Right-derived functors of Hom: Extⁿ_R(A,B) = Hⁿ(Hom_R(P•, B)). Ext¹ classifies extensions 0 → B → E → A → 0; Ext^n classifies n-fold Yoneda extensions modulo an equivalence relation.
Right-derived functors of Hom: Extⁿ_R(A,B) = Hⁿ(Hom_R(P•, B)). Ext¹ classifies extensions 0 → B → E → A → 0; Ext^n classifies n-fold Yoneda extensions modulo an equivalence relation.