Injective resolution

Summary

An injective resolution of an object $X$ is an exact sequence $0 \to X \to I^0 \to I^1 \to \cdots$ in which every $I^n$ is injective. It is the dual of a projective resolution. Right derived functors of a left exact functor are computed by applying the functor to an injective resolution. Examples are Ext in the second variable and sheaf cohomology.