Covers,Envelopes And Relative Flat Dimensions

The main object of this dissertation is to study covers,envelopes and relative flat dimensions.We review the concepts of covers,envelopes,cotorsion pairs,duality pairs,and we study the basic properties and results.The existences of(pre)covers and(pre)envelopes of flat modules and injective modules are studid.Furthermore,left(right)resolutions and right derived functors of of-(?)-are constructed via flat preenvelopes and injective preenvelopes provided that R is left noetherian.As an application,we give some new characterizations of global dimensions of a left noetherian ring R.