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Showing papers by "University of Los Andes published in 1991"


Journal ArticleDOI
TL;DR: In this article, the problem of inducing convergent quasi-sliding regimes on smooth state-space surfaces of non-linear single-input single-output discrete-time controlled systems is addressed in full generality.
Abstract: The problem of inducing convergent quasi-sliding regimes on smooth state-space surfaces of non-linear single-input single output discrete-time controlled systems is addressed in full generality A suitable extension of the notion of relative degree is used in establishing the most salient features of quasi-sliding motions in non-linearly controlled systems Several examples are given

198 citations


Journal ArticleDOI
TL;DR: Sous pression hydrostatique, le GaAs-I cubique subit des transitions de phase vers au moins deux structures orthorhombiques est etudiee par des mesures de transmittance optique, diffusion Raman et absorption des rayons X.
Abstract: Under hydrostatic pressure, cubic GaAs-I undergoes phase transitions to at least two orthorhombic structures. The initial phase transition to GaAs-II has been investigated by optical-transmittance measurements, Raman scattering, and x-ray absorption. The structure of pressurized samples, which are retrieved at ambient, has been studied by x-ray diffraction and high-resolution diffraction microscopy. Various criteria that define the domain of stability of GaAs-I are examined, such as the occurrence of crystalline defects, the local variation in atomic coordination number, or the actual change in crystal structure. These are shown not to occur at the same pressure at 300 K, the latter being observable only several GPa above the actual thermodynamic instability pressure of GaAs-I. Comparison of the evolution of these parameters on increasing and decreasing pressure locates the thermodynamic transition region GaAs-I\ensuremath{\rightarrow}GaAs-II at 12\ifmmode\pm\else\textpm\fi{}1.5 GPa and at 300 K that is lower than generally reported. The use of thermodynamic relations around the triple point, and of regularities in the properties of isoelectronic and isostructural III-V compounds, yields a phase diagram for GaAs which is consistent with this value.

135 citations


Journal ArticleDOI
TL;DR: In this paper, a simple yet physically reasonable model is presented which describes an arbitrary number of interacting particles in a quantum dot and exact analytic expressions are obtained for the energy spectrum as a function of particle number and magnetic field.
Abstract: A simple, yet physically reasonable, model is presented which describes an arbitrary number of interacting particles in a quantum dot. Exact analytic expressions are obtained for the energy spectrum as a function of particle number and magnetic field

112 citations


Journal ArticleDOI
TL;DR: The T(y) phase diagram of the alloys CuInyGa1−ySe2, prepared by the chemical vapor deposition method, is obtained from X-ray diffraction and differential thermal analyses as mentioned in this paper.
Abstract: The T(y) phase diagram of the alloys CuInyGa1−ySe2, prepared by the chemical vapor deposition method, is obtained from X-ray diffraction and differential thermal analyses. It is found that in addition to the chalcopyrite structure, a zincblende In2Se3-rich phase and a zincblende Ga2Se3-rich plus liquid two-phase field, are obtained. Also it is found that the variation of the lattice parameter ratio c/a is not linear with composition but varies from 1.96 to 2.00 as y is increased from 0.0 to 0.6 and c/a being equal to 2.00 in the composition range 0.6 ⪅ y ⪅ 1.0. The energy gap values, obtained from optical absorption measurements, follow a second-order equation in y giving a downward bowing parameter of about 0.15 eV. Mittels Rontgenbeugung und differentieller Thermoanalyse wird das T(y)-Phasendiagramm von mit der CVD-Methode erhaltenen CuInyGa1−ySe2-Legierungen erhalten. Es wird zusatzlich zur Chalkopyritstruktur eine In2Se3-reiche Zinkblendephase und ein Zweiphasenfeld Ga2Se3-reiche Zinkblende plus Flussigkeit gefunden. Es wird ebenfalls gefunden, das die anderung des Gitterparameter-verhaltnisses c/a nicht linear in der Zusammensetzung ist, sondern von 1,96 bis 2,00 variiert, wenn y von 0,0 auf 0,6 ansteigt und c/a, gleich 2,00 im Zusammensetzungsbereich von 0,6 ⪅ y ⪅ 1,0 bleibt. Die Energieluckenwerte aus optischen Absorptionsmessungen folgen einer Gleichung zweiter Ordnung in y und ergeben einen nach unten gerichteten Bowingparameter von etwa 0,15 eV.

