In the branch of abstract mathematics called category theory, a projective cover of an object X is in a sense the best approximation of X by a projective object P. Projective covers are the dual of injective envelopes.
Contents |
Let be a category and X an object in . A projective cover is a pair (P,p), with P a projective object in and p a superfluous epimorphism in Hom(P, X).
If R is a ring, then in the category of R-modules, a superfluous epimorphism is then an epimorphism such that the kernel of p is a superfluous submodule of P.
Projective covers and their superfluous epimorphisms, when they exist, are unique up to isomorphism.
The main effect of p having a superfluous kernel is the following: if N is any proper submodule of P, then [1]. Informally speaking, the superfluous kernel causes P to cover M optimally, in some sense. This does not depend upon the projectivity of P, it is true of all superfluous epimorphisms.
If (P,p) is a projective cover of M, and P' is another projective module with an epimorphism , then there is a split epimorphism α from P' to P such that
Unlike injective envelopes, which exist for every left (right) R-module regardless of the ring R, left (right) R-modules do not in general have projective covers. A ring R is called left (right) perfect if every left (right) R-module has a projective cover in R-Mod (Mod-R).
A ring is called semiperfect if every finitely generated left (right) R-module has a projective cover in R-Mod (Mod-R). "Semiperfect" is a left-right symmetric property.
A ring is called lift/rad if idempotents lift from R/J to R, where J is the Jacobson radical of R. The property of being lift/rad can be characterized in terms of projective covers: R is lift/rad if and only if direct summands of the R module R/J (as a right or left module) have projective covers. (Anderson, Fuller 1992, p. 302)
In the category of R modules: