Example 1. In the category of sets let be a set and a collection of subsets of We also have iff The map is defined to be the inclusion map. Clearly is a direct system.
Example 2. Let be a commutative ring and let be a non-empty subset of such that no element of is nilpotent. The partial order is defined on by iff in i.e. for some For every let be the localization of at We need now to define the morphisms If then for some and so we can define by for all It is easy to see that is a ring homomorphism. Clearly is the identity map over because in this case we may choose Also, if with and then and hence
Thus and so is a direct system in the category of rings.
Example 3. Let be a ring and let be a two-sided ideal of Let be the set of positive integers and put for every For every the ring homomorphism is defined naturally by This ring homomorphisms are well-defined because if then Clearly is the identity map of and if then
Thus is an inverse system in the category of rings.
Similarly, if is an -module and we put then we will have the inverse system in the category of -modules. Here is defined by whenever
Example 4. Let be a group and let be a family of normal subgroups of which have finite index in We define the partial order on by if and only if We also define, whenever the group homomorphism by It is easily seen that is an inverse system in the category of finite groups.
Example 5. Let be a commutative ring, let be an -module and let be a direct system of -modules over some partially ordered index set For every let and for all with define by That means for all and
Claim . is a direct system of -modules.
Proof. So we need to prove two things to show that is a direct system:
1) for all This is obvious because and hence, for every and we have
2) whenever This also follows very easily from