On the minimum of independent collecting processes via the Stirling numbers of the second kind

Abstract

We consider the combinatorial problem where $p$ players aim to a complete set of $N$ different types of items (species) which are uniformly distributed. Let the random variables $T_{N(i)},i=1,2,\cdots ,p$ denoting the number of trials needed until all $N$ types are detected (at least once), respectively for each player. This paper studies the impact of the number $p$ in the asymptotics of the expectation, the second moment, and the variance of the random variable \begin{equation*} M_{N(p)}: = \bigwedge_{i=1}^p T_{N(i)},\,\,\,\,\,\,N\rightarrow \infty. \end{equation*} The main ingredient in the expression of these quantittes are sums involving the Stirling numbers of the second kind; for which the asymptotics are explored. At the end of the paper we conjecture on a remarkable \textit{combinatorial identity}, regarding alternating binomial sums. These sums have been studied (mainly) by P. Flajolet due to their applications to digital search trees and quadtrees.