Projective module

In mathematics, particularly in abstract algebra and homological algebra, the class of projective modules enlarges the class of free modules (that is, modules with basis vectors) over a ring, by keeping some of the main properties of free modules. Various equivalent characterizations of these modules appear below. A free module is a projective module, but the converse may be wrong over some rings, such as Dedekind rings. However, every projective module is a free module over a principal ideal domain, and over a polynomial ring over a field or the integers (this is Quillen–Suslin theorem).

Projective module

In mathematics, particularly in abstract algebra and homological algebra, the class of projective modules enlarges the class of free modules (that is, modules with basis vectors) over a ring, by keeping some of the main properties of free modules. Various equivalent characterizations of these modules appear below. A free module is a projective module, but the converse may be wrong over some rings, such as Dedekind rings. However, every projective module is a free module over a principal ideal domain, and over a polynomial ring over a field or the integers (this is Quillen–Suslin theorem).