Beating the probabilistic lower bound on $q$-perfect hashing (1908.08792v5)

Abstract: 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} $q$.