2,771 research outputs found
Decimated generalized Prony systems
We continue studying robustness of solving algebraic systems of Prony type
(also known as the exponential fitting systems), which appear prominently in
many areas of mathematics, in particular modern "sub-Nyquist" sampling
theories. We show that by considering these systems at arithmetic progressions
(or "decimating" them), one can achieve better performance in the presence of
noise. We also show that the corresponding lower bounds are closely related to
well-known estimates, obtained for similar problems but in different contexts
Faster Inversion and Other Black Box Matrix Computations Using Efficient Block Projections
Block projections have been used, in [Eberly et al. 2006], to obtain an
efficient algorithm to find solutions for sparse systems of linear equations. A
bound of softO(n^(2.5)) machine operations is obtained assuming that the input
matrix can be multiplied by a vector with constant-sized entries in softO(n)
machine operations. Unfortunately, the correctness of this algorithm depends on
the existence of efficient block projections, and this has been conjectured. In
this paper we establish the correctness of the algorithm from [Eberly et al.
2006] by proving the existence of efficient block projections over sufficiently
large fields. We demonstrate the usefulness of these projections by deriving
improved bounds for the cost of several matrix problems, considering, in
particular, ``sparse'' matrices that can be be multiplied by a vector using
softO(n) field operations. We show how to compute the inverse of a sparse
matrix over a field F using an expected number of softO(n^(2.27)) operations in
F. A basis for the null space of a sparse matrix, and a certification of its
rank, are obtained at the same cost. An application to Kaltofen and Villard's
Baby-Steps/Giant-Steps algorithms for the determinant and Smith Form of an
integer matrix yields algorithms requiring softO(n^(2.66)) machine operations.
The derived algorithms are all probabilistic of the Las Vegas type
Regularization strategy for the layered inversion of airborne TEM data: application to VTEM data acquired over the basin of Franceville (Gabon)
Airborne transient electromagnetic (TEM) is a cost-effective method to image
the distribution of electrical conductivity in the ground. We consider layered
earth inversion to interpret large data sets of hundreds of kilometre.
Different strategies can be used to solve this inverse problem. This consists
in managing the a priori information to avoid the mathematical instability and
provide the most plausible model of conductivity in depth. In order to obtain
fast and realistic inversion program, we tested three kinds of regularization:
two are based on standard Tikhonov procedure which consist in minimizing not
only the data misfit function but a balanced optimization function with
additional terms constraining the lateral and the vertical smoothness of the
conductivity; another kind of regularization is based on reducing the condition
number of the kernel by changing the layout of layers before minimizing the
data misfit function. Finally, in order to get a more realistic distribution of
conductivity, notably by removing negative conductivity values, we suggest an
additional recursive filter based upon the inversion of the logarithm of the
conductivity. All these methods are tested on synthetic and real data sets.
Synthetic data have been calculated by 2.5D modelling; they are used to
demonstrate that these methods provide equivalent quality in terms of data
misfit and accuracy of the resulting image; the limit essentially comes on
special targets with sharp 2D geometries. The real data case is from
Helicopter-borne TEM data acquired in the basin of Franceville (Gabon) where
borehole conductivity loggings are used to show the good accuracy of the
inverted models in most areas, and some biased depths in areas where strong
lateral changes may occur
Magnetic Backgrounds from Generalised Complex Manifolds
The magnetic backgrounds that physically give rise to spacetime
noncommutativity are generally treated using noncommutative geometry. In this
article we prove that also the theory of generalised complex manifolds contains
the necessary elements to generate B-fields geometrically. As an example, the
Poisson brackets of the Landau model (electric charges on a plane subject to an
external, perperdicularly applied magnetic field) are rederived using the
techniques of generalised complex manifolds.Comment: Some refs. adde
ON GENERALIZATIONS OF STIRLING NUMBERS AND SOME WELL-KNOWN MATRICES
We introduce a generalization of the Stirling numbers of the first kind and the second kind. By arranging these numbers into matrices, we generalize the Stirling matrices of the first kind and the second kind investigated by Cheon and Kim [Stirling matrix via Pascal matrix, Linear Algebra Appl. 329 (2001) 49–59]. Furthermore, we introduce generalizations of the Pascal matrix and the symmetric Pascal matrix with two real arguments, and generalize earlier results related to the Pascal matrices, Stirling matrices and matrices involving Bell numbers
Structural optimization of thin shells using finite element method
The objective of the present work was the structural optimization of thin shell structures that are subjected to stress and displacement constraints. In order to accomplish this, the structural optimization computer program DESAP1 was modified and improved. In the static analysis part of the DESAP1 computer program the torsional spring elements, which are used to analyze thin, shallow shell structures, were eliminated by modifying the membrane stiffness matrix of the triangular elements in the local coordinate system and adding a fictitious rotational stiffness matrix. This simplified the DESAP1 program input, improved the accuracy of the analysis, and saved computation time. In the optimization part of the DESAP1 program the stress ratio formula, which redesigns the thickness of each finite element of the structure, was solved by an analytical method. This scheme replaced the iterative solution that was previously used in the DESAP1 program, thus increasing the accuracy and speed of the design. The modified program was used to design a thin, cylindrical shell structure with optimum weight, and the results are reported in this paper
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