Showing papers by "University of Los Andes published in 1988"
TL;DR: In this article, a differential geometric approach for the design of sliding modes in non-linear variable-structure feedback systems is presented, which uses basic notions from differential geometry involving vector fields, distributions and 1-forms.
Abstract: This article presents a differential geometric approach for the design of sliding modes in non-linear variable-structure feedback systems. Coordinate-free characterizations of local existence conditions for sliding regimes, and a geometric reformulation of some of its most salient features are presented. The approach uses basic notions from differential geometry involving vector fields, distributions and 1-forms. Both single- and multiple-input cases are treated with some illustrative examples.
224 citations
TL;DR: In this article, an axiomatic approach to the notion of weak noncompactness is presented, and several properties of the defined measures are given, as well as concrete realizations of the accepeted system in some Banach spaces.
Abstract: In this paper an axiomatic approach to the notion of a measure of weak noncompactness is presented. Several properties of the defined measures are given. Moreover, we provide a few concrete realizations of the accepeted axiomatic system in some Banach spaces.
120 citations
TL;DR: Sulpiride-induced drinking was observed in the absence of food, showing that it is not a postprandial phenomenon, and results suggest that hypothalamic D2 receptors might be involved in feeding and drinking regulation.
Abstract: Amphetamine injections into the lateral hypothalamus inhibit feeding. This effect is blocked by local administration of neuroleptics, suggesting a role for dopamine in feeding inhibition. However, the type of dopamine receptor involved in satiety is not known. Therefore, we tested the effect of intrahypothalamic injections of sulpiride, a specific D2 receptor blocker, on amphetamine anorexia in food-deprived rats, and on spontaneous feeding and drinking in satiated rats. Sulpiride attenuated by 36% the anorexia produced by intrahypothalamic injections of amphetamine. In satiated rats, sulpiride (8 μg/0.5 μl) elicited feeding (mean food intake after sulpiride: 5.4 g, and after vehicle 1.6 g, p<0.001), and drinking (mean water intake after sulpiride: 12.3 ml, and after vehicle: 0.9 ml, p<0.001). A dose response relationship was found between sulpiride dose and feeding or drinking. Sulpiride-induced drinking was observed in the absence of food, showing that it is not a postprandial phenomenon. These results suggest that hypothalamic D2 receptors might be involved in feeding and drinking regulation.
102 citations
TL;DR: A microwave oven/flow injection/amotic absorption spectrometric system that permits the determination of zinc and cadmium in small samples of human kidney and liver tissue (ca. 1 g) is described in this paper.
45 citations
TL;DR: In this article, general relationships are established among variable structure control strategies and pulsewidth modulated control schemes leading to sliding modes in non-linear systems, and the results are applied to the design of a sliding regime for the regulation of DC-to-DC switchmode power converters exhibiting a slow integral manifold due to time-scale separation properties among input and output circuits.
Abstract: In this article, general relationships are established among variable structure control strategies and pulse-width modulated control schemes leading to sliding modes in non-linear systems. The results are applied to the design of a sliding regime for the regulation of DC-to-DC switchmode power converters exhibiting a slow integral manifold due to time-scale separation properties among input and output circuits.
43 citations
TL;DR: RuCl 2 (TRIPHOS) as discussed by the authors catalyzes the homogeneous hydrogenation and isomerization of olefins, hydrogenation of alkynes, aldehydes, ketones and nitriles, under moderate reaction conditions in different organic solvents.
41 citations
TL;DR: In this paper, the electrical properties between 80 and 300 K and the optical absorption at room temperature of both n- and p-type CuInTe2 were studied, and the authors attributed the origin of these levels to InCu and VTe, respectively.
Abstract: The electrical properties between 80 and 300 K and the optical absorption at room temperature of both n- and p-type CuInTe2 are studied. The n-type sample is obtained by annealing p-type CuInTe2 in the presence of indium for a prolonged period of time. The p-type samples are grown by programmed directional freezing technique with slight excess of indium in the stoichiometry. Two defect levels with ED = 58 meV and EA around 15 meV are identified from the analysis of the temperature dependence of the carrier concentration. The origin of these levels, consistent with the covalent bonding model, is attributed to InCu and VTe, respectively. From a theoretical fit to the experimental data, the density of states effective mass of the electrons is found to be m = 0.16me. The estimated values of the conduction and valence-band deformation potentials are in agreement with the general trend observed in other I–III–VI2 compounds. Using the model proposed for the band-gap shrinkage, the energy gap of n-type CuInTe2 in the dilute limit is calculated to be 1.026 eV.
