A sequence of modules/groups and maps A → B → C where im(A→B) = ker(B→C). Long exact sequences (homology pair, Mayer–Vietoris) and short exact sequences (ext/Tor) are staples of homological algebra.
A sequence of modules/groups and maps A → B → C where im(A→B) = ker(B→C). Long exact sequences (homology pair, Mayer–Vietoris) and short exact sequences (ext/Tor) are staples of homological algebra.