Free presentation

In algebra, a free presentation of a module M over a commutative ring R is an exact sequence of R-modules:

Note the image of g is a generating set of M. In particular, if J is finite, then M is a finitely generated module. If I and J are finite sets, then the presentation is called a finite presentation; a module is called finitely presented if it admits a finite presentation.

Since f is a module homomorphism between free modules, it can be visualized as an (infinite) matrix with entries in R and M as its cokernel.

A free presentation always exists: any module is a quotient of a free module: , but then the kernel of g is again a quotient of a free module: . The combination of f and g is a free presentation of M. Now, one can obviously keep "resolving" the kernels in this fashion; the result is called a free resolution. Thus, a free presentation is the early part of the free resolution.

A presentation is useful for computaion. For example, since tensoring is right-exact, tensoring the above presentation with a module, say, N gives:

This says that is the cokernel of . If N is an R-algebra, then this is the presentation of the N-module ; that is, the presentation extends under base extension.

For left-exact functors, there is for example

Proposition  Let F, G be left-exact contravariant functors from the category of modules over a commutative ring R to abelian groups and θ a natural transformation from F to G. If is an isomorphism for each natural number n, then is an isomorphism for any finitely-presented module M.

Proof: Applying F to a finite presentation results in

and the same for G. Now apply the snake lemma.

See also


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