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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 76 (1997), S. 309-321 
    ISSN: 0945-3245
    Keywords: Mathematics Subject Classification (1991):65F10, 15A06
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary. Given a nonsingular matrix $A$ , and a matrix $T$ of the same order, under certain very mild conditions, there is a unique splitting $A=B-C$ , such that $T=B^{-1}C$ . Moreover, all properties of the splitting are derived directly from the iteration matrix $T$ . These results do not hold when the matrix $A$ is singular. In this case, given a matrix $T$ and a splitting $A=B-C$ such that $T=B^{-1}C$ , there are infinitely many other splittings corresponding to the same matrices $A$ and $T$ , and different splittings can have different properties. For instance, when $T$ is nonnegative, some of these splittings can be regular splittings, while others can be only weak splittings. Analogous results hold in the symmetric positive semidefinite case. Given a singular matrix $A$ , not for all iteration matrices $T$ there is a splitting corresponding to them. Necessary and sufficient conditions for the existence of such splittings are examined. As an illustration of the theory developed, the convergence of certain alternating iterations is analyzed. Different cases where the matrix is monotone, singular, and positive (semi)definite are studied.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 63 (1992), S. 345-356 
    ISSN: 0945-3245
    Keywords: 65F10
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary Convergence of two-stage iterative methods for the solution of linear systems is studied. Convergence of the non-stationary method is shown if the number of inner iterations becomes sufficiently large. TheR 1-factor of the two-stage method is related to the spectral radius of the iteration matrix of the outer splitting. Convergence is further studied for splittings ofH-matrices. These matrices are not necessarily monotone. Conditions on the splittings are given so that the two-stage method is convergent for any number of inner iterations.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 58 (1990), S. 387-397 
    ISSN: 0945-3245
    Keywords: AMS(MOS): 65F10 ; 65J10 ; 15A48 ; 15A06 ; 46A32 ; 47B55 ; CR: G1.3
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary Comparison theorems for weak splittings of bounded operators are presented. These theorems extend the classical comparison theorem for regular splittings of matrices by Varga, the less known result by Woźnicki, and the recent results for regular and weak regular splittings of matrices by Neumann and Plemmons, Elsner, and Lanzkron, Rose and Szyld. The hypotheses of the theorems presented here are weaker and the theorems hold for general Banach spaces and rather general cones. Hypotheses are given which provide strict inequalities for the comparisons. It is also shown that the comparison theorem by Alefeld and Volkmann applies exclusively to monotone sequences of iterates and is not equivalent to the comparison of the spectral radius of the iteration operators.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 83 (1999), S. 259-278 
    ISSN: 0945-3245
    Keywords: Mathematics Subject Classification (1991):65F10, 65F35, 65M55
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary. Weighted max-norm bounds are obtained for Algebraic Additive Schwarz Iterations with overlapping blocks for the solution of Ax = b, when the coefficient matrix A is an M-matrix. The case of inexact local solvers is also covered. These bounds are analogous to those that exist using A-norms when the matrix A is symmetric positive definite. A new theorem concerning P-regular splittings is presented which provides a useful tool for the A-norm bounds. Furthermore, a theory of splittings is developed to represent Algebraic Additive Schwarz Iterations. This representation makes a connection with multisplitting methods. With this representation, and using a comparison theorem, it is shown that a coarse grid correction improves the convergence of Additive Schwarz Iterations when measured in weighted max norm.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 69 (1994), S. 141-153 
    ISSN: 0945-3245
    Keywords: Mathematics Subject Classification (1991): 65F10
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary. Parallel block two-stage iterative methods for the solution of linear systems of algebraic equations are studied. Convergence is shown for monotone matrices and for $H$ -matrices. Two different asynchronous versions of these methods are considered and their convergence investigated.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 58 (1990), S. 685-702 
    ISSN: 0945-3245
    Keywords: AMS(MOS): 65F10 ; CR: G1.3
