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