Hereditary ring

In mathematics, especially in the area of abstract algebra known as module theory, a ring R is called hereditary if all submodules of projective modules over R are again projective. If this is required only for finitely generated submodules, it is called semihereditary.

For a noncommutative ring R, the terms left hereditary and left semihereditary and their right hand versions are used to distinguish the property on a single side of the ring. To be left (semi-)hereditary, all (finitely generated) submodules of projective left R-modules must be projective, and to be right (semi-)hereditary all (finitely generated) submodules of projective right submodules must be projective. It is possible for a ring to be left (semi-)hereditary but not right (semi-)hereditary, and vice versa.

Equivalent definitions

• The ring R is left (semi-)hereditary if and only if all (finitely generated) left ideals of R are projective modules. (Lam 1999, p.42)
• The ring R is left hereditary if and only if all left modules have projective resolutions of length at most 1. Hence the usual derived functors such as $\mathrm{Ext}_R^i$ and $\mathrm{Tor}_i^R$ are trivial for $i>1$.

Examples

• Semisimple rings are easily seen to be left and right hereditary via the equivalent definitions: all left and right ideals are summands of R, and hence are projective. By a similar token, in a von Neumann regular ring every finitely generated left and right ideal is a direct summand of R, and so von Neumann regular rings are left and right semihereditary.

• For any nonzero element x in a domain R, $R\cong xR$ via the map $r\mapsto xr$. Hence in any domain, a principal right ideal is free, hence projective. This reflects the fact that domains are right Rickart rings. It follows that if R is a right Bézout domain, so that finitely generated right ideals are principal, then R has all finitely generated right ideals projective, and hence R is right semihereditary. Finally if R is assumed to be a principal right ideal domain, then all right ideals are projective, and R is right hereditary.

• A commutative hereditary integral domain is called a Dedekind domain. A commutative semi-hereditary integral domain is called a Prüfer domain.

• An important example of a (left) hereditary ring is the path algebra of a quiver. This is a consequence of the existence of the standard resolution (which is of length 1) for modules over a path algebra.

References

• Crawley-Boevey, William, Notes on Quiver Representations
• Lam, Tsit-Yuen (1999), Lectures on modules and rings, Graduate Texts in Mathematics No. 189, Berlin, New York: Springer-Verlag, ISBN 978-0-387-98428-5, MR1653294 
• Osborne, M. Scott (2000), Basic Homological Algebra, Graduate Texts in Mathematics, 196, Springer-Verlag, ISBN 0-387-98934-X 
• Weibel, Charles A., An introduction to homological algebra. Cambridge Studies in Advanced Mathematics, 38. Cambridge University Press, Cambridge, 1994. xiv+450 pp. ISBN 0-521-43500-5; 0-521-55987-1