Showing papers by "Simón Bolívar University published in 1996"

The Intrinsic Bayes Factor for Model Selection and Prediction

Abstract: In the Bayesian approach to model selection or hypothesis testing with models or hypotheses of differing dimensions, it is typically not possible to utilize standard noninformative (or default) prior distributions. This has led Bayesians to use conventional proper prior distributions or crude approximations to Bayes factors. In this article we introduce a new criterion called the intrinsic Bayes factor, which is fully automatic in the sense of requiring only standard noninformative priors for its computation and yet seems to correspond to very reasonable actual Bayes factors. The criterion can be used for nested or nonnested models and for multiple model comparison and prediction. From another perspective, the development suggests a general definition of a “reference prior” for model comparison.

Adsorption of Water on Activated Carbons: A Molecular Simulation Study

Abstract: We report a molecular simulation study for a model of water adsorption on nonporous and porous activated carbons. The grand canonical Monte Carlo method is used, and the temperature is fixed at 300...

Coordination of directional overcurrent relay timing using linear programming

Abstract: A successive linear programming methodology is presented to treat more effectively those applications where a local structure change is performed to a power system already in operation, and where the modification of the settings of already existent relays is not desirable. The dimension of the optimization problems to be solved is substantially reduced, and a sequence of small linear programming problems is stated and solved in terms of the time dial settings, until a feasible solution is reached. With the proposed technique, the number of relays of the original system to be reset is reduced substantially. It is found that there is a trade-off between the number of relays to be reset and the optimality of the settings of the relays.

Arguing for decisions: a qualitative model of decision making

Abstract: We develop a qualitative model of decision making with two aims: to describe how people make simple decisions and to enable computer programs to do the same. Current approaches based on Planning or Decision Theory either ignore uncertainty and tradeoffs, or provide languages and algorithms that are too complex for this task. The proposed model provides a language based on rules, a semantics based on high probabilities and lexicographical preferences, and a transparent decision procedure where reasons for and against decisions interact. The model is no substitute for Decision Theory, yet for decisions that people find easy to explain it may provide an appealing alternative.

Arbuscular mycorrhizal infection in Cyperaceae and Gramineae from natural, disturbed and restored savannas in La Gran Sabana, Venezuela.

Abstract: The mycorrhizae of a tropical savanna growing in oligotrophic and stony soils were compared with those of a disturbed area that had been reclaimed with introduced species and of an area that was disturbed but not revegetated. All were compared with natural regeneration in a savanna that had been disturbed 12 years previously. Arbuscular mycorrhizae (AM) were common in savannas. Cyperaceae species, which were codominant with Graminaea, showed high levels of infection frequency (45%) like the Gramineae (61%). Arbuscules observed in the Cyperaceae indicated functionality. There were few plants in disturbed, nonrevegetated sites, but those present had AM. Observations of roots from soil monoliths showed that AM were present in disturbed areas, but compared with natural, succesional and revegetated savanna had a lower infection frequency (48–59% vs 75%), lower intensity (10–15% vs 25%) and a lower percentage of arbuscules (0.7–0.8% vs 2.3%). The percentage of vesicles was also lower in succesional savanna than in natural savanna (1.6% vs 4.8%). The revegetated site had the highest percentage of vesicles (6.6%). Although a high frequency of mycorrhizal infection has been reestablished in disturbed areas, the intensity and structure of the infection suggests that mycorrhizal function has not been restored to the original levels. We hypothesize that neither plants nor fungi have adapted to the new edaphic conditions.

Sex ratio at birth deviations in modern Venezuela: The Trivers‐Willard effect

Abstract: This study evaluates the impact of the Trivers‐Willard (T‐W) effect on human populations, using demographic data collected from vital registration data in Venezuela. The evaluation of the sex ratio at birth (SRB) and of fetal and infant deaths supports the existence of T‐W effect in the Venezuelan population in extreme conditions. This T‐W effect was observable in the SRB but not at later ages and is related to the marital status of the mother. The results indicate that the investment in females associated with environmental adversity is greater than the investment in males associated with good environmental conditions.

Parasitic series resistance-independent method for device-model parameter extraction

Abstract: A new method is presented that permits the extraction of a semiconductor device's intrinsic model parameters from its experimental extrinsic forward I-V characteristics, independently of the parasitic resistance that might be present in series within the real device. The extraction is performed from an auxiliary function which contains the integral of the experimentally measured data. Integrating the data also serves as a smoothing procedure. The diode quality factor, reverse current and series resistance parameters of a single exponential diode model are extracted from a real p-n junction diode in order to illustrate the method.

