Published online by Cambridge University Press: 20 November 2018
Let $R$ be an arbitrary ring and let ${{\left( - \right)}^{+}}\,=\,\text{Ho}{{\text{m}}_{\mathbb{Z}}}\left( -,\,{\mathbb{Q}}/{\mathbb{Z}}\; \right)$, where $\mathbb{Z}$ is the ring of integers and $\mathbb{Q}$ is the ring of rational numbers. Let $\mathcal{C}$ be a subcategory of left $R$-modules and $\mathcal{D}$ a subcategory of right $R$-modules such that ${{X}^{+}}\,\in \,\mathcal{D}$ for any $X\,\in \,\mathcal{C}$ and all modules in $\mathcal{C}$ are pure injective. Then a homomorphism $f:\,A\to \,C$ of left $R$-modules with $C\,\in \,\mathcal{C}$ is a $\mathcal{C}$-(pre)envelope of $A$ provided ${{f}^{+}}:\,{{C}^{+}}\,\to \,{{A}^{+}}$ is a $\mathcal{D}$-(pre)cover of ${{A}^{+}}$. Some applications of this result are given.
This research was partially supported by the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20100091110034), NSFC (Grant No. 11171142), NSF of Jiangsu Province of China (Grant Nos. BK2010047, BK2010007) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions