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arXiv 0710.1641cs.CGcs.DMmath.CO
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解缠结几何平面图的一个多项式界

A polynomial bound for untangling geometric planar graphs

Prosenjit Bose, Vida Dujmovic, Ferran Hurtado, Stefan Langerman, Pat Morin, David R. Wood

摘要:

解开一个几何图形意味着移动一些顶点,使得到的几何图形没有交叉。Pach和Tardos(离散计算)Geom.,2002]问是否每个n-顶点几何平面图都可以解开,同时保持至少n^ε个顶点固定。我们用ε=1/4肯定地回答这个问题。之前最好的已知界限是Ω((\log n / \log \log n)^{1/2})。我们还考虑解开几何树。已知每一个n-顶点几何树都能在保持至少(n/3)^{1/2}个顶点不变的情况下解缠结,其最佳上界为O(n \log n)^{2/3}。我们回答了Spillner和Wolff [arXiv:0709.0170 2007]的一个问题,通过关闭这个间隙来解开树。特别地,我们证明了对于无穷多个n值,存在一个n-顶点几何树,当保持多于3(n^{1/2}-1)个顶点固定时,它不能被解开。此外,我们将下界改进为(n/2)^{1/2}。

英文摘要:

To untangle a geometric graph means to move some of the vertices so that the resulting geometric graph has no crossings. Pach and Tardos [Discrete Comput. Geom., 2002] asked if every n-vertex geometric planar graph can be untangled while keeping at least n^εvertices fixed. We answer this question in the affirmative with ε=1/4. The previous best known bound was Ω((\log n / \log\log n)^{1/2}). We also consider untangling geometric trees. It is known that every n-vertex geometric tree can be untangled while keeping at least (n/3)^{1/2} vertices fixed, while the best upper bound was O(n\log n)^{2/3}. We answer a question of Spillner and Wolff [arXiv:0709.0170 2007] by closing this gap for untangling trees. In particular, we show that for infinitely many values of n, there is an n-vertex geometric tree that cannot be untangled while keeping more than 3(n^{1/2}-1) vertices fixed. Moreover, we improve the lower bound to (n/2)^{1/2}.

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