In category theory, the notion of a projective object generalizes the notion of a projective module. Projective objects in abelian categories are used in homological algebra. The dual notion of a projective object is that of an injective object.
Projective objects in abelian categories
An abelian category is said to have enough projectives if, for every object of , there is a projective object of and an epimorphism from P to A or, equivalently, a short exact sequence
The purpose of this definition is to ensure that any object A admits a projective resolution, i.e., a (long) exact sequence
where the objects are projective.
Projectivity with respect to restricted classes
Semadeni (1963) discusses the notion of projective (and dually injective) objects relative to a so-called bicategory, which consists of a pair of subcategories of "injections" and "surjections" in the given category C. These subcategories are subject to certain formal properties including the requirement that any surjection is an epimorphism. A projective object (relative to the fixed class of surjections) is then an object P so that Hom(P, −) turns the fixed class of surjections (as opposed to all epimorphisms) into surjections of sets (in the usual sense).
- The coproduct of two projective objects is projective.
- The retract of a projective object is projective.
The statement that all sets are projective is equivalent to the axiom of choice.
The projective objects in the category of abelian groups are the free abelian groups.
Let be a ring with 1. Consider the (abelian) category of left -modules . The projective objects in are precisely the projective left R-modules. Consequently, is itself a projective object in Dually, the injective objects in are exactly the injective left R-modules.
The category of left (right) -modules also has enough projectives. This is true since, for every left (right) -module , we can take to be the free (and hence projective) -module generated by a generating set for (we can in fact take to be ). Then the canonical projection is the required surjection.
The projective objects in the category of compact Hausdorff spaces are precisely the extremally disconnected spaces. This result is due to Gleason (1958), with a simplified proof given by Rainwater (1959).
In the category of Banach spaces and contractions (i.e., functionals whose norm is at most 1), the epimorphisms are precisely the maps with dense image. Wiweger (1969) shows that the zero space is the only projective object in this category. There are non-trivial spaces, though, which are projective with respect to the class of surjective contractions. In the category of normed linear spaces with contractions (and surjective maps as "surjections"), the projective objects are precisely the -spaces
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'"projective object in nLab". ncatlab.org. Retrieved 2017-10-17.