1,721,072 research outputs found
Didattica e Informatica Una Indagine Statistica relativa alla Provincia di Modena sul rapporto tra Insegnanti e Nuove Tecnologie
Full text linkEvolutionay wavelet networks for statistical time series analysis
No full textIn this paper I describe an evolutionary wavelet network to optimize the filtering of a statistical time series into separate contributions. The wavelet base is regarded as a neural network where the network nodes are discrete wavelet transforms, the wavelon, and the network structure and parameters are selected through evolutionary techniques. With this combined approach I can separate stochastic from structural components within an optimized framework and finally I can perform optimized predictive analysis on the time series components
Algoritmi ibridi per l' ottimizzazione di un Portafoglio Azionario Simulazione stocastica filtrata mediante wavelet decomposition
Full text linkHole density and Tc behaviour in YBa2Cu3O7 - x
No full textThe behaviour of the hole number in YBCO systems as a function of the oxygen deficiency x is studied theoretically by calculating the density of electron states for several different superstructures formed by periodic arrangements of the oxygen vacancies. The characteristic features of the electronic properties for chain copper sites with different coordination are pointed out and their relevance in determining the number of carriers for oxygen planes is illustrated. It is shown that the curve of the hole number as a function of x shows plateaus and jumps similar to those found in the Tc versus x curves if the de-oxygenation process takes place by forming fragmented chains. Structures formed by filled and empty chains only give a smooth hole behaviour as a function of the composition. The results of the numerical calculations can be understood by a simple model giving the Fermi energy shift in terms of the concentrations of Cu(1) sites with different coordination. © 1990
Evolutionary Clustering Analysis
No full textThe determination of the number of groups in a dataset, theircomposition and the most relevant measurements to be considered in clusteringthe data, is a high-demanding task, especially when the a priori information onthe dataset is limited. Three different genetic approaches are introduced in thispaper as tools for automatic data clustering and features selection. They differin the adopted codification of the grouping problem, not in the evolutionaryoperator and parameters. Two of them deals with the grouping problem in adeterministic framework. The first directly approaches the grouping problem asa combinatorial one. The second tries to determine some relevant points in thedata domain to be used in clustering data as group separators. A probabilisticframework is then introduced with the third one, which starts specifying thestatistical model from which data are assumed to be drawn. The evolutionaryapproaches are, finally, compared with respect to classical partitional clusteringalgorithms on simulated data and on Fisher’s Iris dataset used as a benchmark
Correlated one-hole spectra of YBa2Cu3O7 in the bulk and at the surface
No full textThe one-hole valence spectrum of YBa2Cu3O7 is calculated using an accurate tight-binding hamiltonian and a Hubbard model to treat correlation effects. Intra-atomic Coulomb integrals for holes sitting on Cu or O sites are considered and the one-hole self-energy is computed in the low density approximation. The calculated spectrum is compared with photoemission and X-ray emission experiments. This allows to point out a number of problems that arise in assigning the observed structures to a specific atom and to understand some aspects of the data. To evaluate the role of the surface in determining the shape of the photoemission energy distribution curves, the one-hole spectrum is calculated for the Ba-O4 surface. Oxygen induced surface states are shown to be important in determining the intensity of the observed structures. © 1989 Springer-Verlag
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