more rapidly during the 20th century than during earlier centuries. Comparison with the record of LIA fluctuations of Glaciar Frías, an ice mass of similar characteristics located 110 km to the north of GEN, shows a similar pattern of recession over the past 400 yr. Both glacier records have the peak LIA event occurring roughly during the same interval and show a minor readvance during the 1970s, but there are still a few discrepancies in the dating of some inner moraines. These differences may be due to local, specific factors or associated with the inherent uncertainties in the dating of the moraines. The chronologies of GEN and Frías are among the most detailed currently available in Patagonia, but a larger number of study sites is needed to develop robust, regionally representative glacier chronologies. Detailed glaciological, geomorphological and meteorological data are also needed to understand the glacier-climate relationships in this region and develop reliable paleoclimatic reconstructions.
Speeding up Entanglement Degradation
R. B. Mann,V. M Villalba
Physics , 2009, DOI: 10.1103/PhysRevA.80.022305
Abstract: Entanglement between two free bosonic modes can be determined via detection of each mode by different observers and then observing the correlations between their measurements. We show that such entanglement is degraded as a function of time if one observer begins in a state of inertial motion but ends in a state of uniform acceleration while the other remains inertial. At late times we recover previously established results for observers in relative uniform acceleration.
Frequency of viruses associated with acute respiratory infections in children younger than five years of age at a locality of Mexico City
Cabello C,Manjarrez ME,Olvera R,Villalba J
Memórias do Instituto Oswaldo Cruz , 2006,
Abstract: A locality in the district of Tlalpan, Mexico City, was selected in order to identify the viral agents in children younger than 5 years of age with acute respiratory infection (ARI). A total of 300 children were randomly selected and were included in this study for a period of 13 months. During this period nasopharyngeal exudates were collected for the isolation of viral agents. Monoclonal fluorescent antibodies were used for viral identification after cell culture. Viral infection was detected in 65% of the specimens. The respiratory syncytial virus (RSV) was the most common virus agent detected. Children required an average of two consultations during the study period. Two high incidence peaks were observed, one during the summer and the other during winter; the most frequent viruses during these seasons were influenza A and RSV, respectively. The largest number of viruses was isolated in the group of children between 1 and 2 years of age and in the group between 4 and 5 years of age. This study demonstrated the presence of ARI and of different viruses in a period of 13 months, as well as the most frequent viruses in children younger than 5 years of age from a community of Mexico City.
Validating numerical simulations of snow avalanches using dendrochronology: the Cerro Ventana event in Northern Patagonia, Argentina
A. Casteller,M. Christen,R. Villalba,H. Martínez
Natural Hazards and Earth System Sciences (NHESS) & Discussions (NHESSD) , 2008,
Abstract: The damage caused by snow avalanches to property and human lives is underestimated in many regions around the world, especially where this natural hazard remains poorly documented. One such region is the Argentinean Andes, where numerous settlements are threatened almost every winter by large snow avalanches. On 1 September 2002, the largest tragedy in the history of Argentinean mountaineering took place at Cerro Ventana, Northern Patagonia: nine persons were killed and seven others injured by a snow avalanche. In this paper, we combine both numerical modeling and dendrochronological investigations to reconstruct this event. Using information released by local governmental authorities and compiled in the field, the avalanche event was numerically simulated using the avalanche dynamics programs AVAL-1D and RAMMS. Avalanche characteristics, such as extent and date were determined using dendrochronological techniques. Model simulation results were compared with documentary and tree-ring evidences for the 2002 event. Our results show a good agreement between the simulated projection of the avalanche and its reconstructed extent using tree-ring records. Differences between the observed and the simulated avalanche, principally related to the snow height deposition in the run-out zone, are mostly attributed to the low resolution of the digital elevation model used to represent the valley topography. The main contributions of this study are (1) to provide the first calibration of numerical avalanche models for the Patagonian Andes and (2) to highlight the potential of Nothofagus pumilio tree-ring records to reconstruct past snow-avalanche events in time and space. Future research should focus on testing this combined approach in other forested regions of the Andes.
Precipitation changes in the South American Altiplano since 1300 AD reconstructed by tree-rings
M. S. Morales,D. A. Christie,R. Villalba,J. Argollo
Climate of the Past Discussions , 2011, DOI: 10.5194/cpd-7-4297-2011
Abstract: During the second half of the 20th century, the Central Andes has experienced significant climatic and environmental changes characterized by a persistent warming trend, an increase in elevation of the 0 °C isotherm, and a sustained shrinkage of glaciers. These changes have occurred in conjunction with a steady growing demand for water resources. Given the short span of instrumental hidroclimatic records in this region, longer records are needed to understand the nature of climate variability and improve the predictability of precipitation, a key factor modulating the socio-economic development in the South American Altiplano and the adjacent arid lowlands. In this study we present the first quasi-millennial, tree-ring based precipitation reconstruction for the South American Altiplano. This annual (November–October) precipitation reconstruction is based on Polylepis tarapacana tree-ring series and represents the closest dendroclimatological record to the Equator in South America. This high-resolution reconstruction covers the past 707 yr and provides a unique record to characterize the occurrence of extreme events and consistent oscillations in precipitation, as well as to check the spatial and temporal stabilities of the teleconnections between rainfall in the Altiplano and hemispheric forcings such as El Ni o-Southern Oscillation. Since the 1930s up to present a persistent negative trend in precipitation is recorded in the reconstruction, with the three driest years since 1300 AD occurring in the last 70 yr. The reconstruction contains a clear ENSO-like pattern at interannual to multicentennial time scales which determines inter-hemispheric linkages between our reconstruction and other precipitation-sensitive records modulated by ENSO in North America. Our reconstruction points out that century-scale dry periods are a recurrent feature in the Altiplano climate, and that the potential coupling of natural and anthropogenic-induced droughts in the future would have a severe impact on current socio-economical activities in the region. Water resource managers must anticipate these changes to adapt for future climate change, reduce vulnerability and provide water equitably to all users.
Resistencia a herbicidas: Glifosato
Villalba,Andrea;
Ciencia, docencia y tecnolog?-a , 2009,
Abstract: this paper presents a revision of resistance to herbicides mechanisms and factors of those developments in plants. glyphosate resistent plants in argentina and the world are considered. mechanisms of action and development of glyphosate resistance and preventive behaviors are proposed.
The HISPACAT comparative database of syntactic constructions and its applications to syntactic variation research
Xavier Villalba
Oslo Studies in Language , 2011,
Abstract: The HISPACAT database of syntactic constructions in Catalan and Spanish is a dynamic comparative grammar of two closely related languages, which, from a theoretical point of view, offers us a alternative viewpoint to advance in our understanding of the ``atoms'' of linguistic microvariation, and to offer a snapshot of microparametric (in)variance, which will help us to predict less stable parts of the grammar, and hence more sensitive to syntactic change or interference phenomena. Moreover, this tool, which is conceived as a major empirical source for testing syntactic microvariation, may also prove helpful for researchers in bilingualism and language contact studies, and for teachers and students of Catalan or Spanish as L2.
Sobre la dislocació a la dreta
Xavier Villalba
Llengua & Literatura , 1998, DOI: 10.2436/l&l.vi.1401
Abstract:
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