finitely generated module
A module over a ring is said to be finitely generated if there is a finite subset of such that spans . Let us recall that the span of a (not necessarily finite) set of vectors is the class of all (finite) linear combinations
of elements of ; moreover, let us recall that the span of the empty set
is defined to be the singleton consisting of only one vector, the zero vector
. A module is then called cyclic if it can be a singleton.
Examples. Let be a commutative ring with 1 and be an indeterminate.
- 1.
is a cyclic -module generated by .
- 2.
is a finitely-generated -module generated by . Any element in can be expressed uniquely as .
- 3.
is not finitely generated as an -module. For if there is a finite set
, taking to be the largest of all degrees of polynomials in , then would not be in the of , assumed to be , which is a contradiction
. (Note, however, that is finitely-generated as an -algebra.)