On a etudie les proprietes electriques entre 80 et 300 K, et l'absorption optique a temperature ambiante de CuInTe2 type n et type p. L'echantillon type n a ete obtenu par recuit et celuit type p par mise en presence de l'Indium durant une periode de temps prolonge. On a fait croǐtre les echantillons type p par la technique de refroidissement directionnel programme avec un leger exces d'Indium dans la stoechiometrie. Par l'analyse de la dependance de la temperature de concentration des porteurs, on a identifie deux niveaux de defauts avec ED = 58 meV et EA environ 15 meV. L'origine de ces niveaux, compatible avec le modele de liaison covalente, est attribue it a InCu et a VTe, respectivement. Apres un ajustement theorique aux donnees experimentales, la densite des etats de la masse effective des electrons est de m★ = 0,16me. Les valeurs estimees des potentiels de deformation des bandes de conduction et de valence sont en accord avec la tendance generale observee dans les composes I–III–VI2. En utilisant le modele propose pour la contraction de la bande du gap, le resultat de gap d'energie de CuInTe2 type n dans la limite diluee est de 1,026 eV.
40 citations
TL;DR: In this article, the results indicate that in both, the zinc blende and chalcopyrite ranges of composition, ordering of the manganese can occur at lower temperatures.
Abstract: Polycrystalline samples of the semimagnetic semiconductor alloys (AgIn)1−zMn2zTe2 and (CuIn)1−zMn2zTe2 are used in lattice parameter, optical energy gap E0, and differential thermal analysis (DTA) measurements. The results indicate that in both, the zinc blende and chalcopyrite ranges of composition, ordering of the manganese can occur at lower temperatures, and a combination of the results is used to delineate the composition and temperature ranges in which disordered α and ordered α′ chalcopyrite and disordered s and ordered β′ zinc blende is the equilibrium structure. It is shown that while the manganese ordering has little effect on the lattice parameter, it has appreciable effect on the values of E0. This is demonstrated by the different aiming points at z = 1.0 for the E0 vs. z lines in the different fields, the values being 2.8 eV for β, 1.95 eV for β′, and 1.35 eV for α′.
Polykristalline Proben der semimagnetischen Halbleiterlegierungen (AgIn)1−zMn2zTe2 und (CuIn)1−zMn2zTe2 werden fur Messungen der Gitterparameter, optischen Bandlucken E0 und differentiellen Thermoanalyse (DTA) benutzt. Die Ergebnisse zeigen, das sowohl im Zinkblende- als auch im Chalkopyrit-Bereich der Zusammensetzung die Ordnung des Mangans bei tieferen Temperaturen auftritt, und eine Kombination der Ergebnisse wird benutzt, die Zusammensetzungs-und Temperaturbereiche zu zeichnen, in denen fehlgeordnetes α- und geordnetes α′-Chalkopyrit und fehlgeordnete β- und geordnete β′-Zinkblende die Gleichgewichtsstrukutren sind. Wahrend der Manganordnungsprozes nur einen geringen Einflus auf den Gitterparameter hat, wird gezeigt, das er einen wesentlichen Einflus auf die Werte von E0 hat. Dies wird durch die verschiedenen Zielpunkte bei z = 1,0 fur die E0–z-Linien in den verschiedenen Feldern demonstriert, wobei die Werte 2,8 eV fur β, 1,95 eV fur β′ und 1,35 eV fur α′ betragen.
38 citations
TL;DR: In this paper, the use of the closed-flow injection system for the direct and repetitive determination of sulphur dioxide (5.0-300 mg l t−1) in young white wines with chemiluminescence detection is described.
36 citations
TL;DR: Draba chionophila, the plant in which freezing tolerance was found, is the vascular plant which reaches the highest altitudes in the Venezuelan Andes (approximately 4700m), and night cycles of air and leaf temperature were studied in the field to determine the temperature at which leaf freezing began.
