Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Bounds for the Kirchhoff index via majorization techniques

Abstract: Using a majorization technique that identifies the maximal and minimal vectors of a variety of subsets of $${\mathbb{R}^{n}}$$ , we find upper and lower bounds for the Kirchhoff index K(G) of an arbitrary simple connected graph G that improve those existing in the literature. Specifically we show that $$K(G) \geq \frac{n}{d_{1}} \left[ \frac{1}{1+\frac{\sigma}{\sqrt{n-1}}} + \frac{(n-2)^{2}}{n-1-\frac{\sigma}{\sqrt{n-1}}}\right] ,$$ where d 1 is the largest degree among all vertices in G, $$\sigma ^{2} = \frac{2}{n} \sum_{(i, j) \in E} \frac{1}{d_{i}d_{j}} = \left( \frac{2}{n}\right) R_{-1}(G),$$ and R −1(G) is the general Randic index of G for $${\alpha =-1}$$ . Also we show that $$K(G) \leq \frac{n}{d_{n}}\left( \frac{n-k-2}{1-\lambda _{2}}+\frac{k}{2}+\frac{1}{\theta}\right) ,$$ where d n is the smallest degree, $${\lambda _{2}}$$ is the second eigenvalue of the transition probability of the random walk on G, $$k = \left \lfloor \frac{\lambda _{2} \left( n-1\right) +1}{\lambda _{2}+1}\right\rfloor {\rm and}\quad\theta = \lambda _{2} \left( n-k-2\right) -k+2.$$

Leaf Photosynthesis of the Mangrove Avicennia Germinans as Affected by NaCl

Abstract: In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ13C analyses were done. Predawn values of Fv/Fm were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday Fv/Fm values decreased to about 0.55-0.60 which indicated strong down-regulation of photosynthesis in all treatments. Maximum photosynthetic rate (P max) was 14.58 ± 0.22 µmol m-2 s-1 at 0 ‰ it decreased by 21 and 37 % in plants at salinities of 10 and 40 ‰, respectively. Stomatal conductance (g s) was profoundly responsive in comparison to P max which resulted in a high water use efficiency. This was further confirmed by δ13C values, which increased with salinity. From day 3, after salt was removed from the soil solution, P max and g s increased up to 13 and 30 %, respectively. However, the values were still considerably lower than those measured in plants grown without salt addition.

Echocardiographic and Clinical Predictors of Mortality in Chronic Chagas' Disease.

Abstract: BACKGROUND: In a prospective epidemiological study of chronic Chagas' disease, several clinical and echocardiographic variables were analyzed as predictors of mortality. METHODS: Among 960 subjects seropositive for Chagas' disease who were examined between June 1981 and June 1992, 283 had echocardiograms. RESULTS: During a mean follow-up period of 48.3 +/- 36.4 months (range, 1-156 months), 108 subjects died. Echocardiographic end-diastolic and -systolic left ventricular internal dimensions, fractional shortening, radius-to-thickness ratio, left ventricular mass, mitral E-point septal separation, and 17 other nonechocardiographic variables were predictors of death on univariate analysis (P /= 0.55 (P = 0.012), presence of electrocardiographic ST segment elevation on precordial leads (P = 0.014), age >/= 56 years (P = 0.028), and presence of right bundle-branch block (P = 0.045). Patients with an apical aneurysm on two-dimensional echocardiography had an increased mortality (Chi-square = 11.5, P < 0.001). CONCLUSIONS: Echocardiography is a valuable tool to assess the risk of death in prospective studies on chronic Chagas' heart disease.

A Quasi-Exclusive European Ancestry in the Senepol Tropical Cattle Breed Highlights the Importance of the slick Locus in Tropical Adaptation

Abstract: Background The Senepol cattle breed (SEN) was created in the early XXth century from a presumed cross between a European (EUT) breed (Red Poll) and a West African taurine (AFT) breed (N’Dama). Well adapted to tropical conditions, it is also believed trypanotolerant according to its putative AFT ancestry. However, such origins needed to be verified to define relevant husbandry practices and the genetic background underlying such adaptation needed to be characterized.