nLab element in an abelian category



Additive and abelian categories

Homological algebra

homological algebra

(also nonabelian homological algebra)



Basic definitions

Stable homotopy theory notions



diagram chasing

Schanuel's lemma

Homology theories




Not every abelian category is a concrete category, such as Ab or RRMod, hence its objects do not necessarily have underlying sets whose elements one can reason about. To circumvent this and reason by “diagram chases” of elements in general abelian categories, one can instead use generalized elements in a suitable way.

This is closely related to embedding the abelian category fully-faithfully and exactly into a concrete abelian category. See at abelian category the section Embedding theorems for more on this.


One definition goes likes this (MacLane, VIII.4 and also for instance Gelfand-Manin, II.5):

An element of an object WW in a given abelian category 𝒜\mathcal{A} is an equivalence class [X,h][X,h] of pairs (X,h)(X,h) where XX is an object of 𝒜\mathcal{A} and h:XWh:X\to W a morphism (hence a generalized element) and the equivalence is defined as follows: [X,h]=[Y,k][X,h] = [Y,k] iff there exists an object ZZ in 𝒜\mathcal{A} and epimorphisms u:ZXu:Z\to X, v:ZYv:Z\to Y such that hu=kv:ZWh\circ u = k\circ v : Z\to W.

However, beware that the passage to equivalence classes does not respect the abelian group structure and hence generalized elements in this sense cannot be added or subtracted. A more natural approach is discussed in Bergman 1974 where the actual generalized elements are remembered but a refinement of their domain is allowed, much as familiar from topos theory.


Equivalence classes of generalized elements are considered for instance in

  • Sergei Gelfand, Yuri Manin, Methods of homological algebra, transl. from the 1988 Russian (Nauka Publ.) original. Springer 1996. xviii+372 pp.; 2nd corrected ed. 2002.

Genuine generalized elements are considered in

  • George Bergman, A note on abelian categories – translating element-chasing proofs, and exact embedding in abelian groups (1974) [pdf, pdf]

Last revised on January 13, 2024 at 05:18:26. See the history of this page for a list of all contributions to it.