Davenport constant of the multiplicative semigroup of the ring   $\mathbb{Z}_{n_1}\oplus\cdots\oplus \mathbb{Z}_{n_r}$
  Given a finite commutative semigroup $\mathcal{S}$ (written additively), denoted by ${\rm D}(\mathcal{S})$ the Davenport constant of $\mathcal{S}$, namely the least positive integer $\ell$ such that for any $\ell$ elements $s_1,\ldots,s_{\ell}\in \mathcal{S}$ there exists a set $I\subsetneq [1,\ell]$ for which $\sum_{i\in I} s_i=\sum_{i=1}^{\ell} s_i$.   Then, for any integers $r\geq 1, n_1,\ldots,n_r>1$, let $R=\mathbb{Z}_{n_1}\oplus\cdots\oplus \mathbb{Z}_{n_r}$ be the direct sum of these $r$ residue class rings $\mathbb{Z}_{n_1}, \ldots,\mathbb{Z}_{n_r}$. Moreover, let $\mathcal{S}_R$ be the multiplicative semigroup of the ring $R$, and ${\rm U}(\mathcal{S}_R)$ the group of units of $\mathcal{S}_R$. In this paper, we prove that $${\rm D}({\rm U}(\mathcal{S}_R))+P_2\leq {\rm D}(\mathcal{S}_R)\leq {\rm D}({\rm U}(\mathcal{S}_R))+\delta,$$ where $P_2=\sharp\{i\in [1,r]: 2 \parallel n_i\}$ and $\delta=\sharp\{i\in [1,r]: 2\mid n_i\}.$ This corrects our previous published wrong result on this problem. 
Guoqing Wang and Weidong Gao
math.CO math.NT