Showing papers by "University of Los Andes published in 2001"
TL;DR: In a recent workshop with 88 Latin American herpetologists and conservationists, a signed resolution was presented to better understand the geographic extent of amphibian population declines, their possible causes, and the measures needed to improve Latin American scientists' ability to research the phenomenon and make effective management recommendations as mentioned in this paper.
Abstract: Although dramatic amphibian population declines have been reported worldwide, our understanding of the extent of the declines in Latin America, where amphibian diversity is high, is limited to a few well-documented studies. To better understand the geographic extent of declines, their possible causes, and the measures needed to improve Latin American scientists' ability to research the phenomenon and make effective management recommendations, we convened three regional workshops with 88 Latin American herpetologists and conservationists. Population declines are widespread in Latin America. At least 13 countries have experienced declines, and in 40 cases species are now thought to be extinct or extirpated in a country where they once occurred. Declines or extinctions have affected 30 genera and nine families of amphibians. Most declines have occurred in remote highlands, above 500 m in elevation in Central America and above 1000 m in the Andes. Most documented declines occurred in the 1980s. Of the possible causes studied to date, climate change appears to be important at one site and chytrid fungal disease has been identified at sites in three countries. Although many monitoring studies are currently underway in a variety of habitats, most studies are recent and of short duration. In a signed resolution, workshop participants called for greater collaboration and communication among scientists working in Latin America to understand the geographic extent of population declines and the distribution of possible causal factors. In situ conservation is important to protect habitats, but captive-rearing programs for species subject to imminent extinction are also needed. Better understanding of the taxonomy and natural history of amphibians and more funding for research and monitoring are critical to developing a scientific basis for management action to arrest and reverse population declines.
388 citations
TL;DR: In this paper, a general noncommutative quantum mechanical system in a central potential (V=V(r)$ in two dimensions) is considered and the spectrum is bounded from below.
Abstract: A general noncommutative quantum mechanical system in a central potential $V=V(r)$ in two dimensions is considered. The spectrum is bounded from below and, for large values of the anticommutative parameter $\ensuremath{\theta},$ we find an explicit expression for the eigenvalues. In fact, any quantum mechanical system with these characteristics is equivalent to a commutative one in such a way that the interaction $V(r)$ is replaced by $V=V({H}_{\mathrm{HO}},{L}_{z}),$ where ${H}_{\mathrm{HO}}$ is the Hamiltonian of the two-dimensional harmonic oscillator and ${L}_{z}$ is the z component of the angular momentum. For other finite values of $\ensuremath{\theta}$ the model can be solved by using perturbation theory.
385 citations
TL;DR: In this paper, the periods and light curves of 148 RR Lyrae variables from V = 13.5 to 19.7 were measured from the first 100 deg2 of the Quasar Equatorial Survey Team RR LyRAe survey, and 55% of these stars belong to the clump of stars detected earlier by the Sloan Digital Sky Survey.
Abstract: We have measured the periods and light curves of 148 RR Lyrae variables from V = 13.5 to 19.7 from the first 100 deg2 of the Quasar Equatorial Survey Team RR Lyrae survey. Approximately 55% of these stars belong to the clump of stars detected earlier by the Sloan Digital Sky Survey. According to our measurements, this feature has ~10 times the background density of halo stars, spans at least 375 by 35 in α and δ (≥30 by ≥3 kpc), lies ~50 kpc from the Sun, and has a depth along the line of sight of ~5 kpc (1 σ). These properties are consistent with the recent models that suggest that it is a tidal stream from the Sagittarius dwarf spheroidal galaxy. The mean period of the type ab variables, 0.58 days, is also consistent. In addition, we have found two smaller overdensities in the halo, one of which may be related to the globular cluster Pal 5.
246 citations
TL;DR: The magnitude and duration of the induction of COX-2 mRNA, protein, and PGE2 release by rhIL-1β is primarily the result of PGE 2-dependent stabilization of COx-2 RNA and stimulation of translation, a process involving a positive feedback loop mediated by the EP4 receptor and the downstream kinases p38 MAPK and, perhaps, cAMP-dependent protein kinase.
196 citations
TL;DR: Indicators of disk accretion such as Halpha emission and near-infrared emission from dusty disks fall sharply from Ori OB1b to Ori OB 1a, indicating that the time scale for disk dissipation and possibly the onset of planet formation is a few million years.
