1,720,967 research outputs found
Design of asymptotic estimators: An approach based on neural networks and nonlinear programming
No full textA methodology to design state estimators for a class of nonlinear continuous-time dynamic systems that is based on neural networks and nonlinear programming is proposed. The estimator has the structure of a Luenberger observer with a linear gain and a parameterized (in general, nonlinear) function, whose argument is an innovation term representing the difference between the current measurement and its prediction. The problem of the estimator design consists in finding the values of the gain and of the parameters that guarantee the asymptotic stability of the estimation error. Toward this end, if a neural network is used to take on this function, the parameters (i.e., the neural weights) are chosen, together with the gain, by constraining the derivative of a quadratic Lyapunov function for the estimation error to be negative definite on a given compact set. It is proved that it is sufficient to impose the negative definiteness of such a derivative only on a suitably dense grid of sampling points. The gain is determined by solving a Lyapunov equation. The neural weights are searched for via nonlinear programming by minimizing a cost penalizing grid-point constraints that are not satisfied. Techniques based on low-discrepancy sequences are applied to deal with a small number of sampling
points, and, hence, to reduce the computational burden required
to optimize the parameters. Numerical results are reported and comparisons with those obtained by the extended Kalman filter are
made
Ensemble Aggregation Approaches for Functional Optimization
No full textIn this work we investigate the use of ensemble methods, consisting in the aggregation of several approximating models, in the context of functional optimization. In fact, while ensemble techniques are routinely employed in the machine learning literature for classification and regression, there is little research on their application to general optimization problems. Here we consider two strategies to aggregate different solutions to a functional optimization problem, based on optimized weighted averaging and aggregation over the minimum, the latter also in approximate version. A theoretical analysis of approximate functional optimization in the context of ensemble aggregation is provided. Then, simulation results are reported to showcase the advantages of ensembles for functional optimization, in terms of better accuracy and improved robustness with respect to single solutions
Regulation of p27kip1 and p57kip2 functions by natural polyphenols
Full text linkIn numerous instances, the fate of a single cell not only represents its peculiar outcome but also contributes to the overall status of an organism. In turn, the cell division cycle and its control strongly influence cell destiny, playing a critical role in targeting it towards a specific phenotype. Several factors participate in the control of growth, and among them, p27Kip1 and p57Kip2, two proteins modulating various transitions of the cell cycle, appear to play key functions. In this review, the major features of p27 and p57 will be described, focusing, in particular, on their recently identified roles not directly correlated with cell cycle modulation. Then, their possible roles as molecular effectors of polyphenols’ activities will be discussed. Polyphenols represent a large family of natural bioactive molecules that have been demonstrated to exhibit promising protective activities against several human diseases. Their use has also been proposed in association with classical therapies for improving their clinical effects and for diminishing their negative side activities. The importance of p27Kip1 and p57Kip2 in polyphenols’ cellular effects will be discussed with the aim of identifying novel therapeutic strategies for the treatment of important human diseases, such as cancers, characterized by an altered control of growth
A decision support tool based on a queueing model for performance analysis and optimization of container terminals
No full textThe constant growth of container traffic has posed the problem of devising efficient tools for the management of logistics activities at container terminals, where a number of tasks are carried out, including loading, unloading and storing. To this purpose, a decision support system (DSS) based on a discrete-time dynamic model of container flows in maritime terminals is proposed for performance analysis and simulation.
The tool can be used both for planning and real-time decision support in the management of the handling resources. More specifically, on the one hand, it is possible to evaluate the performance of a container terminal (in terms of different indexes of interest) and the improvements attainable by possible changes in the resources (for example, to understand the impact of adding or removing a crane) in various scenarios. On the other hand, the DSS is also able to compute a real-time strategy of resource allocation aimed at optimizing the overall efficiency of the operations dynamically. The model at the core of the DSS has control inputs that represent the percentages of capacities of the available handling machines used to move containers inside a terminal. We regard such capacities as limited resources to be allocated to the various operations they are required for. By suitably choosing different cost functions, it is possible to address various optimization problems that can take into account different performance indexes. The model is based on a discrete-time state equation that describes the dynamic behaviour of the quantities of containers stored or transferred across a maritime terminal. It is basically a system of queues that represent either temporal storages or delays that may occur during the container transfers due to the unavailability of resources.
The proposed tool integrates simulation and optimization in an easy-to-use fashion, thus making it useful also for those with no experience in simulation. An example of the possible uses of the tool is reported to show its effectiveness in evaluation of the terminal performance.
The paper is structured as follows: Section 1 concerns introduction; Section 2 describes the main features of the tool, specifically for performance analysis and container flow optimization; Section 3 contains a simulation case study that shows possible uses of the DSS; Section 4 illustrates the basic ideas of the dynamic model of the container flows that is the basis of the proposed DSS; finally, Section 5 addresses conclusions and further comments
Optimization of an eMule-like modifier strategy
No full textOne of the main issues in peer-to-peer file-sharing services is to ensure balanced transfers of data among
peers. For instance, the popular eMule file-sharing client interface implements techniques based on a
credit system and time modifiers, in order to guarantee a fair degree of contribution/reward, e.g., to prevent
leeching. However, fixed heuristic modifiers may not be able to guarantee a fair treatment for every
peer participating in the service. This paper introduces an optimized strategy to compute modifiers in
order to promote cooperation among peers, while guaranteeing a suitable degree of fairness. Simulations
are provided to compare the proposed mechanism to the standard one adopted by the popular eMule client
interface
QuantTree: Histograms for change detection in multivariate data streams
No full textWe address the problem of detecting distribution changes in multivariate data streams by means of histograms. Histograms are very general and flexible models, which have been relatively ignored in the change-detection literature as they often require a number of bins that grows unfeasibly with the data dimension. We present QuantTree, a recursive binary splitting scheme that adaptively defines the histogram bins to ease the detection of any distribution change. Our design scheme implies that i) we can easily control the overall number of bins and ii) the bin probabilities do not depend on the distribution of stationary data. This latter is a very relevant aspect in change detection, since thresholds of tests statistics based on these histograms (e.g., the Pearson statistic or the total variation) can be numerically computed from univariate and synthetically generated data, yet guaranteeing a controlled false positive rate. Our experiments show that the proposed histograms are very effective in detecting changes in high dimensional data streams, and that the resulting thresholds can effectively control the false positive rate, even when the number of training samples is relatively small
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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