aceptación: 15 de agosto de 2013. ResumenEn este trabajo se analizan algunos factores relevantes que intervienen en la configuración del sistema de distribución de alimentos en México y en los cuales se basa el liderazgo y dominio corporativo de éste. Se ubican las causas y efectos que operan en la expansión de un nuevo esquema empresarial de distribución de alimentos más competitivo y mejor organizado en comparación con los canales tradicionales que operaban antes del surgimiento de esta hegemonía. La importancia de este proceso, asociado a la apertura económica, se refleja en un conjunto de estrategias de expansión territorial mediante múltiples modalidades de operación y tamaños de establecimientos para dominar cuotas importantes del consumo alimentario ubicados principalmente en las ciudades mexicanas. Palabras clave: comercio alimentario, canales minoristas, supermercados, corporativos comerciales, abasto alimentario, México.
In this article, we consider an extension of the classical Curie-Weiss model in which the global and deterministic external magnetic field is replaced by local and random external fields which interact with each spin of the system. We prove a Large Deviations Principle for the so-called magnetization per spin S n /n with respect to the associated Gibbs measure, where S n /n is the scaled partial sum of spins. In particular, we obtain an explicit expression for the rate function, which enables an extensive study of the phase diagram in some examples. It is worth mentioning that the model considered in this article covers, in particularly, both the case of i. i. d. random external fields (also known under the name of random field Curie-Weiss models) and the case of dependent random external fields generated by e. g. Markov chains or dynamical systems.
The discovery of dipole-induced exchange bias (EB), switching from negative to positive sign, is reported in systems where the antiferromagnet and the ferromagnet are separated by a paramagnetic spacer (AFM-PM-FM). The magnitude and sign of the EB is determined by the cooling field strength and the PM thickness. The same cooling field yields negative EB for thin spacers, and positive EB for thicker ones. The EB decay profile as a function of the spacer thickness, and the change of sign, are attributed to long-ranged dipole coupling. Our model, which accounts quantitatively for the experimental results, ignores the short range interfacial exchange interactions of the usual EB theories. Instead, it retains solely the long range dipole field that allows for the coupling of the FM and AFM across the PM spacer. The experiments allow for novel switching capabilities of long range EB systems, while the theory allows description of the structures where the FM and AFM are not in atomic contact. The results provide a new approach to design novel interacting heterostructures.
<p>El sobrepeso y la obesidad representan un problema de salud pública que afecta al desarrollo económico de México. Su estudio desde el enfoque médico-epidemiológico<br />resulta hoy insuficiente debido a su magnitud y multifactores que los originan. Por<br />lo tanto, es necesario incorporar la perspectiva económica al análisis de tal manera que ubique sus causas estructurales, según las modalidades del consumo de alimentos que conforman un patrón hegemónico oferta-demanda impuesto por la globalización de los mercados que transgrede los patrones locales. La evidencia empírica muestra que el tránsito de una dieta tradicional a una industrializada, con altos contenidos de azúcares y harinas refinadas, influye en la prevalencia actual del problema de salud pública.</p>
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