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arXiv 1609.00744math.COmath.LO
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无限随机图的哪些子集看起来是随机的?

Which subsets of an infinite random graph look random?

Will Brian

摘要:

给定一个可数图,如果它包含每个可数图作为导出子图,我们称它的顶点集$A$为\emph{universal};如果它包含每个有限图作为诱导子图,我们称它的顶点集$A$为\emph{weakly universal}.证明了对几乎所有的$\mathbb N$,$(1)$上的正上密度集是泛图,$(2)$上的倒数和发散集是弱泛图.我们证明了第二个结果是尖锐的(即,$\mathbb N$上的随机图几乎必然包含具有发散倒数和的非泛集),并且更一般地,这两个结果对于一大类划分正则族都不成立。

英文摘要:

Given a countable graph, we say a set $A$ of its vertices is \emph{universal} if it contains every countable graph as an induced subgraph, and $A$ is \emph{weakly universal} if it contains every finite graph as an induced subgraph. We show that, for almost every graph on $\mathbb N$, $(1)$ every set of positive upper density is universal, and $(2)$ every set with divergent reciprocal sums is weakly universal. We show that the second result is sharp (i.e., a random graph on $\mathbb N$ will almost surely contain non-universal sets with divergent reciprocal sums) and, more generally, that neither of these two results holds for a large class of partition regular families.

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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