Abstract: We are conducting a large-scale, multiepoch, optical photometric survey [Centro de Investigaciones de Astronomia-Quasar Equatorial Survey Team (CIDA-QUEST)] covering about 120 square degrees to identify the young low-mass stars in the Orion OB1 association. We present results for an area of 34 square degrees. Using photometric variability as our main selection criterion, as well as follow-up spectroscopy, we confirmed 168 previously unidentified pre-main sequence stars that are about 0.6 to 0.9 times the mass of the sun (Mo), with ages of about 1 million to 3 million years (Ori OB1b) and about 3 million to 10 million years (Ori OB1a). The low-mass stars are spatially coincident with the high-mass (at least 3 Mo) members of the associations. Indicators of disk accretion such as Halpha emission and near-infrared emission from dusty disks fall sharply from Ori OB1b to Ori OB1a, indicating that the time scale for disk dissipation and possibly the onset of planet formation is a few million years.
150 citations
TL;DR: In this paper, the Higgs sector of a SO(10) grand unified theory was studied and the see-saw mechanism was used to lower the breaking scale, allowing the d = 6 proton decay operators to be comparable in magnitude to the d=5 ones.
134 citations
04 Dec 2001
TL;DR: A cost function is introduced as the difference between pricing and utility functions, and the existence of a unique Nash equilibrium is established, and two update algorithms are shown to be globally stable under specific conditions.
Abstract: We present a game-theoretic treatment of distributed power control in CDMA wireless systems. We make use of the conceptual framework of noncooperative game theory to obtain a distributed and market-based control mechanism. We address not only the power control problem, but also pricing and allocation of a single resource among several users. A cost function is introduced as the difference between pricing and utility functions, and the existence of a unique Nash equilibrium is established. Furthermore, two update algorithms, namely parallel update and random update, are shown to be globally stable under specific conditions. Convergence properties and robustness of each algorithm are also studied through extensive simulations.
118 citations
TL;DR: It is suggested that manufacturer's leadership reduces inefficiency in a channel and is more beneficial to the consumer.
Abstract: This paper provides an answer to the question who should, if any, lead a marketing channel? We consider a channel consisting of one manufacturer and one retailer where each player controls his advertising rate and margin. Supposing that advertising has a carry over effect on demand, we adopt a dynamic model. Nash and Stackelberg equilibria are characterised and outcomes compared with an efficient coordinated solution. Our findings suggest that manufacturer's leadership reduces inefficiency in a channel and is more beneficial to the consumer.
110 citations
TL;DR: Espectros de Marx puede leerse como un texto que, siguiendo this principio operativo, no se limita a reflexionar en tono a las ideas de Marx, sino que elabora una contextualizacion de dicho pensamiento dentro del ambito politico occidental actual, en el cual predomina un discurso de tipo neoliberal as mentioned in this paper.
Abstract: El pensamiento de Jaques Derrida se ocupa de cuestionar y desplazar todas las oposiciones clasicas. Espectros de Marx puede leerse como un texto que, siguiendo este principio operativo, no se limita a reflexionar en tono a las ideas de Marx, sino que elabora una contextualizacion de dicho pensamiento dentro del ambito politico occidental actual, en el cual predomina un discurso de tipo neoliberal. Se puede afirmar, en principio, que Espectros de Marx se articula a partir de dos ejes argumenta...
106 citations
TL;DR: The demonstration that recessive Z‐linked loci cause hybrid sterility in a female heterogametic species supports the contention that dominance theory provides a general explanation of Haldane's rule.