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary Classical iterative methods for the solution of algebraic linear systems of equations proceed by solving at each step a simpler system of equations. When this system is itself solved by an (inner) iterative method, the global method is called a two-stage iterative method. If this process is repeated, then the resulting method is called a nested iterative method. We study the convergence of such methods and present conditions on the splittings corresponding to the iterative methods to guarantee convergence forany number of inner iterations. We also show that under the conditions presented, the spectral radii of the global iteration matrices decrease when the number of inner iterations increases. The proof uses a new comparison theorem for weak regular splittings. We extend our results to larger classes of iterative methods, which include iterative block Gauss-Seidel. We develop a theory for the concatenation of such iterative methods. This concatenation appears when different numbers of inner interations are performed at each outer step. We also analyze block methods, where different numbers of inner iterations are performed for different diagonal blocks.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 82 (1999), S. 1-20 
    ISSN: 0945-3245
    Keywords: Mathematics Subject Classification (1991):65H10, 65F10
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract. Block parallel iterative methods for the solution of mildly nonlinear systems of equations of the form $Ax=\Phi(x)$ are studied. Two-stage methods, where the solution of each block is approximated by an inner iteration, are treated. Both synchronous and asynchronous versions are analyzed, and both pointwise and blockwise convergence theorems provided. The case where there are overlapping blocks is also considered. The analysis of the asynchronous method when applied to linear systems includes cases not treated before in the literature.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Numerische Mathematik 69 (1994), S. 61-82 
    ISSN: 0945-3245
    Keywords: Mathematics Subject Classification (1991): 65F10, 65F15, 65J10, 15A48, 15A06, 46A22, 90A15
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary. The iterative aggregation method for the solution of linear systems is extended in several directions: to operators on Banach spaces; to the method with inexact correction, i.e., to methods where the (inner) linear system is in turn solved iteratively; and to the problem of finding stationary distributions of Markov operators. Local convergence is shown in all cases. Convergence results apply to the particular case of stochastic matrices. Moreover, an argument is given which suggests why the iterative aggregation method works so well for nearly uncoupled Markov chains, as well as for Markov chains with other zero-nonzero structures.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Numerical algorithms 25 (2000), S. 347-361 
    ISSN: 1572-9265
    Keywords: nonlinear multisplittings ; asynchronous parallel methods
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract Frommer's nonlinear multisplitting methods for solving nonlinear systems of equations are extended to the asynchronous setting. Block methods are extended to include overlap as well. Several specific cases are discussed. Sufficient conditions to guarantee their local convergence are given. A numerical example is presented illustrating the performance of the new approach.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Economic change & restructuring 15 (1979), S. 142-167 
    ISSN: 1574-0277
    Source: Springer Online Journal Archives 1860-2000
    Topics: Economics
    Notes: Conclusions Combining the concepts described in this paper makes it possible to solve inter-regional input-output systems (and other types of large, sparse, linear systems) with considerable efficiency in storage and computation. The exact number of operations and corresponding savings in computational time and storage depend on the particular zero-non zero structure of each matrix in the system, but in any case the savings can be enormous. A recommended procedure is summarized below. 1. Take the entire inter-regional IO system which includes the representation of each of the regions and of the links among them and express it in the formMx=Bz or, more simply,Mx=b (since bothB andz are known). It may be helpful for this purpose to draw a picture of the large matrixM. 2. PartitionM into blocks, exploiting the structure of the particular system. For example, at the very least, the matrix or matrices corresponding to each region will probably be separate blocks. The analysis required for this step may lead to reformulating the matrix representation of the given economic system by, for example, replacing a set of equations with linear combinations of these same equations, particularly for the equations representing the links among regions. 3. Identify an appropriate partition of the blocks ofM, and a corresponding partition of the vectorsx andb, for performing a block factorization. This solution algorithm, which solves forx inMx=b (whereM now represents the entire system) is not available as a packaged program and so for the foreseeable future must be written in ‘home-made’ computer code that assumes a sparse matrix storage scheme compatible with the package to be used (as in 4 below). The algorithm for block factorization in the 2×2 case relevant to our particular inter-regional IO system is given in Section 8 of this paper. 4. Some steps of the algorithm require solving smaller systems of the formHu=v foru, given some matrixH and some vectorv. Efficient packaged subroutines can be obtained at low cost in order to solve these systems by finding the block triangular form corresponding to a particularH, then performing theLU factorization of the diagonal blocks only. The home-made code will interact with these subroutines.
    Type of Medium: Electronic Resource
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