Abstract: Freezing tolerance as a cold resistance mechanism is described for the first time in a plant growing in the tropical range of the Andean high mountains. Draba chionophila, the plant in which freezing tolerance was found, is the vascular plant which reaches the highest altitudes in the Venezuelan Andes (approximately 4700m). Night cycles of air and leaf temperature were studied in the field to determine the temperature at which leaf freezing began. In the laboratory, thermal analysis and freezing injury determinations were also carried out. From both field and laboratory experiments, it was determined that freezing of the leaf tissue, as well as root and pith tissue, initiated at a temperature of approximately-5.0°C, while freezing injury occurred at approximately-12.0°C for the pith, and below-14.0°C for roots and leaves. This difference in temperature suggests that the plant still survives freezing in the-5.0 to-14.0°C range. Daily cycles of leaf osmotic potential and soluble carbohydrate concentration were also determined in an attempt to explain some of the changes occurring in this species during the nighttime temperature period. A comparison between Andean and African high mountain plants from the point of view of cold resistance mechanisms is made.
36 citations
TL;DR: The ratio between maximal and minimal R-R intervals measured during the Valsalva maneuver is an adequate means of studying the functional state of the cardiac parasympathetic nervous system and results are in discordance with the commonly accepted hypothesis that cardiac parASYmpathetic denervation causes a dilated myocardiopathy in patients with chronic Chagasic myocarditis.
TL;DR: The results indicate that myocardial damage and the degree of left ventricular dilatation are more severe in chagasic patients with an abnormal response of their heart rate to atropine.
TL;DR: In this paper, the diversity and dispersion of sandflies species found in 15 localities between 175 m and 1,960 m.a.s.l., are presented as part of an epidemiological study on leishmaniasis in Merida, Venezuela.
Abstract: As part of an epidemiological study on leishmaniasis in Merida, Venezuela, the diversity and dispersion of sandflies species found in 15 localities between 175 m and 1,960 m.a.s.l., are presented. From 7,126 collected sandflies (5,132 men and 1,994 women), 24 species were identified, 10 of them recognized as anthropophilic. The relation species-altitud is presented, and the species composition found in human dwellings, peridomestic and sylvatic areas, are recorded. The possible role of the identified species on the transmission of leishmaniasis in the andean region, is discussed.
TL;DR: The soil lead content decreased with distance from the edge of the roadside and with depth of sampling, but where the motor vehicle traffic volume was greater than 10,000 motor vehicles per day, levels of lead increased by a factor of up to approximately 18.
TL;DR: The highest value of RNA was obtained at day 18 of chick embryo development coinciding with the maximum period of synaptic formation and consequently of the PSDs, which seems to reinforce the hypothesis of the ribosomal origin of the postsynaptic densities during chick cerebellar maturation.
TL;DR: In males, the same treatment tended to decrease body weight in the peripubertal and adult groups while in the prepUbertal animals an increase of body weight without a significant hyperphagia was observed.
TL;DR: In this paper, it was found that the range of single phase solid solution in CuInSe2 extends to 0.32 and that the transition temperature from chalcopyrite to zinc blende decreases from 810 °C at z = 0 to 510°C at the limit of the single phase behavior.
Abstract: Polycrystalline samples of (CuIn)1−zMn2zSe2 alloys are prepared by the melt and anneal technique and are used in differential thermal analysis, lattice parameter, optical energy gap, and magnetic susceptibility measurements. It is found that the range of single phase solid solution in CuInSe2 extends to z = 0.32 and that the transition temperature from chalcopyrite to zinc blende decreases from 810 °C at z = 0 to 510 °C at the limit of single phase behaviour. Measurements of optical energy gap and magnetic susceptibility as a function of temperature show that in all of the chalcopyrite samples ordering of the manganese on the cation sublattice occurs, while for the zincblende samples, the manganese may be ordered or disordered depending upon the heat treatment used. The magnetic data indicates that the ordering is the same as that observed previously in similar telluride alloys.