Spectrally-resolved white-light interferometry as a profilometry tool

Abstract: Phase-shifting interferometry and white-light interferometry are reliable techniques for surface analysis in which the optical path difference has to be changed by some transducer to evaluate the phase. We present here a different procedure in which optical path modulation is completely avoided. This technique is based on the spectral analysis of white-light interferograms. By means of a spectroscopic device, a non-visible interferogram is split into its monochromatic components and absolute, unambiguous values of the phase are obtained along the spectral axis. Only one interferogram is required to obtain the profile of one-dimensional surfaces with nanometric resolution.

Effects of Story Reading on Language

Abstract: This article reports the results of an experimental investigation of the effects of a systematic, story-reading-aloud program on some language variables in 222 preschool and first-grade children. They were randomly assigned to 3 groups: 2 experimental and 1 control. The program was carried out by the parents, at home, for the children in the Home-Based Experimental group and by the teacher, at school, for the School-Based Experimental children. I assessed the effects of the programs both on language comprehension (the understanding of syntactic structures and story comprehension) and on expressive language (syntactic structures and sentence repetition). Results indicated that both preschool and first-grade children significantly increased their language comprehension and expression when listening to stories read aloud, either at home or at school. The program is discussed in terms of its usefulness in language stimulation and as an instrument for prevention of language difficulties.

Influence of Aging and Crystallinity on the Molecular Motions in Bisphenol-A Polycarbonate

Abstract: Thermally Stimulated Depolarization Current technique, Differential Scanning Calorimetry, and Dynamic Mechanical Analysis have been applied to amorphous and semicrystalline bisphenol-A polycarbonate with crystallinity degrees up to 21.8%, in a temperature interval covering the α and β relaxations. The secondary β transition is found to be the sum of three components whose variations in aged and annealed specimens have shown the cooperative character of the β1 and β2 modes, contrary to the localized nature of the β3 component. A Tg decrease was observed by both TSDC and DSC as a function of Xc and has been related to the possible confinement of the mobile amorphous phase in regions whose sizes are smaller than the correlation lengths of the cooperative movements that characterize the motions occurring at Tg. The α relaxation intensity variations with crystallinity show the existence of an abundant rigid amorphous phase in the semicrystalline material. The relaxation parameters deduced from the Direct Signal Analysis of the α relaxation for the mobile amorphous phase do not show significant deviations from those found for the amorphous material. The existence of the rigid amorphous phase has been associated to the ductile-to-brittle transition experienced by the material at low crystallinity levels. © 1996 John Wiley & Sons, Inc.

Dynamics of energy and nutrient concentration and construction cost in a native and two alien C4 grasses from two neotropical savannas.

Abstract: In Venezuela, the alien grasses Melinis minutiflora Beauv. and Hyparrhenia rufa (Nees.) Stapf tend to displace the native savanna plant community dominated by Trachypogon plumosus (Humb. and Bonpl.) Nees. This occurs in either relatively wetter and fertile highland savannas or in drier and less fertile lowland savannas. Although the native and aliens are perennial C4 grasses, higher net assimilation leaf biomass per plant and germination rate of the latter are some causes for their higher growth rates and for their competitive success. The objective of this study is to compare seasonal tissue energy, N, P and K concentrations and the calculated construction costs (CC) between the native grass and either one of the alien grasses from lowland and highland savannas. We predict that, in order to out-compete native plants, alien grasses should be more efficient in resource use as evidenced by lower tissue energy and nutrient concentrations and CC.

The 60 kDa heat shock protein in human semen: relationship with antibodies to spermatozoa and Chlamydia trachomatis.

Abstract: The presence of the 60 kDa heat shock protein (hsp60) in seminal fluid and its relationship to sperm autoimmunity or a localized immune response to Chlamydia trachomatis were examined. Semen from 64 male partners of infertile couples with no history of a chlamydial infection were investigated. Hsp60 was identified by an enzyme-linked immunosorbent assay (ELISA) using a monoclonal anti-hsp60 antibody bound to wells of a microtitre plate and a polyclonal anti-hsp60 antibody for detection. Antisperm antibodies on motile spermatozoa were detected by immunobead binding, while antichlamydial immuno-globulin (Ig) A and IgG in seminal fluid were identified by a commercial ELISA (SeroELISA: Savyon Diagnostics, Beer-Sheva, Israel). RNA was purified from isolated seminal round mononuclear cells and tested for hsp60-specific mRNA by a reverse transcription polymerase chain reaction ELISA. Hsp60 was present in semen from nine (14.1%) men, 12 (18.8%) men had antisperm autoantibodies. 16 (25.0%) were positive for antichlamydial IgA and 17 (26.6%) had detectable hsp60-specific mRNA. The presence of hsp60 in semen correlated with the occurrence of antichlamydial IgA (P = 0.0005), hsp60 mRNA (P = 0.04) and antisperm antibodies (P = 0.05). Thus, hsp60 was present in a soluble form in semen primarily in men with evidence of immune system activation within their genital tract. The role of hsp60 in promoting or inhibiting immune responses within the genital tract remains to be determined.