107 citations


Journal ArticleDOI
TL;DR: It is suggested that a combination of freezing tolerance and avoidance by insulation is least expensive and is a more secure mechanism for high tropical mountain plants than supercooling alone.
Abstract: Freezing tolerance and avoidance were studied in several different sized species of the tropical high Andes (4200 m) to determine whether there was a relationship between plant height and cold resistance mechanisms. Freezing injury and supercooling capacity were determined in ground level plants (i.e. cushions, small rosettes and a perennial herb), intermediate height plants (shrubs and perennial herbs) and arborescent forms (i.e. giant rosettes and small trees). All ground-level plants showed tolerance as the main mechanism of resistance to cold temperatures. Arborescent forms showed avoidance mechanisms mainly through supercooling, while intermediate plants exhibited both. Insulation mechanisms to avoid low temperatures were present in the two extreme life-forms. We suggest that a combination of freezing tolerance and avoidance by insulation is least expensive and is a more secure mechanism for high tropical mountain plants than supercooling alone.

89 citations


Journal ArticleDOI
TL;DR: Recent research is reviewed on the ecological, biogeographical, taxonomic, morphological and physiological traits that have led to the adaptive radiation of Espeletia in this extreme habitat: the cold tropics.
Abstract: The genus Espeletia (Asteraceae) underwent an accelerated adaptive radiation in the new habitats of the high tropical Andes after the retreat of the glaciers. From the ancestral rainforest species, with tree-like forms, the genus diversified at high altitude, developing morphological and physiological adaptations to the peculiar combination of low-temperature, energy and nutrient stresses of the tropical periglacial environments. Espeletia offers an exceptional example of a taxon undergoing a rapid evolutionary process through the colonization of a totally original environment: the cold tropics. Here we review recent research on the ecological, biogeographical, taxonomic, morphological and physiological traits that have led to the adaptive radiation of Espeletia in this extreme habitat.

86 citations


Journal ArticleDOI
TL;DR: In this article, the authors consider the Volterra-Lotka equations for n -competing species (n ⩾ 2) in which the right-hand sides are periodic in time.

73 citations


Journal ArticleDOI
01 Oct 1991
TL;DR: In this article, the authors present an onepistemological foundation for interpretive systemology with the principle of noncontradiction as the source of the merging together the most fundamental ontological and epistemological principles ruling Western thought.
Abstract: Systems thinking is presented as the antithesis of “reductionism.” This article—the first in a trilogy which intends to present an ontoepistemological foundation for interpretive systemology—is concerned with understanding the ontoepistemological roots of reductionism. The immediate purpose of such an understanding is to provide an interpretive contrasting context (a counter-ontoepistemology) against which an ontoepistemology for systems thinking can dialectically be drawn (the second and third papers in the trilogy published in this issue ofSystems Practice). The inquiry into the ontoepistemological roots of reductionism leads to the principle of noncontradiction. Such a principle is shown to be the source of the merging together the most fundamental ontological and epistemological principles ruling Western thought. As such, they are shown to have brought about reductionism in modern science. Finally, the “form of essential recursiveness” is put forward as a logical antithetical form with regard to the principle of noncontradiction and which will serve as a logical instrument for developing an ontoepistemology for the systems approach.

72 citations


Journal ArticleDOI
01 Oct 1991
TL;DR: In this article, the notion of truth is interpreted in terms of both the ontoepistemology of "reductionism" and the ontology of interpretive systemology, and the general shape of a systemic interpretive inquiring process is outlined.
Abstract: Both an ontoepistemology for reductionist modern science (counter-ontoepistemology) and an ontology for interpretive Systemology have been outlined in the two preceding papers in this special issue ofSystems Practice. In the present article, the notion of “truth” is interpreted in terms of both the ontoepistemology of “reductionism” and the ontology of interpretive systemology. Both interpretations are discussed. Such a discussion represents the objective of this paper, that is, to outline the epistemological “face” of the ontoepistemology of interpretive systemology. In order to design that “epistemological face,” the relation between ontology and epistemology must be clarified. Such a relation is seen from the standpoint already provided by the ontology. After the discussion on the notion of truth, the general shape of a systemic-interpretive inquiring process is outlined.

68 citations


Journal ArticleDOI
01 Oct 1991
TL;DR: In this paper, the authors present an onepistemology for the systems approach which, in contrast with the former counter-ontoepistemic approach, can explain wholeness.
Abstract: This is the second article in a trilogy. In the preceding paper, an ontoepistemology for reductionism was presented. It thus constituted a contrasting counter-ontoepistemology for the systems approach. The present article is devoted to outlining an ontoepistemology for the systems approach which, in contrast with the former counter-ontoepistemology, can explain wholeness. Such an outline is organized around the description of the essentially dynamic structure of an everyday-living situation, so that the holistic character of what-ever-is-the-case may be put forward. The discussion concerning the structure of an everyday-living situation is performed under a phenomenological approach. Thus, the paper begins by presenting a very brief introduction to phenomenology.