Polykristalline (CuIn)1−zMn2zSe2-Legierungsproben werden in der Schmelze und mittels Temperung hergestellt und in der differentiellen Thermoanalyse sowie bei Messungen der optischen Bandlucke, Gitterparameter und der magnetischen Suszeptibilitat benutzt. Es wird gefunden, das sich der Bereich der Einphasen-Festkorperlosung in CuInS2 bis zu z = 0,32 ausdehnt und das die Ubergangstemperatur von Chalkopyrit zu Zinkblende von 810 °C bei z = 0 auf 510 °C an der Grenze des Einphasenverhaltens abnimmt. Meassungen der optischen Bandlucke und der magnetischen Suszeptibilitat also Funktion der Temperatur zeigen, das in allen Chalkopyritproben Ordnungsprozesse von Mangan im Kationensubgitter auftreten, wahrend fur Zinkblendeproben, Mangan geordnet oder fehlgeordnet sein kann, in Abhagigkeit von der benutzten Warmebehandlung. Die magnetischen Werte zeigen, das der Ordnungsvorgang der gleiche ist, der kurzlich in ahnlichen Telluridlegierungen beobachtet wurde.
TL;DR: Recently, Schemske & Lande (1984) found that captive male Euglossa imperialis show elevated levels of territorial behaviour when provided with cineole, a fragrant chemical that readily attracts male bees, compared to control bees maintained without a chemical source.
Abstract: Bees in the tribe Euglossini (Apidae) form an important component of the pollinator fauna in lowland, Neotropical forests (Dressler 1982, Janzen 1971, Williams 1982). The community structure and seasonality of male euglossine bees are relatively well known because they can be attracted to chemical baits (Ackerman 1983a, Dodson et al. 1969, Janzen et al. 1982, Pearson & Dressler 1985). The male bees are important pollinators of many orchids (Dressler 1968, Williams 1982) from which they collect fragrant chemicals; these chemicals are also collected from other plants and decaying vegetation (Ackerman 1983b, Dressler 1982). Male bees are known to store collected chemicals in hind tibial organs (Dressler 1982) and, it is presumed, use these chemicals to synthesize sex pheromones (Williams & Whitten 1983). Mating occurs in territories to which males attract female bees (Dodson 1966, Kimsey 1980). Recently, Schemske & Lande (1984) found that captive male Euglossa imperialis show elevated levels of territorial behaviour when provided with cineole, a fragrant chemical that readily attracts male bees, compared to control bees maintained without a chemical source. Although the exact function of the fragrant chemicals is still unknown, these were the first results that clearly implicated fragrance collection in male reproductive behaviour. Schemske & Lande (1984) noted that the male E. imperialis provided with a daily source of cineole actively collected the fragrance only on the first day of the 11 day experiment. One potential explanation for this observation is that males require only one collection of chemical fragrance before establishing territories. Thus, only the youngest bees in the population would be expected
TL;DR: In this article, it was found that in the equilibrium diagram the range of single-phase solid solution at room temperature is very small with the chalcopyrite phase existing for 0 ≤ z ≤ 0.04 and no zinc blende phase occurring.
TL;DR: In this article, the location of zeros of polynomials y2 + 7~’ and y2 - II´ has been shown to be a determinant of the growth of the problem x = g(x), x(0)=x(1)=0.
TL;DR: Investigation of the relationships between architecture, seasonal aerial biomass growth and interference from neighbours in three savanna grass species differing in time of flowering concluded that neighbour interference is probably reducing both tillering and leafing in the basal species but only culm elongation and leafed in the erect species.
Abstract: In a field experiment we studied the relationships between architecture, seasonal aerial biomass growth and interference from neighbours in three savanna grass species differing in time of flowering: a precocious species (Elyonurus adustus), an early species (Leptocoryphium lanatum) and a late species (Andropogon semiberbis). To detect the neighbour interference upon architecture and seasonal regrowth, we measured the leaf interception of a plane at different heights, when the species grew alone, in pairs and in groups of three. Although the three species differed widely in the spatial and temporal patterns of occupation of above-ground space, important levels of interference among neighbouring plants were detected. The species differ in: a) the pattern of space occupation when growing alone; b) the magnitude of the interference effect by the companion species; c) the change in pattern as a consequence of interference; d) their reaction to fire.
