共查询到20条相似文献,搜索用时 15 毫秒
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Masoud Hassani 《Comptes Rendus Mathematique》2017,355(11):1133-1137
In this paper, we study the irreducible representation of in . This action preserves a quadratic form with signature . Thus, it acts conformally on the 3-dimensional Einstein universe . We describe the orbits induced in and its complement in . This work completes the study in [2], and is one element of the classification of cohomogeneity one actions on [5]. 相似文献
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A Steiner 2- trade is a pair of disjoint partial Steiner triple systems, each on the same set of points, such that each pair of points occurs in if and only if it occurs in . A Steiner 2- trade is called d-homogeneous if each point occurs in exactly d blocks of (or ). In this paper we construct minimal d-homogeneous Steiner 2- trades of foundation and volume for sufficiently large values of . (Specifically, if is divisible by 3 and otherwise.) 相似文献
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A.J.W. Hilton 《Discrete Mathematics》2008,308(5-6):645-669
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A note on two source location problems 总被引:1,自引:1,他引:0
We consider Source Location () problems: given a capacitated network , cost and a demand for every , choose a min-cost so that holds for every , where is the maximum flow value from v to S. In the directed variant, we have demands and and we require and . Undirected is (weakly) NP-hard on stars with for all v except the center. But, it is known to be polynomially solvable for uniform costs and uniform demands. For general instances, both directed an undirected admit a -approximation algorithms, where D is the sum of the demands; up to constant this is tight, unless P = NP. We give a pseudopolynomial algorithm for undirected on trees with running time , where . This algorithm is used to derive a linear time algorithm for undirected with . We also consider the Single Assignment Source Location () where every should be assigned to a single node . While the undirected is in P, we give a -approximation algorithm for the directed case, and show that this is tight, unless P = NP. 相似文献
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Sizhong Zhou 《Comptes Rendus Mathematique》2009,347(21-22):1223-1226
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An configuration is a set of points and lines such that each point lies on lines while each line contains points. The configuration is geometric, topological, or combinatorial depending on whether lines are considered to be straight lines, pseudolines, or just combinatorial lines. The existence and enumeration of configurations for a given has been subject to active research. A current front of research concerns geometric configurations: it is now known that geometric configurations exist for all , apart from sporadic exceptional cases. In this paper, we settle by computational techniques the first open case of configurations: we obtain all topological configurations among which none are geometrically realizable. 相似文献
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《Comptes Rendus Mathematique》2008,346(11-12):667-670
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