Standardization of activated sludge for biodegradation tests

Activated sludges are an inoculum source commonly used in biodegradation studies, as wastewater treatment facilities constitute an entry point to the environment for many chemicals. In this paper, the main issues relating to the use of activated sludge in biodegradability tests are presented. Special attention is also devoted to discussing the factors affecting both the activity of the microbial communities and the test results. After a short survey of the state of the art of microbiology of activated sludge, the paper focuses on the methods used to reduce the variations in the diversity, quality and quantity of these communities. Finally, use of surrogates as reference materials in biodegradability tests is discussed

Dynamic scaling approach to study time series fluctuations

We propose a new approach for properly analyzing stochastic time series by mapping the dynamics of time series fluctuations onto a suitable nonequilibrium surface-growth problem. In this framework, the fluctuation sampling time interval plays the role of time variable, whereas the physical time is treated as the analog of spatial variable. In this way we found that the fluctuations of many real-world time series satisfy the analog of the Family-Viscek dynamic scaling ansatz. This finding permits to use the powerful tools of kinetic roughening theory to classify, model, and forecast the fluctuations of real-world time series.Comment: 25 pages, 7 figures, 1 tabl

Premios Nobel de Economía

Con esta sección, la revista Semestre Económico desea entregar a sus lectores un espacio informativo donde se consignen apartes del pensamiento económico y sus aportes a esta disciplina, teniendo en cuenta el momento y los contextos donde cambios estructurales se han dado, para que tales trabajos hayan contribuido al desarrollo de la Economía.Inicialmente, se ha pensado en quienes han trabajado desde la década de los sesenta, cuyos planteamientos se han perfilado a la solución de algunos problemas económicos que generalmente han brillado por su carácter teórico, técnico, de formalización matemática de modelos y, últimamente, por atención a la parte social. Son, en resumidas cuentas, aquellos trabajos que se identifican con los Premios Nobel de Economía

Variable order Mittag-Leffler fractional operators on isolated time scales and application to the calculus of variations

We introduce new fractional operators of variable order on isolated time scales with Mittag-Leffler kernels. This allows a general formulation of a class of fractional variational problems involving variable-order difference operators. Main results give fractional integration by parts formulas and necessary optimality conditions of Euler-Lagrange type.Comment: This is a preprint of a paper whose final and definite form is with Springe