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arXiv 1202.5569math.PRcs.DM
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图上随机游动的覆盖时间

The Cover Time of Random Walks on Graphs

Mohammed Abdullah

摘要:

图上的简单随机游动是从一个顶点到另一个顶点的运动序列,其中在每一步中,从当前顶点上的边集合中随机均匀地选择一条边,然后过渡到下一个顶点。本论文的核心是行走的覆盖时间,即访问每个顶点所需的步数的期望,在所有起始顶点上最大化。在我们的第一个贡献中,我们建立了一对图的覆盖时间与它们的笛卡儿积的覆盖时间之间的关系。这扩展了以前的工作的特殊情况下的笛卡儿积,特别是平方的一个图。我们表明,当其中一个因素是在某种意义上大于其他,其覆盖时间占主导地位,并可以成为一个对数因子的覆盖时间的产品作为一个整体。我们的主要定理有效地给出了这种情况成立的条件。我们介绍的技术和引理可能是独立的利益。在我们的第二个贡献中,我们确定了具有给定度序列的随机图的覆盖时间的精确渐近值。这是从所有具有该度序列的单图中随机均匀选取的一个图。我们还表明,与高概率,称为电导图的结构特性,是有界的常数。这是独立的利益。最后,我们探讨加权随机边选择的随机游动。我们提出了一个加权方案,有一个更小的最坏情况下的覆盖时间比一个简单的随机游动。给出了度序列按本文方案加权的随机图的一个上界。我们证明了简单随机游动的加速(即覆盖时间的比率)可以是无界的

英文摘要:

A simple random walk on a graph is a sequence of movements from one vertex to another where at each step an edge is chosen uniformly at random from the set of edges incident on the current vertex, and then transitioned to next vertex. Central to this thesis is the cover time of the walk, that is, the expectation of the number of steps required to visit every vertex, maximised over all starting vertices. In our first contribution, we establish a relation between the cover times of a pair of graphs, and the cover time of their Cartesian product. This extends previous work on special cases of the Cartesian product, in particular, the square of a graph. We show that when one of the factors is in some sense larger than the other, its cover time dominates, and can become within a logarithmic factor of the cover time of the product as a whole. Our main theorem effectively gives conditions for when this holds. The techniques and lemmas we introduce may be of independent interest. In our second contribution, we determine the precise asymptotic value of the cover time of a random graph with given degree sequence. This is a graph picked uniformly at random from all simple graphs with that degree sequence. We also show that with high probability, a structural property of the graph called conductance, is bounded below by a constant. This is of independent interest. Finally, we explore random walks with weighted random edge choices. We present a weighting scheme that has a smaller worst case cover time than a simple random walk. We give an upper bound for a random graph of given degree sequence weighted according to our scheme. We demonstrate that the speed-up (that is, the ratio of cover times) over a simple random walk can be unbounded

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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