$C(\mathcal{X^{*}})$-Cover and $C(\mathcal{X^{*}})$-Envelope
  Let $R$ be any associative ring with unity and $\mathcal{X}$ be a class of $R$-modules of closed under direct sum (and summands) and with extension closed. We prove that every complex has an $C(\mathcal{X^{*}})$-cover ($C(\mathcal{X^{*}})$-envelope) if every module has an $\mathcal{X}$-cover ($\mathcal{X}$-envelope) where $C(\mathcal{X^{*}})$ is the class of complexes of modules in $\mathcal{X}$ such that it is closed under direct and inverse limit. 
Tah\.ire \"Ozen, Em\.ine Y{\i}ld{\i}r{\i}m
math.RA