Exact sequence … → P₁ → P₀ → A → 0 with each Pᵢ projective. Unique up to chain-homotopy, so derived functors depend only on A. Injective resolutions play the dual role for right-derived functors like Ext.
Exact sequence … → P₁ → P₀ → A → 0 with each Pᵢ projective. Unique up to chain-homotopy, so derived functors depend only on A. Injective resolutions play the dual role for right-derived functors like Ext.