讨论范畴R-mod,R-gr,Re-mod,R#G-mod中的拉回性质之间的关系,得到分次范畴中的交换图成为拉回图当且仅当其也是模范畴中的拉回图等结论.
Discusses the relationships among properties of pullbacks in R-mod,R-gr,Re-mod and R#G-mod,and obtains some conclusions such as commutative diagram in R-gr is pullback if and only if it's in R-mod.