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151.
152.
We give a complete characterization of countable primitive groups in several settings including linear groups, subgroups of mapping class groups, groups acting minimally on trees and convergence groups. The latter category includes as a special case Kleinian groups as well as subgroups of word hyperbolic groups. As an application we calculate the Frattini subgroup in many of these settings, often generalizing results that were only known for finitely generated groups. In particular, we answer a question of G. Higman and B.H. Neumann on the Frattini group of an amalgamated product. Received: January 2006, Revision: May 2006, Accepted: May 2006 相似文献
153.
Yair Shapira 《Numerical Methods for Partial Differential Equations》2002,18(3):280-295
Finite difference schemes for the numerical solution of singularly perturbed convection problems on uniform grids are studied in the limit case where the viscosity and the meshsize approach zero at the same time. The present error estimates are given in terms of order of magnitude in the above limit process and are useful in a priori choosing adequate schemes and meshsizes for boundary‐layer problems and problems with closed characteristics. Published 2002 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 18: 280–295, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/num.10007 相似文献
154.
A main result proved in this paper is the following. Theorem. Let G be a noncomplete graph on n vertices with degree sequence d1 ≥ d2 ≥ · · · ≥ dn and t ≥ 2 be a prime. Let m = gcd{t, di − dj: 1 ≤ i < j ≤ n} and set Then R(tG, ℤt) = t(n + d) − d, where R is the zero-sum Ramsey number. This settles, almost completely, problems raised in [Bialostocki & Dierker, J Graph Theory, 1994; Y. Caro, J Graph Theory, 1991]. © 1999 John Wiley & Sons, Inc. J Graph Theory 32: 207–216, 1999 相似文献
155.
Yair Shapira 《Numerical Linear Algebra with Applications》1999,6(8):655-685
A multilevel method for the solution of sparse linear systems is introduced. The method is defined in terms of the coefficient matrix alone; no underlying PDE or mesh is assumed. An upper bound for the condition number is available for a class of SPD problems. In particular, for certain discretizations of diffusion boundary value problems, this bound grows only polynomially with the number of levels used, regardless of whether or not the discontinuities in the diffusion coefficient align with the coarse grids. Numerical results in line with the analysis are presented. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
156.
Let d be a positive integer. A graph G is called d-divisible if d divides the degree of each vertex of G. G is called nowhere d-divisible if no degree of a vertex of G is divisible by d. For a graph H, gcd(H) denotes the greatest common divisor of the degrees of the vertices of H. The H-packing number of G is the maximum number of pairwise edge disjoint copies of H in G. The H-covering number of G is the minimum number of copies of H in G whose union covers all edges of G. Our main result is the following: For every fixed graph H with gcd(H) = d, there exist positive constants ϵ(H) and N(H) such that if G is a graph with at least N(H) vertices and has minimum degree at least (1 − ϵ(H))|G|, then the H-packing number of G and the H-covering number of G can be computed in polynomial time. Furthermore, if G is either d-divisible or nowhere d-divisible, then there is a closed formula for the H-packing number of G, and the H-covering number of G. Further extensions and solutions to related problems are also given. © 1998 John Wiley & Sons, Inc. J Combin Designs 6: 451–472, 1998 相似文献
157.
Yair Shapira 《Numerical Linear Algebra with Applications》1998,5(3):165-202
Convergence theory for a multigrid method with matrix-dependent restriction, prolongation and coarse-grid operators is developed for a class of SPD problems. It motivates the construction of improved multigrid versions for diffusion problems with discontinuous coefficients. A computational two-level analysis method for a class of separable problems is also available. It motivates the design of matrix-dependent multigrid algorithms and, in particular, multiple coarse-grid correction algorithms for highly indefinite equations. Numerical experiments show the advantage of the present methods for several examples. © 1998 John Wiley & Sons, Ltd. 相似文献
158.
In many fields of psychology, it may be interesting to measure the potential number of structure‐preserving transformations that exist between succeeding structures. The aim of this article is to present a methodology for measuring the potential number of structure‐preserving transformations between succeeding structures and to illustrate the applicability of the methodology through a case study. The article concludes by discussing the lessons and implications of the proposed methodology for microgenetic research. © Wiley Periodicals, Inc. Complexity, 2012 相似文献
159.
We consider pollution that arises from the leakage of a container into a contiguous water region. It is shown that if one considers this to be an “effective diffusion” process, the creation of a “Controlled Pollution Buffer Zone” significantly decreases the degradation of the water quality in the adjacent region. 相似文献
160.
The problem of finding a point in the intersection of a finite family of convex sets in the Euclidean space R″ is considered here. We present a general algorithmic scheme which employs projections onto separating hyperplanes instead of projections onto the convex sets. This scheme includes the method of successive projections of Gubin et al., USSR Comp. Math. and Math. Phys. 7 (1967), 1–24, as a special case. A different realization proposed here is capable of handling the problem when the sets are solid and an interior point of each set is available. This alternative algorithm may, in certain cases, be more attractive than the method of Gubin et al. 相似文献