Compositional evolution with mass transfer in closed systems

Evolution of compositions in time, space, temperature or other covariates is frequent in practice. For instance, the radioactive decomposition of a sample changes its composition with time. Some of the involved isotopes decompose into other isotopes of the sample, thus producing a transfer of mass from some components to other ones, but preserving the total mass present in the system. This evolution is traditionally modelled as a system of ordinary di erential equations of the mass of each component. However, this kind of evolution can be decomposed into a compositional change, expressed in terms of simplicial derivatives, and a mass evolution (constant in this example). A rst result is that the simplicial system of di erential equations is non-linear, despite of some subcompositions behaving linearly. The goal is to study the characteristics of such simplicial systems of di erential equa- tions such as linearity and stability. This is performed extracting the compositional dif ferential equations from the mass equations. Then, simplicial derivatives are expressed in coordinates of the simplex, thus reducing the problem to the standard theory of systems of di erential equations, including stability. The characterisation of stability of these non-linear systems relays on the linearisation of the system of di erential equations at the stationary point, if any. The eigenvelues of the linearised matrix and the associated behaviour of the orbits are the main tools. For a three component system, these orbits can be plotted both in coordinates of the simplex or in a ternary diagram. A characterisation of processes with transfer of mass in closed systems in terms of stability is thus concluded. Two examples are presented for illustration, one of them is a radioactive decay

Ventajas e inconvenientes del uso del preservativo con una pareja afectiva heterosexual

Se analiza la validez de constructo y la consistencia interna de un cuestionario sobre ventajas e inconvenientes del uso del preservativo con una pareja afectiva heterosexual, así como su capacidad para predecir el uso del preservativo en el pasado y la intención de uso en los próximos tres meses. Participan en el estudio 1711 estudiantes universitarios (46.3% varones y 60% menores de 21 años) elegidos mediante muestreo de conveniencia. Se emplea un diseño transversal y los datos se recogen mediante autoinforme voluntario y anónimo. Los resultados muestran que la consistencia interna del cuestionario es equiparable a la de otros instrumentos semejantes (alfas de Cronbach en escala de ventajas: .605 y en escala de inconvenientes: .725). Tanto las ventajas como los inconvenientes del uso del preservativo predicen significativamente su uso sistemático en el pasado, con un mayor peso de las ventajas. Entre los jóvenes que no lo han utilizado siempre, y únicamente en los varones, las ventajas predicen la intención de uso en los próximos tres meses. Los resultados apoyan la realización de programas preventivos que destaquen los beneficios derivados del uso de preservativo y, así mismo, reduzcan las percepciones negativas asociadas a su empleo, con una atención preferente a los primeros