Categorical and Homological Algebra

Branches that study algebra through maps between structures and through invariants computed from chain complexes. An entry belongs here when its cited source describes categories, functors and natural transformations as a language that unifies mathematics, or the homological methods that measure how far algebraic constructions fail to be exact. It does not belong here when the branch concerns one concrete structure and its internal properties, which is the business of the other algebra groups. The group reflects the Mathematics Subject Classification, which treats category theory and homological algebra as their own part of the algebraic landscape.