Abstract: Recent studies, primarily in Drosophila, have greatly advanced our understanding of Haldane's rule, the tendency for hybrid sterility or inviability to affect primarily the heterogametic sex (Haldane 1922). Although dominance theory (Turelli and Orr 1995) has been proposed as a general explanation of Haldane's rule, this remains to be tested in female-heterogametic taxa, such as the Lepidoptera. Here we describe a novel example of Haldane's rule in Heliconius melpomene (Lepidoptera; Nymphalidae). Female F1 offspring are sterile when a male from French Guiana is crossed to a female from Panama, but fertile in the reciprocal cross. Male F1s are fertile in both directions. Similar female F1 sterility occurs in crosses between French Guiana and eastern Colombian populations. Backcrosses and linkage analysis show that sterility results from an interaction between gene(s) on the Z chromosome of the Guiana race with autosomal factors in the Panama genome. Large X (or Z) effects are commonly observed in Drosophila, but to our knowledge have not been previously demonstrated for hybrid sterility in Lepidoptera. Differences in the abundance of male versus female or Z-linked versus autosomal sterility factors cannot be ruled out in our crosses as causes of Haldane's rule. Nonetheless, the demonstration that recessive Z-linked loci cause hybrid sterility in a female heterogametic species supports the contention that dominance theory provides a general explanation of Haldane's rule (Turelli and Orr 2000).
103 citations
TL;DR: In this paper, the frequency of occurrence and long term evolution of subclinical carditis in patients with acute rheumatic fever was determined using Doppler echocardiography.
Abstract: Objective—To determine the frequency of occurrence and long term evolution of subclinical carditis in patients with acute rheumatic fever. Design—Valvar incompetence was detected by clinical examination and Doppler echocardiographic imaging during the acute and quiescent phases of rheumatic fever. Patients were followed prospectively and submitted to repeat examinations at one and five years after the acute attack. Persistence of acute mitral and aortic lesions detected solely by echocardiography (subclinical disease) was compared with that of disease detected by clinical examination as well (thereby fulfilling the latest 1992 Jones criteria for rheumatic carditis). Setting—Three general hospitals with a university aYliation in Chile. Patients—35 consecutive patients fulfilling the revised Jones criteria for rheumatic fever. Clinical and echocardiographic examination was repeated in 32 patients after one year and in 17 after five years. Ten patients had subclinical carditis on admission, six of whom were followed for five years. Main outcome measures—Auscultatory and echocardiographic evidence of mitral or aortic regurgitation during the acute attack or at follow up. Results—Mitral or aortic regurgitation was detected by Doppler echocardiographic imaging in 25/35 rheumatic fever patients as opposed to 5/35 by clinical examination (p = 0.03). Doppler echocardiography revealed acute valvar lesions in 10 of 20 rheumatic fever patients who had no auscultatory evidence of rheumatic carditis (subclinical carditis). Three of these subclinical lesions and three of the clinical or auscultatory lesions detected on admission were still present after five years of follow up, emphasising that subclinical lesions are not necessarily transient. Conclusions—Doppler echocardiographic imaging improves the detection of rheumatic carditis. Subclinical valve lesions, detected only by Doppler imaging, can persist. Echocardiographic findings should be accepted as a major criterion for the diagnosis of rheumatic fever. (Heart 2001;85:407‐410)
TL;DR: In this paper, it was shown that the phonon energy involved in electron-exciton-phonon interaction is not that of the ordered compound longitudinal or transverse optical modes.
Abstract: From the analysis of the Urbach tail in ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuInTe}}_{2},$ it is found that the energy $h{\ensuremath{
u}}_{p}$ involved in the electron--exciton-phonon interaction is not that of the ordered compound longitudinal or transverse optical modes. It is established that this energy depends on the structural and cations disorders. An expression of the form ${E}_{U}(T,P,N)=K\ensuremath{\Theta}/{\ensuremath{\sigma}}_{0}{(1+P)/2+N[\mathrm{exp}(\ensuremath{\Theta}/T)\ensuremath{-}1{]}^{\ensuremath{-}1}},$ where P and N are adjustable structural and order-disorder parameters and $\ensuremath{\Theta}$ the Einstein characteristic temperature, accurately explains the temperature dependence of the Urbach energy. From extrapolation of the linear variation of N and P with $h{\ensuremath{
u}}_{p},$ the phonon energy for a completely ordered and disordered systems can be estimated. We discuss the physical meaning of the parameter N as due to the contribution of localized modes produced by substitutional disorder of low energy of formation. We find an intriguing relation between the temperature of the chalcopyrite-sphalerite transition and the phonons contributing to the formation of Urbach tails in the strong-disorder limit.
TL;DR: It is suggested that intense synaptic activity associated with seizures modifies the composition of postsynaptic densities and has profound consequences on the function of the N-methyl-D-aspartate receptors present in them.