50 citations


Journal ArticleDOI
TL;DR: An expression to predict the nonlinear variation of the fundamental energy gap E g v-x curve for these alloys is proposed and good agreement with experimental data is found.
Abstract: The nonlinear variation of the fundamental energy gap ${\mathit{E}}_{\mathit{g}}$ as a function of composition x observed in ${\mathit{A}}^{\mathrm{I}}$${\mathit{B}}^{\mathrm{III}}$${\mathit{C}}_{2}^{\mathrm{VI}}$ chalcopyrite-structure alloys is discussed. The downward optical-bowing parameter scrc observed in these alloys is analyzed in terms of current models given in the literature. A relationship between scrc and the difference in electronegativities \ensuremath{\Delta}X of the atoms mixed to form the alloy is obtained. Also, an expression to predict the ${\mathit{E}}_{\mathit{g}\mathrm{\ensuremath{-}}}$vs-x curve for these alloys is proposed and good agreement with experimental data is found.

Journal ArticleDOI
01 Oct 1991
TL;DR: In this paper, the conditions under which this trend in systems thinking has come to life are briefly depicted, and a wide international perspective of systems thinking and practice is presented. But the focus is on the Third World and underdeveloped countries.
Abstract: In this introductory paper to the special issue ofSystems Practice devoted to Interpretive Systemology, some of the conditions under which this trend in systems thinking has come to life are briefly depicted. For that purpose a “wider” and a “narrower” scene are presented. The “wider scene” presents the general questions and problems that are to be tackled by Interpretive Systemology, within a wide international perspective of systems thinking and practice. The narrower scene, which is related to more particular conditions, shows how some circumstances connected to the socalled “Third World” or “underdeveloped” countries have helped to trigger the launching of Interpretive Systemology. Finally, a brief outline of the research program for Interpretive Systemology is introduced.

Journal ArticleDOI
TL;DR: From a methylene chloride extract of the roots of Krameria ixina, nine new neolignans/ nor -neolignants besides 24-methylenecycloartanol and eight further lignan-type compounds already known from other Krameriaceae were isolated as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, either steroidal sapogenins or steroidal alkaloids have been isolated from the unripe fruits of Solanum hazenii, S, meridense, S. scorpioideum, S. sycophanta and S. torvum.

Journal ArticleDOI
TL;DR: It is proposed that the cardiac parasympathetic abnormalities arise as a compensating mechanism for the progressive left ventricular dilatation in chagasic cardiac patients.

Journal ArticleDOI
TL;DR: In this paper, the authors used the restricted primitive model (RPM) for electrolytes, which takes into account the ionic size, to calculate several electrokinetic quantities for a 1:1 ionic solution in a charged slit.
Abstract: The equilibrium density profiles for the restricted primitive model (RPM) for electrolytes, which takes into account the ionic size, are used to calculate several electrokinetic quantities for a 1:1 ionic solution in a charged slit. The three‐point‐extension hypernetted‐chain–mean‐spherical equation is used to calculate the RPM density profiles. The results are compared to those obtained through the Poisson–Boltzmann equation for a point‐ion model. Important quantitative and qualitative differences are found for ultrafine slits. These differences, as well as the general behavior of the electrokinetic quantities, are explained in terms of the structure of the electrical double layer inside the slit.

Journal ArticleDOI
TL;DR: The effect of lithium on body weight and feeding was additive to the effects of sulpiride and insulin, indirect evidence that lithium enhances body weight in rats by a different mechanism than the one described for sulpirides or insulin.
Abstract: Two experiments are reported here. First, the effect of lithium chloride (1, 2 and 4 mEq/kg IP for 21 days) on body weight was assessed in female and male rats. Food intake was measured in the rats treated with 2 mEq/kg. All the doses tested significantly increased body weight in female rats. A linear relationship between body weight gain and lithium dose was also observed. In contrast, in male rats, the low doses of lithium (1 and 2 mEq/kg) did not affect body weight, whereas the high dose (4 mEq/kg) decreased body weight. These results confirm previous reports on a sex-dependent effect of lithium on body weight in rats. In the second experiment, body weight and food intake were assessed in female rats treated with lithium alone, or in combination with insulin or sulpiride, a D2 dopamine receptor blocker. It was found that the effect of lithium on body weight and feeding was additive to the effects of sulpiride and insulin. These findings are indirect evidence that lithium enhances body weight in rats by a different mechanism than the one described for sulpiride or insulin.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the evolution of formal management education and development in Latin America and its present nature and its evolution over the past 30 years in a number of Latin American countries.
Abstract: Formal management education and development in Latin America is a relatively recent phenomenon. Its present nature, and its evolution over the past 30 years in a number of Latin American countries, is examined. The issues are set, first, within the broader context of underdevelopment in Latin America. Second, the origins of management education and development are then analysed in terms of their relationship with the education and development of professionals in general. Third, possible links are explored between management education and the transition in Latin America from traditional to modern societies. This is followed, by a discussion of the foundation of a number of important Latin American business schools in the 1960s. Finally, the article speculates about the future of management education and development in the region.

