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arXiv 1302.1634math.COmath.PR
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超图极限:一种正则性方法

Hypergraph limits: a regularity approach

Yufei Zhao

摘要:

一个$k$ -一致超图序列$H_1, H_2, \dots$是收敛的,如果对于每个$k$ -一致超图$F$,同态密度序列$t(F, H_1), t(F, H_2), \dots$是收敛的.对于图,Lovász和Szegedy证明了每个收敛序列都有一个极限,其形式为对称可测函数$W \colon [0,1]^2 \to [0,1]$。对于超图,Elek和Szegedy使用超积构造了类似的极限$W \colon [0,1]^{2^k-2} \to [0,1]$。胡佛、奥尔德斯和卡伦伯格在可交换随机阵列的背景下也研究过这些极限。本文给出了超图极限的一个新的证明和构造。我们的方法受到Lovász和Szegedy的原始方法的启发,关键成分是弱Frieze-Kannan型正则性引理。

英文摘要:

A sequence of $k$-uniform hypergraphs $H_1, H_2, \dots$ is convergent if the sequence of homomorphism densities $t(F, H_1), t(F, H_2), \dots$ converges for every $k$-uniform hypergraph $F$. For graphs, Lovász and Szegedy showed that every convergent sequence has a limit in the form of a symmetric measurable function $W \colon [0,1]^2 \to [0,1]$. For hypergraphs, analogous limits $W \colon [0,1]^{2^k-2} \to [0,1]$ were constructed by Elek and Szegedy using ultraproducts. These limits had also been studied earlier by Hoover, Aldous, and Kallenberg in the setting of exchangeable random arrays. In this paper, we give a new proof and construction of hypergraph limits. Our approach is inspired by the original approach of Lovász and Szegedy, with the key ingredient being a weak Frieze-Kannan type regularity lemma.

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