On a Problem of Erd\H{o}s, Herzog and Sch\"onheim

Let $p_1, p_2,..., p_n$ be distinct primes. In 1970, Erd\H os, Herzog and Sch\"{o}nheim proved that if $\cal D$ is a set of divisors of $N=p_1^{\alpha_1}...p_n^{\alpha_n}$, $\alpha_1\ge \alpha_2\ge...\ge \alpha_n$, no two members of the set being coprime and if no additional member may be included in $\cal D$ without contradicting this requirement then $|{\cal D}|\ge \alpha_n \prod_{i=1}^{n-1} (\alpha_i +1)$. They asked to determine all sets $\cal D$ such that the equality holds. In this paper we solve this problem. We also pose several open problems for further research.