Journal ArticleDOI
TL;DR: In this paper, uncertainty measures are defined in a general manner as morphism between certain posets which permits a unified treatment of different measures, for classical and for fuzzy events, making use of quasilinear entropies.

Journal ArticleDOI
TL;DR: A new species of water rat of the genus Neusticomys is described, based on two specimens collected on the southern slope of the Venezuelan Andean Cordillera in the Rio Potosi, 14 km SE Pregonero (Tachira State).
Abstract: A new species of water rat of the genus Neusticomys is described, based on two specimens collected on the southern slope of the Venezuelan Andean Cordillera in the Rio Potosi, 14 km SE Pregonero (Tachira State). The diagnostic characters of this new taxon include small size, short molar series, a reduction of the pollex, and poorly developed masseteric tubercles. By comparison with other ichthyomyines, the new species appears less specialized for aquatic life as indicated by relatively narrower hindfeet (with shorter specialized hairs and webs), moderately developed vibrissae, a reduced gnathic process of the premaxillae, and a more compressed infraorbital foramen.

Journal Article
TL;DR: The higher prevalence of complex ventricular arrhythmias in patients with chronic Chagas' disease may explain the prognostic differences between the groups.
Abstract: Purpose To determine the prognostic significance of several parameters of left ventricular mechanics and of complex ventricular arrhythmias. Patients and methods 184 chronic chagasic and 85 primary dilated cardiomyopathy patients were submitted to non-invasive and invasive studies, including Holter monitoring and "M" mode echocardiogram and followed for a maximum of 8 years. Results Finding of a depressed left ventricle (LV) systolic function (% AC) was the most important independent prognostic factor in both groups of patients. The presence of complex ventricular arrhythmias had an additional, but very limited, independent prognostic importance, observed only in chagasic patients with depressed LV systolic function. When patients with similar degrees of left ventricular systolic dysfunction, inadequate hypertrophy or increased afterload were compared, chagasic patients had significantly worse prognosis than those with primary dilated cardiomyopathy. This is partly attributed to higher prevalence of complex ventricular arrhythmias. Conclusion The higher prevalence of complex ventricular arrhythmias in patients with chronic Chagas' disease may explain the prognostic differences between the groups.


Journal ArticleDOI
TL;DR: The polyesterification reaction of maleic anhydride and 1,4-butylene glycol was carried out, using p-toluensulphonic acid as catalyst The resulting material was characterized by IR and H1-NMR spectroscopy, end group analysis, and gel permeation chromatography as mentioned in this paper.
Abstract: The polyesterification reaction of maleic anhydride and 1,4-butylene glycol was carried out, using p-toluensulphonic acid as catalyst The resulting material was characterized by IR and H1-NMR spectroscopy, end group analysis, and gel permeation chromatography It is shown that there is a relationship between maleate to fumarate isomerization and steric interaction among condensed groups on the one hand and the tendency of the reacting system to crosslink or gel

Journal ArticleDOI
TL;DR: The maximal rise in T c diminished if the procedure was repeated daily for 43 days and increased abruptly again when the T c measurements were taken, in one session, in the presence of a rat.

Journal ArticleDOI
01 Oct 1991
TL;DR: In this paper, the Regional Planning Corporation of Los Andes is studied as a system-whole (as opposed to a set of interrelated parts and functions), and different interpretive contexts are built, and an attempt is made to interpret with them the actuality of the Corporation.
Abstract: This interpretive systemic study is part of a major research project whose primary goal is to unfold the holistic meaning of Regional Planning Corporations in Venezuelan society. Using the methodological guidelines of Interpretive Systemology, the Regional Planning Corporation of Los Andes is studied as a system-whole (as opposed to a set of interrelated parts and functions). Accordingly, different interpretive contexts are built, and an attempt is made to interpret with them the actuality of the Corporation. However, the attempt is not successful because its actual organizational activities cannot be fully accounted for in these contexts. Hence the need to search for more powerful interpretations arises. In so doing, the holistic character of the organization is displayed in its multiple and contrasting interpretations.

