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.

A delay differential equation model for tumor growth

Abstract: We present a competition model of tumor growth that includes the immune system response and a cycle-phase-specific drug. The model considers three populations: Immune system, population of tumor cells during interphase and population of tumor during mitosis. Delay differential equations are used to model the system to take into account the phases of the cell cycle. We analyze the stability of the system and prove a theorem based on the argument principle to determine the stability of a fixed point and show that the stability may depend on the delay. We show theoretically and through numerical simulations that periodic solutions may arise through Hopf Bifurcations.

Marine biodiversity in the Atlantic and Pacific coasts of South America: knowledge and gaps.

Abstract: The South American region The marine areas of the South American continent extend for almost 30,000 km of coastline and encompass three different oceanic domains—the Caribbean, the Pacific, and the Atlantic. The latitudinal and longitudinal ranges within this region are similarly wide, from 12°N to 55°S, and from 34° to 81°W. Ten countries border on these coasts, each with different research capabilities and taxonomic traditions; therefore, taxonomic knowledge differs among countries. Coastal biodiversity is strongly influenced by the physical and geological history of these coasts. The eastern tropical Pacific region, which encompasses the continental coasts of southern Central America (Costa Rica and Panama) and of northwestern South America (Colombia and Ecuador) is characterized by cliffs alternating with pocket beaches, alluvial and deltaic plains with extensive sandy beaches, well-developed mangrove forests, estuaries, lagoons, and, reefs. It also includes important offshore island systems such as the Pearl and Galapagos islands [1], [2]. The Peruvian coast also is diverse with bays, cliffs, kelp and macroalgal beds, rocky shores and sandy beaches, islands, and peninsulas, as well as wetlands, which include the southernmost limit to the tropical Pacific mangrove ecosystem [3], [4]. The Chilean coast is 4,500 km of mainly rocky shores, but does include some sandy-beach bays with channels and archipelagos toward the south (Patagonian region) [5], [6]. Some of the most diverse ecosystems in Chile are the beds of kelp (Lessonia and Macrosystis) and macroalgae (Gracillaria and Ulva). The combination of the unique oceanographic conditions and coastal heterogeneity in the Chilean coast has resulted in high levels of endemism (near 40%) in many invertebrate groups [5], and several marine invertebrate taxa show latitudinal biodiversity patterns, some of them explained by the presence of Antarctic fauna [7]–[9]. Ecuador, Peru, and Chile are under the influence of the Humboldt upwelling system and subject to high environmental variability caused by the ENSO (El Nino Southern Oscillation) and LNSO (La Nina Southern Oscillation), which cause important changes in community composition and abundance, particularly of the plankton [1], [10]. The Atlantic coast of the South American continent is distinctly different from the Pacific coast. It includes three major rivers (Orinoco, Amazon, and La Plata), which discharge enormous amounts of freshwater and sediment to the ocean, and the coast has an extensive continental platform. Argentina's coast has mostly sandy beaches [6], [11] and some rocky formations located mainly at Mar del Plata and at Peninsula Valdes. At Mar del Plata, these rocky shores are dominated by two mussel species and by a diverse macroalgal community with a clear tidal zonation [12], [13]. The Uruguayan coast is dominated by sandy beaches with a narrow portion of rocky habitats known to sustain a rich biological diversity [14]. Observed variations in community composition and distribution may be related to the salinity gradient caused by La Plata River discharge [15]. The coast of Brazil, extending almost 7,500 km, is under the influence of the warm Brazil Current, the cold Malvinas/Falklands Current, and many rivers and upwelling regions [16]. The warm northern coast, where the Amazon discharges into the ocean, is characterized by a combination of freshwater, estuarine, and marine ecosystems, with diverse but poorly known habitats [17]. The colder southern coast is characterized by a variety of ecosystems, including mangrove forests, seagrass beds, coral reefs, sandy beaches, rocky shores, lagoons, and estuaries. Because of its vastness, extensive areas of Brazil's coast remain unexplored. North of Brazil are Suriname, French Guiana, Guyana, and the Venezuelan Atlantic Front. This area, including about 1,900 km of coastline, is under the strong influence of the Amazon River. Therefore, the typical ecosystems are estuaries, mudflats, sandy beaches, and mangrove forests, which extend along most of the coastline [18]. The Venezuelan Atlantic coast is also under the influence of the Orinoco River, with coastal mudflats and extensive mangrove forests [19]. In this paper, we analyze the status of knowledge of marine biodiversity in five subregions along the Atlantic and Pacific coasts of South America. As most of the information is based in national reports, these subregions were based in the Large Marine Ecosystem boundaries as defined for South America, with a few practical adaptations, based in country political borders. The paper also provides an updated review of ecosystem threats, such as invasive species, and the marine conservation strategies employed by South American countries with access to the coast, excluding the Caribbean coasts of Venezuela and Colombia, as these are included in another paper of this collection [20].

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.