Case of Commutative Rings
For any multiplicatively closed subset S of R, the localization ring is flat as an R-module.
When R is Noetherian and M is a finitely-generated R-module, being flat is the same as being locally free in the following sense: M is a flat R-module if and only if for every prime ideal (or even just for every maximal ideal) P of R, the localization is free as a module over the localization .
If S is an R-algebra, i.e., we have a homomorphism, then S has the structure of an R-module, and hence it makes sense to ask if S is flat over R. If this is the case, then S is faithfully flat over R if and only if every prime ideal of R is the inverse image under f of a prime ideal in S. In other words, if and only if the induced map is surjective.
Flat modules over commutative rings are always torsion-free. Projective modules (and thus free modules) are always flat. For certain common classes of rings, these statements can be reversed (for example, every torsion-free module over a Dedekind ring is automatically flat and flat modules over perfect rings are always projective), as is subsumed in the following diagram of module properties:
Read more about this topic: Flat Module
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