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Capizzano S.S.,University Dellinsubria Sede Of Como | Tablino-Possio C.,University of Milan Bicocca
Computers and Mathematics with Applications

In recent contributions, algebraic multigrid methods have been designed and studied from the viewpoint of spectral complementarity. In this note, we focus our efforts on specific applications and, more precisely, on large linear systems arising from the approximation of the weighted Laplacian with various boundary conditions. We adapt the multigrid idea to this specific setting and we present and critically discuss a wide set of numerical experiments showing the potentiality of the considered approach. © 2010 Elsevier Ltd. All rights reserved. Source

Cinosi A.,Sasol Limited | Cinosi A.,GNR S.R.L. | Andriollo N.,Sasol Limited | Pepponi G.,Fondazione Bruno Kessler | Monticelli D.,University Dellinsubria Sede Of Como
Analytical and Bioanalytical Chemistry

A total reflection X-ray fluorescence (TXRF) procedure was developed for the determination of metal traces in petrochemical end products or intermediates for surfactant synthesis. The method combines a fast and straightforward sample preparation, i.e. deposition on the sample holder and evaporation of the sample matrix, with an efficient quantification method based on internal standardization (organic gallium standard). The method developed showed detection limits below 0.05 μg g-1 and in most cases below 0.005 μg g-1. Fifteen elements (Ca, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Rh, Sn, Sr, V and Zn) were determined in matrices such as paraffins, n-olefins, linear alkylbenzenes, long-chain alkyl alcohols and esters: typical metal contents were below 1 μg g-1. The results were compared with the reference method ASTM D5708 (test method B) based on inductively coupled plasma optical emission spectroscopy: advantages and drawbacks of the two procedures were critically evaluated. The TXRF method developed showed comparable precision and absence of bias with respect to the reference method. A comparison of the performances of the two methods is presented. © 2010 Springer-Verlag. Source

Roberts G.P.,University of London | Raithatha B.,University of London | Sileo G.,University Dellinsubria Sede Of Como | Pucci S.,Italian National Institute of Geophysics and Volcanology | And 12 more authors.
Geophysical Journal International

The shallow subsurface structure of the 2009 April 6 Mw 6.3 L'Aquila earthquake surface rupture at Paganica has been investigated with ground penetrating radar to study how the surface rupture relates spatially to previous surface displacements during the Holocene and Pleistocene. The discontinuous surface rupture stepped between en-echelon/parallel faults within the overall fault zone that show clear Holocene/Pleistocene offsets in the top 10 m of the subsurface. Some portions of the fault zone that show clear Holocene offsets were not ruptured in 2009, having been bypassed as the rupture stepped across a relay zone onto a fault across strike. The slip vectors, defined by opening directions across surface cracks, indicate dip-slip normal movement, whose azimuth remained constant between 210° and 228° across the zone where the rupture stepped between faults. We interpret maximum vertical offsets of the base of the Holocene summed across strike to be 4.5 m, which if averaged over 15 kyr, gives a maximum throw-rate of 0.23-0.30 mm yr-1, consistent with throw-rates implied by vertical offsets of a layer whose age we assume to be ~33 ka. This compares with published values of 0.4 mm yr-1 for a minimum slip rate implied by offsets of Middle Pleistocene tephras, and 0.24 mm yr-1 since 24.8 kyr from palaeoseismology. The Paganica Fault, although clearly an important active structure, is not slipping fast enough to accommodate all of the 3-5 mm yr-1 of extension across this sector of the Apennines; other neighbouring range-bounding active normal faults also have a role to play in the seismic hazard. © 2010 The Authors Journal compilation © 2010 RAS. Source

Donatelli M.,University Dellinsubria Sede Of Como | Serra-Capizzano S.,University Dellinsubria Sede Of Como | Sesana D.,University of Piemonte Orientale
BIT Numerical Mathematics

Starting from the spectral analysis of g-circulant matrices, we study the convergence of a multigrid method for circulant and Toeplitz matrices with various size reductions. We assume that the size n of the coefficient matrix is divisible by g≥2 such that at the lower level the system is reduced to one of size n/g, by employing g-circulant based projectors. We perform a rigorous two-grid convergence analysis in the circulant case and we extend experimentally the results to the Toeplitz setting, by employing structure preserving projectors. The optimality of the two-grid method and of the multigrid method is proved, when the number θ ∈ ℕ of recursive calls is such that 1 < θ < g. The previous analysis is used also to overcome some pathological cases, in which the generating function has zeros located at "mirror points" and the standard two-grid method with g=2 is not optimal. The numerical experiments show the correctness and applicability of the proposed ideas, both for circulant and Toeplitz matrices. © 2011 Springer Science + Business Media B.V. Source

Donatelli M.,University Dellinsubria Sede Of Como | Serra-Capizzano S.,University Dellinsubria Sede Of Como
Journal of Electrical and Computer Engineering

This survey paper deals with the use of antireflective boundary conditions for deblurring problems where the issues that we consider are the precision of the reconstruction when the noise is not present, the linear algebra related to these boundary conditions, the iterative and noniterative regularization solvers when the noise is considered, both from the viewpoint of the computational cost and from the viewpoint of the quality of the reconstruction. In the latter case, we consider a reblurring approach that replaces the transposition operation with correlation. For many of the considered items, the anti-reflective algebra coming from the given boundary conditions is the optimal choice. Numerical experiments corroborating the previous statement and a conclusion section end the paper. © 2010 Marco Donatelli and Stefano Serra-Capizzano. Source

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