A molecular dynamics study of structure, stability and fragmentation patterns of sodium bis(2-ethylhexyl)sulfosuccinate positively charged aggregates in vacuo

Positively charged supramolecular aggregates formed in vacuo by n AOTNa (sodium bis(2-ethylhexyl)sulfosuccinate) molecules and n(c) additional sodium ions, i.e. [AOT(n)Na(n+n(c))](n(c)), have been investigated by molecular dynamics (MD) simulations for n = 1-20 and n(c) = 0-5. Statistical analysis of physical quantities like gyration radii, atomic B-factors and moment of inertia tensors provides detailed information on their structural and dynamical properties. Even for n(c) = 5, all stable aggregates show a reverse micelle-like structure with an internal solid-like core including sodium counterions and surfactant polar heads surrounded by an external layer consisting of the surfactant alkyl chains. Moreover, the aggregate shapes may be approximated by rather flat and elongated ellipsoids whose longer axis increases with n and n(c). The fragmentation patterns of a number of these aggregates have also been examined and have been found to markedly depend on the aggregate charge state. In one particular case, for which experimental findings are available in the literature, a good agreement is found with the present fragmentation data.     

Graph problems arising from parameter identification of discrete dynamical systems

This paper focuses on combinatorial feasibility and optimization problems that arise in the context of parameter identification of discrete dynamical systems. Given a candidate parametric model for a physical system and a set of experimental observations, the objective of parameter identification is to provide estimates of the parameter values for which the model can reproduce the experiments. To this end, we define a finite graph corresponding to the model, to each arc of which a set of parameters is associated. Paths in this graph are regarded as feasible only if the sets of parameters corresponding to the arcs of the path have nonempty intersection. We study feasibility and optimization problems on such feasible paths, focusing on computational complexity. We show that, under certain restrictions on the sets of parameters, some of the problems become tractable, whereas others are NP-hard. In a similar vein, we define and study some graph problems for experimental design, whose goal is to support the scientist in optimally designing new experiments.     

Log‐symmetric regression models: information criteria and application to movie business and industry data with economic implications

This work deals with log‐symmetric regression models, which are particularly useful when the response variable is continuous, strictly positive, and following an asymmetric distribution, with the possibility of modeling atypical observations by means of robust estimation. In these regression models, the distribution of the random errors is a member of the log‐symmetric family, which is composed by the log‐contaminated‐normal, log‐hyperbolic, log‐normal, log‐power‐exponential, log‐slash and log‐Student‐t distributions, among others. One way to select the best family member in log‐symmetric regression models is using information criteria. In this paper, we formulate log‐symmetric regression models and conduct a Monte Carlo simulation study to investigate the accuracy of popular information criteria, as Akaike, Bayesian, and Hannan‐Quinn, and their respective corrected versions to choose adequate log‐symmetric regressions models. As a business application, a movie data set assembled by authors is analyzed to compare and obtain the best possible log‐symmetric regression model for box offices. The results provide relevant information for model selection criteria in log‐symmetric regressions and for the movie industry. Economic implications of our study are discussed after the numerical illustrations.     

Regularity in parabolic phase transition problems

We describe the main results obtained in a joint work with Athanasopoulos and Caffarelli on the regularity of viscosity solutions and of their free boundaries for a rather general class of parabolic phase transition problems. To a great unforgettable Master.     

On the mechanical modelling of masonry structures using the F.E. technique

A non-linear finite element for the mechanical modelling of masonry structures is presented. Both material and geometrical non-linearities are introduced, developing the expression of the potential strain energy through appropriate average functions. The characteristics of the numerical model are explained and the approximations introduced in the analysis are discussed.

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Sommario Viene sviluppata la formulazione di un nuovo elemento finito per la modellazione meccanica di strutture murarie in campo non lineare. Vengono prese in esame sia non-linearita' geometriche che materiali, sviluppando l'espressione dell'energia potenziale elastica del solido murario attraverso l'introduzione di opportune funzioni medie sulle deformazioni. Vengono discusse le caratteristiche del modello numerico e le approssimazioni introdotte nell'analisi.

     

Time behaviour of non-linear stochastic processes in the presence of multiplicative noise: From Kramers' to Suzuki's decay

The diffusional regime of a Brownian particle in a double-well potential in the presence of both additive and multiplicative noise is explored. As a relevant effect of the multiplicative noise, the escape rate from a well is shown to change from the small value of the Kramers theory into the large relaxation rate of the Suzuki regime. It is shown, furthermore, that the time required to get equilibrium in a well after sudden application of multiplicative noise (the activation time) is very much shorter than the Kramers relaxation time. We envisage therefore an operational scheme making available multiplicative noise for a short interval of time (for example using a light pulse) as an efficient tool to get a fast process of escape from a well. These results are obtained by using a continued-fraction algorithm which makes it possible even to successfully deal with the decay of an unstable state at the critical point.     

The palladium-catalyzed reaction of aryl iodides with mono and disubstituted acetylenes: a new synthesis of trisubstituted alkenes.

Mono and disubstituted acetylenes react with aryl iodides containing a variety of functional groups in the presence of palladium catalyst, formic acid and a tertiary amine to give trisubstituted alkenes.     

Equilibrium states for a plane incompressible perfect fluid

We associate to the plane incompressible Euler equation with periodic conditions the corresponding Hopf equation, as an equation for measures on the space of solenoidal distributions. We define equilibrium states as the solutions of the stationary Hopf equation. We find a class of equilibrium states which corresponds to a class of infinitely divisible distributions, and investigate the properties of gaussian and poissonian states. Equilibrium dynamics for a class of poissonian states is constructed by means of the Onsager vortex equations.Research partially supported by C.N.R., G.N.F.M.