arXiv 1602.035692016-02-10math.CO

非一致非拥挤超图的独立数和一个反Ramsey型结果

The independence number of non-uniform uncrowded hypergraphs and an anti-Ramsey type result

Sang June Lee, Hanno Lefmann

备注：: 17 pages

摘要：

我们证明如下：固定一个整数$k\geq 2$，并让$T$是一个实数与$T\geq 1.5$。设$\cH=(V,\cE_2\cup \cE_3\cup\dots\cup\cE_k)$是一个顶点集为$V$，边集为$i=2,\ldots , k$的非均匀超图.假设$\cH$没有$2$ -圈（不管边的大小），并且既不包含由$2$ -元素边组成的$3$ -圈也不包含由$2$ -元素边组成的$4$ -圈。如果平均度$t_i^{i-1} := i |\cE_i|/ |V|$满足$i= 2, \dots , k$的$t_i^{i-1} \leq T^{i-1} (\ln T)^{\frac{k-i}{k-1}}$，则存在一个常数$C_k > 0$，仅取决于$k$，使得$α(\cH)\geq C_k \frac{|V|}{T} (\ln T)^{\frac{1}{k-1}}$，其中$α(\cH)$表示$\cH$的独立数。这扩展了Ajtai，Komlós，Pintz，Spencer和Szemerédi以及Duke，Rödl和第二作者关于一致超图的结果。作为应用，我们考虑了非一致超图上的一个反Ramsey型问题。设$\cH=\cH(n;2,\ldots,\ell)$是$n$ -顶点集$V$上的超图，其中对$s=2,\ldots,\ell$，$V$的每个$s$ -子集是$\cH$的超边.设$Δ$是$\cH$的边着色，满足：（a）共享一个顶点的两个超边有不同的颜色;（b）两个大小不同的超边有不同的颜色;（c）用于大小为$s$的超边的颜色最多出现$u_s$次。对于这样的着色$Δ$，令$f_Δ(n;u_2,\ldots,u_{\ell})$是$V$的子集$U$的最大大小，使得$\cH[U]$的每个超边具有不同的颜色，并且令$f(n;u_2,\ldots,u_{\ell}):=\min_Δ f_Δ(n;u_2,\ldots,u_{\ell}).$我们确定$f(n;u_2,\ldots,u_{\ell})$直到乘法对数因子。

英文摘要：

We prove the following: Fix an integer $k\geq 2$, and let $T$ be a real number with $T\geq 1.5$. Let $\cH=(V,\cE_2\cup \cE_3\cup\dots\cup\cE_k)$ be a non-uniform hypergraph with the vertex set $V$ and the set $\cE_i$ of edges of size $i=2,\ldots , k$. Suppose that $\cH$ has no $2$-cycles (regardless of sizes of edges), and neither contains $3$-cycles nor $4$-cycles consisting of $2$-element edges. If the average degrees $t_i^{i-1} := i |\cE_i|/ |V|$ satisfy that $t_i^{i-1} \leq T^{i-1} (\ln T)^{\frac{k-i}{k-1}}$ for $i= 2, \dots , k$, then there exists a constant $C_k > 0$, depending only on $k$, such that $α(\cH)\geq C_k \frac{|V|}{T} (\ln T)^{\frac{1}{k-1}}$, where $α(\cH)$ denotes the independence number of $\cH$. This extends results of Ajtai, Komlós, Pintz, Spencer and Szemerédi and Duke, Rödl and the second author for uniform hypergraphs. As an application, we consider an anti-Ramsey type problem on non-uniform hypergraphs. Let $\cH=\cH(n;2,\ldots,\ell)$ be the hypergraph on the $n$-vertex set $V$ in which, for $s=2,\ldots,\ell$, each $s$-subset of $V$ is a hyperedge of $\cH$. Let $Δ$ be an edge-coloring of $\cH$ satisfying the following: (a) two hyperedges sharing a vertex have different colors; (b) two hyperedges with distinct size have different colors; (c) a color used for a hyperedge of size $s$ appears at most $u_s$ times. For such a coloring $Δ$, let $f_Δ(n;u_2,\ldots,u_{\ell})$ be the maximum size of a subset $U$ of $V$ such that each hyperedge of $\cH[U]$ has a distinct color, and let $f(n;u_2,\ldots,u_{\ell}):=\min_Δ f_Δ(n;u_2,\ldots,u_{\ell}).$ We determine $f(n;u_2,\ldots,u_{\ell})$ up to a multiplicative logarithm factor.

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