TL;DR: In this paper, Tungsten carbide/diamond like (W-C/DLC) multilayers have been investigated as low friction coatings on high-speed steel substrates.
Abstract: Tungsten carbide/diamond like (W–C/DLC) multilayers have been investigated as low friction coatings on high-speed steel substrates. The coatings are composed of a W–C multilayer base and an upper lubricious DLC layer and they are obtained by reactive r.f. magnetron sputtering from a single target comprising of two equal halves; one-half carbon and the other half tungsten. The whole coating structure was obtained in situ, without any interruption of the sputtering process. Transmission electron microscopy (TEM) and SIMS were used to assess the multilayer structure and XPS, XRD, and Raman spectroscopy was used to analyze its composition. The tribological properties of the coatings in sliding wear were investigated by means of scratch test and ball-on-disc test measurements. It was found that the multilayer W–C base improves the adhesion of the upper DLC layer to steel substrates while maintaining its low friction coefficient.
TL;DR: Brain glial cells secrete several molecules that can modulate the survival of neurons after various types of damage to the CNS, and the events occurring in response to injury may lead to changes in the nature and relative concentration of the various factors involved.
Abstract: Brain glial cells secrete several molecules that can modulate the survival of neurons after various types of damage to the CNS. Activated microglia and astrocytes closely associate to amyloid plaques in Alzheimer Disease (AD). They could have a role in the neurotoxicity observed in AD because of the inflammatory reaction they generate. There is controversy regarding the individual part played by the different glial cells, and the interrelationships between them. Both astrocytes and microglia produce several cytokines involved in the inflammatory reaction. Moreover, the same cytokines may have different effects, depending on their concentration and the type of cells in the vicinity. In turn, the events occurring in response to injury may lead to changes in the nature and relative concentration of the various factors involved. To learn about these putative glial interrelationships, we examined some effects of astrocytes on microglial activation.
TL;DR: In this article, a vision of epistemology is provided which clarifies the relationships among knowledge, power, will and intervention, and a normative framework for systemic intervention is then presented.
Abstract: This paper interrogates four perspectives (structuralist community psychology, deconstruction, interpretive systemology, and critical systems thinking) to inform the unfolding of a theory of systemic intervention. A vision of epistemology is provided which clarifies the relationships among knowledge, power, will and intervention, and a normative framework for systemic intervention is then presented. Finally, the theory unfolded in this paper is deconstructed to reveal a second theory, yet to be explored, of systemic life projects. This provides an exciting agenda for future research.
TL;DR: In this article, an online flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS) system was developed to study the effect of acid concentration on the arsine generation from arsenite (As(III)), arsenate (As (V)), monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) pretreated in different reaction media.
TL;DR: In this article, the authors used micro-climatic measurements at two altitudes, 4300 and 4850 m, during both the dry cold and wet warm seasons to determine low-temperature resistance mechanisms in P. tarapacana.
Abstract: The Polylepis tarapacana forests found in Bolivia are unique with respect to their altitudinal distribution (4200–5200 m). Given the extreme environmental conditions that characterize these altitudes, this species has to rely on distinct mechanisms to survive stressful temperatures. The purpose of this study was to determine low-temperature resistance mechanisms in P. tarapacana. Tissue was sampled for carbohydrate and proline contents and micro-climatic measurements were made at two altitudes, 4300 and 4850 m, during both the dry cold and wet warm seasons. Supercooling capacity (−3 to −6 °C for the cold dry and −7 to −9 °C for the wet warm season) and injury temperatures (−18 to −23 °C for both seasons), determined in the laboratory, indicate that P. tarapacana is a frost-tolerant species. On the other hand, an increase in supercooling capacity, as the result of significant increase in total soluble sugar and proline contents, occurs during the wet warm season as a consequence of higher metabolic activity. Hence, P. tarapacana, a frost-tolerant species during the colder unfavourable season, is able to avoid freezing during the more favourable season when minimum night-time temperatures are not as extreme.
TL;DR: In this article, three phases of the Ag-In-S system were simultaneously refined from a single X-ray powder diffraction pattern using the Rietveld method using a single Rietvld method.
TL;DR: In this article, the formation of nano-emulsions has been studied in water/ mixed nonionic surfactant/oil systems using two emulsification methods, and the droplet size and stability were determined as a function of surfactants mixing ratio, W1, at 25 °C.
