Krull–Schmidt category

In category theory, a Krull–Schmidt category is a generalization of categories in which the Krull–Schmidt theorem holds. They arise, for example, in the study of finite-dimensional modules over an algebra.

Definition

Let C be an additive category, or more generally an additive R-linear category for a commutative ring R. We call C a Krull-Schmidt category provided that every object decomposes into a finite direct sum of objects having local endomorphism rings. Equivalently, C has split idempotents and the endomorphism ring of every object is semiperfect.

Properties

One has the analogue of the Krull–Schmidt theorem in Krull–Schmidt categories:

An object is called indecomposable if it is not isomorphic to a direct sum of two nonzero objects. In a Krull-Schmidt category we have that

One can define the Auslander–Reiten quiver of a Krull–Schmidt category.

Examples

A non-example

The category of finitely-generated projective modules over the integers has split idempotents, and every module is isomorphic to a finite direct sum of copies of the regular module, the number being given by the rank. Thus the category has unique decomposition into indecomposables, but is not Krull-Schmidt since the regular module does not have a local endomorphism ring.

See also

Notes

  1. This is the classical case, see for example Krause (2012), Corollary 3.3.3.
  2. A finite R-algebra is an R-algebra which is finitely generated as an R-module.
  3. Reiner (2003), Section 6, Exercises 5 and 6, p. 88.
  4. Atiyah (1956), Theorem 2.

References

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