3,369 research outputs found
A Backward Stable Algorithm for Computing the CS Decomposition via the Polar Decomposition
We introduce a backward stable algorithm for computing the CS decomposition
of a partitioned matrix with orthonormal columns, or a
rank-deficient partial isometry. The algorithm computes two polar
decompositions (which can be carried out in parallel) followed by an
eigendecomposition of a judiciously crafted Hermitian matrix. We
prove that the algorithm is backward stable whenever the aforementioned
decompositions are computed in a backward stable way. Since the polar
decomposition and the symmetric eigendecomposition are highly amenable to
parallelization, the algorithm inherits this feature. We illustrate this fact
by invoking recently developed algorithms for the polar decomposition and
symmetric eigendecomposition that leverage Zolotarev's best rational
approximations of the sign function. Numerical examples demonstrate that the
resulting algorithm for computing the CS decomposition enjoys excellent
numerical stability
Maxwell's Theory of Solid Angle and the Construction of Knotted Fields
We provide a systematic description of the solid angle function as a means of
constructing a knotted field for any curve or link in . This is a
purely geometric construction in which all of the properties of the entire
knotted field derive from the geometry of the curve, and from projective and
spherical geometry. We emphasise a fundamental homotopy formula as unifying
different formulae for computing the solid angle. The solid angle induces a
natural framing of the curve, which we show is related to its writhe and use to
characterise the local structure in a neighborhood of the knot. Finally, we
discuss computational implementation of the formulae derived, with C code
provided, and give illustrations for how the solid angle may be used to give
explicit constructions of knotted scroll waves in excitable media and knotted
director fields around disclination lines in nematic liquid crystals.Comment: 20 pages, 9 figure
QR-Factorization Algorithm for Computed Tomography (CT): Comparison With FDK and Conjugate Gradient (CG) Algorithms
[EN] Even though QR-factorization of the system matrix for tomographic devices has been already used for medical imaging, to date, no satisfactory solution has been found for solving large linear systems, such as those used in computed tomography (CT) (in the order of 106 equations). In CT, the Feldkamp, Davis, and Kress back projection algorithm (FDK) and iterative methods like conjugate gradient (CG) are the standard methods used for image reconstruction. As the image reconstruction problem can be modeled by a large linear system of equations, QR-factorization of the system matrix could be used to solve this system. Current advances in computer science enable the use of direct methods for solving such a large linear system. The QR-factorization is a numerically stable direct method for solving linear systems of equations, which is beginning to emerge as an alternative to traditional methods, bringing together the best from traditional methods. QR-factorization was chosen because the core of the algorithm, from the computational cost point of view, is precalculated and stored only once for a given CT system, and from then on, each image reconstruction only involves a backward substitution process and the product of a vector by a matrix. Image quality assessment was performed comparing contrast to noise ratio and noise power spectrum; performances regarding sharpness were evaluated by the reconstruction of small structures using data measured from a small animal 3-D CT. Comparisons of QR-factorization with FDK and CG methods show that QR-factorization is able to reconstruct more detailed images for a fixed voxel size.This work was supported by the Spanish Government under Grant TEC2016-79884-C2 and Grant RTC-2016-5186-1.Rodríguez-Álvarez, M.; Sánchez, F.; Soriano Asensi, A.; Moliner Martínez, L.; Sánchez Góez, S.; Benlloch Baviera, JM. (2018). QR-Factorization Algorithm for Computed Tomography (CT): Comparison With FDK and Conjugate Gradient (CG) Algorithms. IEEE Transactions on Radiation and Plasma Medical Sciences. 2(5):459-469. https://doi.org/10.1109/TRPMS.2018.2843803S4594692
Pseudospectral Shattering, the Sign Function, and Diagonalization in Nearly Matrix Multiplication Time
We exhibit a randomized algorithm which given a square complex
matrix with and , computes with high probability
invertible and diagonal such that and
in
arithmetic operations on a floating point machine with bits of precision. Here is the number of arithmetic
operations required to multiply two complex matrices numerically
stably, with for every , where
is the exponent of matrix multiplication. The algorithm is a variant
of the spectral bisection algorithm in numerical linear algebra (Beavers and
Denman, 1974). This running time is optimal up to polylogarithmic factors, in
the sense that verifying that a given similarity diagonalizes a matrix requires
at least matrix multiplication time. It significantly improves best previously
provable running times of arithmetic operations for
diagonalization of general matrices (Armentano et al., 2018), and (w.r.t.
dependence on ) arithmetic operations for Hermitian matrices
(Parlett, 1998).
The proof rests on two new ingredients. (1) We show that adding a small
complex Gaussian perturbation to any matrix splits its pseudospectrum into
small well-separated components. This implies that the eigenvalues of the
perturbation have a large minimum gap, a property of independent interest in
random matrix theory. (2) We rigorously analyze Roberts' Newton iteration
method for computing the matrix sign function in finite arithmetic, itself an
open problem in numerical analysis since at least 1986. This is achieved by
controlling the evolution the iterates' pseudospectra using a carefully chosen
sequence of shrinking contour integrals in the complex plane.Comment: 78 pages, 3 figures, comments welcome. Slightly edited intro from
previous version + explicit statement of forward error Theorem (Corolary
1.7). Minor corrections mad
Refresher course in maths and a project on numerical modeling done in twos
These lecture notes accompany a refresher course in applied mathematics with a focus on numerical concepts (Part I), numerical linear algebra (Part II), numerical analysis, Fourier series and Fourier
transforms (Part III), and differential equations (Part IV). Several numerical projects for group work are provided in Part V. In these projects, the tasks are threefold: mathematical modeling, algorithmic design, and implementation. Therein, it is important to draw interpretations
of the obtained results and provide measures (Parts I-IV) how to build confidence into numerical findings such intuition, error analysis, convergence analysis, and comparison to manufactured solutions. Both authors have been jointly teaching over several years this class and
bring in a unique mixture of their respective teaching and research fields
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