Abstract: The formation of nano-emulsions has been studied in water/ mixed nonionic surfactant/oil systems using two emulsification methods. In one method, the composition was kept constant and the temperature was changed (phase-inversion temperature, PIT, method), while in the other method, water was added dropwise to a solution of the mixed surfactants in oil at constant temperature (method B). The droplet size and stability were determined as a function of surfactant mixing ratio, W1, at 25 °C. The droplet size of nano-emulsions obtained by the PIT method is practically independent of W1 and falls in the range 60-80 nm. In contrast, the droplet size of nano-emulsions prepared by method B, is highly dependent on W1 and varies between 60 and 300 nm. At W1 values where the PIT or the hydrophile—lipophile balance temperature (Thlb) of the system is close to 25 °C, the droplet sizes of the nano-emulsions are similar for both emulsification methods. There are three equilibrium phases of the latter compositions: an aqueous micellar solution or oil-in-water microemulsion (W m), a lamellar liquid-crystalline phase and an oil phase (O) in addition, these nano-emulsions showed higher kinetic stability than those with lower W 1 values (higher T hlb) and consisting of two liquid phases (W m + O).
TL;DR: It is concluded that consumption of ptaquiloside‐contaminated milk may contribute to human gastric cancer in the Andean states of Venezuela.
Abstract: To explore a possible connection between specific environmental factors that might explain the high rates of stomach cancer in people living in the highlands of western Venezuela, an epidemiologic study was conducted in 2 regions of contrasting topography. The regions embrace 3 Andean states, Merida, Tachira and Trujillo, and the vicinal lowland surrounding the Maracaibo lake basin of Zulia State. Statistical sanitary records from 1986 to 1996 comprising 5.5 million people in the study area indicated that age-sex-adjusted gastric cancer death rate per 100,000 people (DR) was up to 3.64 times higher in highland than lowland areas, although total cancer-related DRs were comparable in both regions. DRs of other less frequent cancers from the upper alimentary tract [esophagous (1.18/0.99) and mouth-throat (1.39/2.64)] showed comparable values in both regions as well as colorectal, breast, and uterus-cervix cancers, suggesting that the stomach cancer DRs were related to geographically determined factors. Comparison of some nutrition issues, incidence of Helicobacter pylori infection in selected areas, the discovery of the bracken carcinogen ptaquiloside in milk from bracken-fed cows, the prevalence of this plant in mountain cattle households and pasturelands and the rates of bracken-evoked bovine enzootic hematuria led us to conclude that consumption of ptaquiloside-contaminated milk may contribute to human gastric cancer in the Andean states of Venezuela.
TL;DR: A search for effects of large extra spatial dimensions in pp collisions at a center-of-mass energy of 1.8 TeV with the D0 detector is reported, using events containing a pair of electrons or photons and does not exhibit evidence for large extra dimensions.
Abstract: We report a search for effects of large extra spatial dimensions in ppbar collisions at a center-of-mass energy of 1.8 TeV with the DZero detector, using events containing a pair of electrons or photons. The data are in good agreement with the expected background and do not exhibit evidence for large extra dimensions. We set the most restrictive lower limits to date, at the 95% confidence level, on the effective Planck scale between 1.0 TeV and 1.4 TeV for several formalisms and numbers of extra dimensions.
TL;DR: Several dynamic policy adjustment schemes for the online computation of the Nash equilibrium of noncooperative networks are obtained and their local convergence properties are studied.
TL;DR: In this paper, an electroactive film of poly(o-phenylenediamine) (PPD) was obtained by electropolymerization of o-PD on 304 stainless steel under potentiostatic conditions.
Abstract: Adherent electroactive films of poly(o-phenylenediamine) (PPD) were obtained by electropolymerization of o-phenylenediamine (o-PD) on 304 stainless steel under potentiostatic conditions. The in situ Fourier transform infrared reflectance spectroscopy study suggests a phenazine ring ladder-structure polymer. Open circuit potential and potentiodynamic studies of PPD-coated steel electrodes, in chloride aqueous solution show a positive shift of the corrosion potential.
TL;DR: In this paper, the results of analyses of jet data produced in pp collisions at √s=630 and 1800 GeV collected with the DO detector during the 1994-1995 Fermilab Tevatron Collider run are presented.