TL;DR: The structure of meridinol, a lignan isolated from Zanthoxylum fagara, was determined by the evidence of its spectroscopic data and X-ray single crystal structure as (1 S, 2R)-1,2-bis (3,4-methylenedioxy benzyl)- 1-hydroxy butyrolactone as mentioned in this paper.
TL;DR: Two groups of patients were examined for anti-Trypanosoma cruzi antibodies by immunofluorescence and ELISA, representing the nucleus of a group in which prospective studies will identify the effect of T. rangeli infection on the course of subsequent South American trypanosomiasis and Chagas' disease.
Abstract: Two groups of patients were examined for anti-Trypanosoma cruzi antibodies by immunofluorescence and ELISA (i) inhabitants of the village and surrounding rural area of Tibu, Norte de Santander, Columbia (n = 327) and (ii) employees of the Empresa Colombiana de Petroleos (ECOPETROL, n = 849). The latter group had a lower rate of positive serology (12 as compared to 29%) but the distributions of antibody titres were very similar in the two groups. A total of 119 serum samples (37 village and 82 ECOPETROL, including 25 seronegative controls) were analysed for their ability to immunoprecipitate the 7 major polypeptides of T. cruzi trypomastigotes of Mr greater than 72 kDa. Although 10 sera from positive patients showed no immunoprecipitation, all of the remaining positive sera contained antibodies which reacted with the 150, 90 and 85 kDa polypeptides. When the T. cruzi immunofluorescence positive, immunoprecipitation negative sera were retested by ELISA using GP90, all were negative thus suggesting that the patients had had a misdiagnosed T. rangeli infection. The new diagnosis was confirmed by immunofluorescence and ELISA with T. rangeli epimastigotes. Longitudinal studies were carried out on 19 patients from the ECOPETROL group for up to 3.5 years. Five seropositive patients showed a change in their anti-trypomastigote immunoprecipitation profiles over this period; one by loss of a previously recognized high molecular weight band and four others by conversion from a negative to a positive immunoprecipitation profile. These latter patients presented initially with uncomplicated T. rangeli infection but then acquired a T. cruzi superinfection. These patients represent the nucleus of a group in which prospective studies will identify the effect of T. rangeli infection on the course of subsequent South American trypanosomiasis and Chagas' disease.
TL;DR: The observability of non-linear analytic systems can be assessed by means of Kalman's rank condition, when the system is represented in terms of the evolution equations of an associated infinite family of tensor powers of the state vector as discussed by the authors.
Abstract: The observability of non-linear analytic systems can be assessed by means of Kalman's rank condition, when the system is represented in terms of the evolution equations of an associated infinite family of tensor powers of the state vector. Carleman's exact linearization provides the necessary link to transform differential-geometric observability conditions into Kalman's algebraic rank condition.
TL;DR: The diversity and dispersion of sandflies species found in 15 localities between 175 m and 1,960 m.a.s.l., and the species composition found in human dwellings, periodomestic and sylvatic areas, are recorded.
Abstract: As part of an epidemiological study on leishmaniasis in Merida, Venezuela, the diversity and dispersion of sandflies species found in 15 localities between 175 m and 1,960 m.a.s.l., are presented. From 7,126 collected sandflies (5,132 female and 1,994 male), 24 species were identified, 10 of them recognized as anthropophilic. The relation species-altitude is presented, and the species composition found in human dwellings, periodomestic and sylvatic areas, are recorded. The possible role of the identified species on the transmission of leishmaniasis in the andean region, is discussed.
TL;DR: The results are discussed in terms of the existence of postsynaptic D2 receptors in the lateral hypothalamus involved in inhibition of locomotion in rats in order to quantitate the locomotor activity induced by sulpiride injected in the perifornical region of theateral hypothalamus.
TL;DR: In this paper, both chalcopyrite α and zinc blende β single-phase fields are found and the results indicate that in both these cases ordering of the manganese can occur at lower temperatures to give ordered α and β′ structures.
TL;DR: In this article, the deformation potentials of the valence and conduction band extrema of the ternary compound CuInSe 2 were estimated to be 7.78 and 9.48 eV per unit dilation, respectively, and it was suggested that electron-electron scattering combined with polar optical modes has an important influence on the mobility in n -type CuSe 2.