Supported by the Natural Science Foundation of Fujian Province (Grant No. 2009J01003);the Foundation of Fujian Normal University (Grant No. 2008100209);partially by the Open Funds of Key Lab of Fujian Province University Network Security and Cryptology (Grant No. 09A004)
In a pushout-pullback diagram, which consists of four morphisms f : A → B, g : A → C, α : C → D and β : B → D, we give some relations among the covers of these four modules. If ker f∈I( ), then g : A → ...