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P.D Seymour 《Journal of Combinatorial Theory, Series B》1980,28(3):305-359
It is proved that every regular matroid may be constructed by piecing together graphic and cographic matroids and copies of a certain 10-element matroid. 相似文献
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Mowat JP Seymour VR Griffin JM Thompson SP Slawin AM Fairen-Jimenez D Düren T Ashbrook SE Wright PA 《Dalton transactions (Cambridge, England : 2003)》2012,41(14):3937-3941
The scandium analogue of the flexible terephthalate MIL-53 yields a novel closed pore structure upon removal of guest molecules which has unusual thermal behaviour and stepwise opening during CO(2) adsorption. By contrast, the nitro-functionalised MIL-53(Sc) cannot fully close and the structure possesses permanent porosity for CO(2). 相似文献
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Maria Chudnovsky Alexandra FradkinPaul Seymour 《Journal of Combinatorial Theory, Series B》2012,102(1):93-101
A (loopless) digraph H is strongly immersed in a digraph G if the vertices of H are mapped to distinct vertices of G, and the edges of H are mapped to directed paths joining the corresponding pairs of vertices of G, in such a way that the paths used are pairwise edge-disjoint, and do not pass through vertices of G that are images of vertices of H. A digraph has cutwidth at most k if its vertices can be ordered {v1,…,vn} in such a way that for each j, there are at most k edges uv such that u∈{v1,…,vj−1} and v∈{vj,…,vn}.We prove that for every set S of tournaments, the following are equivalent:
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- there is a digraph H such that H cannot be strongly immersed in any member of S,
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- there exists k such that every member of S has cutwidth at most k,
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- there exists k such that every vertex of every member of S belongs to at most k edge-disjoint directed cycles.
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Given k pairs of vertices (si,ti)(1≤i≤k) of a digraph G, how can we test whether there exist k vertex-disjoint directed paths from si to ti for 1≤i≤k? This is NP-complete in general digraphs, even for k=2 [2], but for k=2 there is a polynomial-time algorithm when G is a tournament (or more generally, a semicomplete digraph), due to Bang-Jensen and Thomassen [1]. Here we prove that for all fixed k there is a polynomial-time algorithm to solve the problem when G is semicomplete. 相似文献
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