Abstract: Results are presented from analyses of jet data produced in pp collisions at √s=630 and 1800 GeV collected with the DO detector during the 1994–1995 Fermilab Tevatron Collider run. We discuss the details of detector calibration, and jet selection criteria in measurements of various jet production cross sections at √s=630 and 1800 GeV. The inclusive jet cross sections, the dijet mass spectrum, the dijet angular distributions, and the ratio of inclusive jet cross sections at √s=630 and 1800 GeV are compared to next-to-leading-order QCD predictions. The order αs3 calculations are in good agreement with the data. We also use the data at √s=1800 GeV to rule out models of quark compositeness with a contact interaction scale less than 2.2 TeV at the 95% confidence level.
TL;DR: In this paper, the authors present a search for electroweak production of single top quarks in approximate to 90 pb(-1) of data collected with the D(null set) detector at the Fermilab Tevatron collider.
TL;DR: In this article, a quasi-model-independent strategy was applied to search for new high p(T) physics in approximate to 100 pb(-1) of p (p) over bar collisions at roots = 1.8 TeV collected by the D empty set experiment during 1992-1996 at thr Fermilab Tevatron.
Abstract: We apply a quasi-model-independent strategy ("SLEUTH") to search for new high p(T) physics in approximate to 100 pb(-1) of p (p) over bar collisions at roots = 1.8 TeV collected by the D empty set experiment during 1992-1996 at thr Fermilab Tevatron. Over 32 e muX, W+jets-like, Z+jets-like, and (ll gamma)(ll gamma)(ll gamma )X exclusive final states are systematically analyzed for hints of physics beyond the standard model. Simultaneous sensitivity to a variety of models predicting new phenomena at the electroweak scale is demonstrated by testing the method on a particular signature in each set of final states. No evidence of new high p(T) physics is observed in the course of this search, and we find that 89% of an ensemble of hypothetical similar experimental runs would have produced a final state with a candidate signal more interesting than the most interesting observed in these data.
TL;DR: This review focuses mostly on the applications of CE–LIFD, to measure drugs and endogenous neuroactive substances such as amino acids and monoamines, especially in microdialysis samples from experimental animals and humans.
Abstract: Capillary electrophoresis (CE) is a high-efficiency analytical technique that has had a great impact as a tool in biomedical research, clinical and forensic practice in the last ten years. Only in one of the applications, the DNA analysis, it has had an explosive exponential growth in the last few years. This impact is expressed in an enormous amount of CE articles and many reviews. The CE advantages with respect to other analytical techniques: the required very small sample volume, rapid analysis, great resolution power and low costs, have made this technique ideal for the analysis of a numerous endogenous and exogenous substances present in biological fluids. The different modes of CE have been coupled to different detection techniques such as UV-absorbance, electrochemical, mass spectrometry and laser-induced fluorescence detection (LIFD) to detect different nature and molecular size separated analytes. This review focuses mostly on the applications of CE-LIFD, to measure drugs and endogenous neuroactive substances such as amino acids and monoamines, especially in microdialysis samples from experimental animals and humans. CE-LIFD trends are discussed: automated faster analysis with capillary array systems, resolution power improvement, higher detection sensitivity, and CE systems miniaturization for extremely small sample volume, in order to make CE easier and affordable to the lab bench or the clinical bed.
TL;DR: It is shown that axiomatic extensions of intuitionistic propositional calculus defining univocally new connectives, including those proposed by Gabbay, are strongly complete with respect to valuations in Heyting algebras with additional operations.
Abstract: It is shown that axiomatic extensions of intuitionistic propositional calculus defining univocally new connectives, including those proposed by Gabbay, are strongly complete with respect to valuations in Heyting algebras with additional operations In all cases, the double negation of such a connective is equivalent to a formula of intuitionistic calculus Thus, under the excluded third law it collapses to a classical formula, showing that this condition in Gabbay's definition is redundant Moreover, such connectives can not be interpreted in all Heyting algebras, unless they are already equivalent to a formula of intuitionistic calculus These facts relativize to connectives over intermediate logics In particular, the intermediate logic with values in the chain of length n may be “completed” conservatively by adding a single unary connective, so that the expanded system does not allow further axiomatic extensions by new connectives