For an integer $q\ge 2$, a perfect $q$-hash code $C$ is a block code over $[q]:=\{1,\ldots,q\}$ of length $n$ in which every subset $\{\mathbf{c}_1,\mathbf{c}_2,\dots,\mathbf{c}_q\}$ of $q$ elements is separated, i.e., there exists $i\in[n]$ such that $\{\mathrm{proj}_i(\mathbf{c}_1),\dots,\mathrm{proj}_i(\mathbf{c}_q)\}=[q]$, where $\mathrm{proj}_i(\mathbf{c}_j)$ denotes the $i$th position of $\mathbf{c}_j$. Finding the maximum size $M(n,q)$ of perfect $q$-hash codes of length $n$, for given $q$ and $n$, is a fundamental problem in combinatorics, information theory, and computer science. In this paper, we are interested in asymptotic behavior of this problem. Precisely speaking, we will focus on the quantity $R_q:=\limsup_{n\rightarrow\infty}\frac{\log_2 M(n,q)}n$. A well-known probabilistic argument shows an existence lower bound on $R_q$, namely $R_q\ge\frac1{q-1}\log_2\left(\frac1{1-q!/q^q}\right)$ \cite{FK,K86}. This is still the best-known lower bound till now except for the case $q=3$ \cite{KM}. The improved lower bound of $R_3$ was discovered in 1988 and there has been no progress on the lower bound of $R_q$ for more than $30$ years. In this paper we show that this probabilistic lower bound can be improved for $q$ from $4$ to $15$ and all odd integers between $17$ and $25$, and \emph{all sufficiently large} $q$.

翻译：对于整数$q\ge 2$，一个完美$q$哈希码$C$是定义在字母表$[q]:=\{1,\ldots,q\}$上的长度为$n$的分组码，其中任意$q$个元素构成的子集$\{\mathbf{c}_1,\mathbf{c}_2,\dots,\mathbf{c}_q\}$均被分离，即存在$i\in[n]$使得$\{\mathrm{proj}_i(\mathbf{c}_1),\dots,\mathrm{proj}_i(\mathbf{c}_q)\}=[q]$，这里$\mathrm{proj}_i(\mathbf{c}_j)$表示$\mathbf{c}_j$的第$i$个位置。在给定$q$和$n$的条件下，寻找长度为$n$的完美$q$哈希码的最大尺寸$M(n,q)$是组合学、信息论和计算机科学中的一个基本问题。本文关注该问题的渐近行为。具体而言，我们将聚焦于量$R_q:=\limsup_{n\rightarrow\infty}\frac{\log_2 M(n,q)}n$。一个著名的概率论证给出了$R_q$的存在性下界，即$R_q\ge\frac1{q-1}\log_2\left(\frac1{1-q!/q^q}\right)$ \cite{FK,K86}。除$q=3$的情况外，这至今仍是最优已知下界\cite{KM}。$R_3$下界的改进发现于1988年，此后三十余年$R_q$的下界再无进展。本文证明：该概率下界可对$q=4$至$15$、$17$至$25$之间的所有奇数以及\textit{所有充分大的}$q$加以改进。