On The Justification of Default and Intrinsic Bayes Factors

Abstract: In Bayesian model selection or hypothesis testing, it is difficult to develop default Bayes factors, since (improper) noninformative priors cannot typically be used. In developing such default Bayes factors, we feel that it is important to keep several principles in mind. The first is that the default Bayes factor should correspond, in some sense, to an actual Bayes factor with a (sensible) prior, which we call an intrinsic prior. The second principle is that such priors should be properly calibrated across models, in the sense of being “predictively matched.” These notions will be described and illustrated, primarily using examples involving the intrinsic Bayes factor, a recently proposed default Bayes factor. It will be seen that intrinsic Bayes factors seem to correspond to actual Bayes factors with proper priors, at least for nested model scenarios. The corresponding intrinsic priors are specifically given for the normal linear model.

Silver electrocrystallization from a nonpolluting aqueous leaching solution containing ammonia and chloride

Abstract: Silver electrocrystallization from aqueous solutions at pH 11, pC10 and pNH3 - 0.2, where Ag(NH3) + is the dominant Ag(i) species, has been studied. In spite of the complexities of this medium, the experimental results can be satisfactorily described in terms of multiple nucleation and diffusioncontrolled growth of hemispherical nuclei. Nucleation rates, A, and number densities of active sites on the electrode surface, No, were determined from potentiostatic current transients as a function of overpotential. Saturation number densities of silver nuclei on the electrode surface obtained from the A and No values were found to be in excellent agreement with those obtained from the direct, microscopic observation of the electrode surface. Spatial distributions of nuclei were also analysed for silver electrodeposited at different potentials. It was found that nuclei were uniformly distributed when electrodeposited at low overpotentiats, whereas inhibition of nucleation close to already established nuclei occurred at higher overpotentials. From the change of the true nucleation rate with overpotential, it was found that the critical nucleus is formed by a single atom within the - 100 to -300 mV overpotential range.

Digestive physiology of wild capybara

Abstract: The capybara (Hydrochaeris hydrochaeris) is a hindgut fermenter whose digestive efficiency is comparable to that of ruminants on similar diets. It is an interesting case for study because it is the largest caecum fermenter and uses coprophagy as part of its digestive strategy. It practices coprophagy in the early morning and forages and defaecates in the evening. Its anatomy is well known but the limited information available about its digestive physiology has been obtained from captive animals. In this work we studied the capybara's digestive physiology, using microbial and chemical information from samples taken from wild capybaras in the early wet season in the morning (0600–0700 hours), noon (1200–1300 hours) and evening (1800–1900 hours), key points in the digestive cycle. Bacteria (cellulolytic and non-cellulolytic) and protozoa were present in high concentrations in the caecum and colon. There were no significant differences in nitrogen concentrations between digestive tract compartments in the coprophagy period (0600 hours), but in the other two periods nitrogen concentrations were significantly higher in the caecum than in the stomach and colon. This is suggestive of selective retention of microbial cells with fluid digesta in the caecum and of cecotrophy (the production of two distinctly different kinds of faeces — one kind called cecotrophes formed from caecal contents and ingested). The capybara hindgut (caecum and colon) with its content, was heavier during the dry season (period of poor diet quality) than in the wet season, but there were no significant seasonal differences between the stomach or small intestine and their contents. This suggests changes in the capacity of the hindgut, the site of microbial fermentation, related to seasonal variation in resource quality.

The glass transition in linear low density polyethylene determined by thermally stimulated depolarization currents

Abstract: New thermally stimulated depolarization currents (TSDC) results on LLD polyethylene functionalized with diethylmaleate polar groups are precisely computer fitted with the direct signal analysis technique. It is shown that the TSDC spectrum consists, with increasing temperatures, of a sub-γ peak, a sharp γ peak, and a β and an α relaxation. The first peak is analyzed in terms of Arrhenius relaxation times, whereas the γ and β transitions could only be fitted by using Vogel-Fulcher temperature dependence for the relaxation times. The best value for To obtained from both fittings is 69.7 K. This is a quantitative proof for the identification of the γ transition as one of the dielectric manifestations of the glass-rubber transition for polyethylenes, Tg = 136.5 K, which has been discussed extensively in the literature. The β relaxation, Tgβ = 237 K, has also the expected characteristic of a glass transition; the existence of two Tgs in polyethylene could explain our results. © 1996 John Wiley & Sons, Inc.

