Given a short exact sequence 0 → A' → A → A'' → 0 and projective resolutions P'_• → A' and P''_• → A'', the horseshoe lemma constructs a projective resolution of A whose n-th term is P'_n ⊕ P''_n and which fits between the two given…
Given a short exact sequence 0 → A' → A → A'' → 0 and projective resolutions P'_• → A' and P''_• → A'', the horseshoe lemma constructs a projective resolution of A whose n-th term is P'_n ⊕ P''_n and which fits between the two given…