Book ChapterDOI
01 Jan 1991
TL;DR: In this article, the authors used a stochastic model to predict the droplet size distribution of emulsions generated in a mixer, based on the two extreme diameters (dmin, dmax) of the states in which the droplets are classified during the formation of emulsion.
Abstract: We use a stochastic model to predict the droplet size distribution of emulsions generated in a mixer. The emulsions are made of two immiscible liquids, and do not contain surfactants. The intensity of mixing is measured by the speed of rotation of the propeller stirrer. - The model depends on the two extreme diameters (dmin, dmax) of the states in which the droplets are classified during the formation of the emulsion. It predicts a lognormal stationary distribution, the two independent parameters of which can be analytically related to dmin and dmax, by assuming that during the emulsification the breaking/coalescence probabilities of the droplets are time independent and linear functions of the droplet diameter. It can also be related to the speed of rotation of the stirrer by means of the isotropic turbulence theory. — The predicted volume median diameters and standard deviations of the lognormal stationary distributions are in agreement with experiments especially designed to test the model.

Journal ArticleDOI
01 Oct 1991
TL;DR: In this paper, an interpretive-systemic study of the University of Los Andes in Venezuela was presented, where three interpretive contextual systems were designed in order to "comprehend" the sense of the university.
Abstract: This article is a summary of an interpretive-systemic study of the University of Los Andes in Venezuela. Following the methodological guidelines of interpretive systemology, three interpretive contextual systems were designed in order to “comprehend” the sense of the university. These three interpretive contexts were derived from possible different interpretations of the University Law (Statutes). Results obtained from the “thematic interpretation” of university activities and decision taking reveal an institution removed from any of the missions depicted in the interpretive contexts. A fourth interpretive context was designed to provide an interpretation of the regulative role the institution plays in order to maintain a particular social order and power structure. This fourth interpretive context shows greater interpretive power with regard to actual university activities than the other three.

Journal ArticleDOI
TL;DR: These findings are unexpected considering that Sphiggurus and Coendou resemble morphologically more to each other than either of them to Erethizon, and are most parsimoniously explained by retaining the hypothesis of a closer cladistic relationship between Sphigguru and Coendschildi and by proposing the 2n‐74 and FN=82 karyotype to be autapomorphic.
Abstract: Sphiggurus vestitus pruinosus, a porcupine from western Venezuela, has a karyotype with a diploid number (2n) of 42, and a fundamental number (FN) of 76. The autosomes consist of 34 metacentric and submetacentric, 2 subacrocentric, and 4 acrocentric chromosomes. The X‐chromosome is submetacentric, and the Y‐chromosome is acrocentric. This karyotype is identical in gross morphology to that of the North American porcupine, Erethizon dorsatum, but differs markedly from the 2n=74 and FN=82 karyotype of the Panamanian porcupine, Coendou rothschildi. These findings are unexpected considering that Sphiggurus and Coendou resemble morphologically more to each other than either of them to Erethizon, and are most parsimoniously explained by retaining the hypothesis of a closer cladistic relationship between Sphiggurus and Coendou and by proposing the 2n‐74 and FN=82 karyotype to be autapomorphic. The karyological information is complemented with observations on cranial and external characters. The results support re...

Book ChapterDOI
01 Jan 1991
TL;DR: It follows that the principal function of Move-α in the grammar is not reordering or otherwise rewriting of an input phrase marker for the derivation of S-structure, but rather the above noted establishment of coindexing relations between the nodes in the output representation.
Abstract: In Government-Binding (GB) theory, the chief empirical effect of the principle Move-α is the definition of coindexing relations between nodes in phrase structure trees. This situation is explained by the extreme generality of the base component and several substantive constraints on transformations. The constraints on transformations, in particular Emonds’ (1976) Structure Preserving Hypothesis, imply that the base may generate all S-structures of the core constructions of the language in question. The two conditions combine with a shift away from a grammatical model with both conditions on derivations, such as transformational rule ordering, and conditions on representations, such as the Subjacency condition on adjacent chain elements, to a grammatical model that favors representations.1 It follows that the principal function of Move-α in the grammar is not reordering or otherwise rewriting of an input phrase marker for the derivation of S-structure, but rather the above noted establishment of coindexing relations between the nodes in the output representation. The relations in question are trace-antecedent relations. This trend in the role of Move-α in the grammar has been noted in the literature, especially by Koster (1978), Chomsky (1982), and Barss (1983).

Journal ArticleDOI
TL;DR: This study consists of the synthesis, separation, and stereochemical analysis of oximino ethers, followed by a preliminary pharmacological evaluation for neuromuscular blockade, based on 1HNMR and 13CNMR spectroscopy.