On the Construction of Strong Complementarity Slackness Solutions for DEA Linear Programming Problems Using a Primal-Dual Interior-Point Method

Abstract: A novel approach for solving the DEA linear programming problems using a primaldual interior-point method is presented. The solution found by this method satisfies the Strong Complementarity Slackness Condition (SCSC) and maximizes the product of the positive components among all SCSC solutions. The first property is critical in the use of DEA and the second one contributes significantly to the reliability of the solution.

Vocal fold pathology assessment using AM autocorrelation analysis of the Teager energy operator

Abstract: Traditional speech processing methods for laryngeal pathology assessment assume linear speech production, with measures derived from an estimated glottal flow waveform. They normally require the speaker to achieve complete glottal closure, which for many vocal fold pathologies cannot be accomplished. To address this, a nonlinear signal processing approach is proposed which employs a differential Teager energy operator and the energy separation algorithm to obtain formant AM and FM modulations from bandpass-filtered speech recordings. A new speech measure is proposed, based on parameterization of the autocorrelation envelope of the AM response. Using a cubic model of the autocorrelation envelope, a 3D space is formed to assess changes in speech quality. This approach is shown to achieve exemplary detection performance for a set of muscular tension dysphonias. Unlike flow characterization using numerical solutions of Navier-Stokes equations, this method is extremely computationally attractive, requiring only NlogN+8N multiplications and N square roots for N samples, and is therefore suitable for real-time applications due to its computational simplicity. The new non-invasive method shows conclusively that a fast, effective digital speech processing technique can be developed for vocal fold pathology assessment, without the need for (i) direct glottal flow, estimation or (ii) complete glottal closure by the speaker. The proposed method also confirms that alternative nonlinear methods can begin to address the limitations of previous linear approaches for speech pathology assessment.

Effect of intermolecular cross links on drag reduction by polymer solutions

Abstract: In this work we explore the effect of a cross-linking agent on the drag reduction capability of hydroxypropyl guar and guar gum solutions in turbulent flow through horizontal pipes. The results show that the addition of a cross-linking agent (borax) to solutions with concentrations below those required for gel formation enhances drag reduction due to the increased dimensions of the macromolecules in the presence of intermolecular cross links. The flow-induced degradation of the polymers is not appreciably affected by the addition of cross-linking agent.

Immune Regulation in the Male Genital Tract

Abstract: Spermatozoa are not produced until puberty, long after the establishment of tolerance to selfantigens. Therefore, sperm-specific antigens are immunogenic in men. Most men, however, do not produce antibodies to their own gametes. Development of mechanisms to prevent or limit autoimmune responses to spermatozoa were essential for preservation of reproductive capacity. Tight junctions between adjacent Sertoli cells, as part of the blood-testis barrier, prevent sperm-immune cell contact. In some portions of the genital tract this barrier is thin or incomplete. Immune mechanisms have evolved to actively suppress the autoimmune response to spermatozoa within the genital tract. Unlike in the circulation where CD4 + helperT lymphocytes predominate, CD8+ suppressor/ cytotoxic T lymphocytes are the most prominant T cells in the epididymis and vas deferens. In addition, spermatozoa suppress pro-inflammatorylymphocyteimmune responses, possiblyby inducing production of anti-inflammatory cytokines. Antisperm antibody production is induced in the male genital tract when a local infection or disruption in the genital tract physical barrier leads to an influx ofCD4 + T cells. In response to induction ofa productive immune response, two additional mechanisms downregulate humoral immunity within the genital tract. T lymphocytes possessing the/ form of the antigen receptor (/ T cells) are concentrated in the male genital tract and in semen. These cells become activated and proliferate in men with evidence of sperm autoimmunity. Activated/ T cells inhibit production of antibodies by activated B lymphocytes, thereby limiting antisperm antibody production. Heat shock proteins (hsps) are also present in semen in association with infection and antisperm antibody formation. Hsp gene transcription leads to inhibition of transcription of the genes coding for pro-inflammatory cytokines and, conversely, to activation of / T cells. Activated/ T cells also promote hsp synthesis. The mechanisms to inhibit immunity to sperm may hinder effective immune elimination of microoganisms in the male genital tract. (C) 1996 Wiley-Liss, Inc.

Stabilized multistep methods for index 2 Euler-Lagrange DAEs

Abstract: We consider multistep discretizations, stabilized by β-blocking, for Euler-Lagrange DAEs of index 2. Thus we may use “nonstiff” multistep methods with an appropriate stabilizing difference correction applied to the Lagrangian multiplier term. We show that orderp =k + 1 can be achieved for the differential variables with orderp =k for the Lagrangian multiplier fork-step difference corrected BDF methods as well as for low orderk-step Adams-Moulton methods. This approach is related to the recently proposed “half-explicit” Runge-Kutta methods.