More precisely:
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2 General theory 3 In general topology |
Assuming one has a category like that of abelian groups, one can in fact form direct sums of copies of G until the morphism f:Sum(G) -> H is a surjection; and one can form direct products of C until the morphism f:H-> Prod(C) is one to one.
For example, the integers are a generator of the category of abelian groups (since every abelian group is a quotient of a free abelian group). This is the origin of the term generator. The approximation here is normally described as generators and relations.
As an example of a cogenerator in the same category, we have Q/Z, the rationals modulo the integers, which is a divisible abelian group. Given any abelian group A, there is an isomorphic copy of A contained inside the product of |A| copies of Q/Z. This approximation is close to what is called the divisible envelope - the true envelope is subject to a minimality condition.
In topological language, we try to find covers of unfamiliar objects.
Finding a generator of an abelian category allows one to express every object as a quotient of a direct sum of copies of the generator. Finding a cogenerator allows one to express every object as a subobject of a direct product of copies of the cogenerator. One is often interested in projective generators (even finitely generated projective generators, called progenerators) and minimal injective cogenerators. Both examples above have these extra properties.
The cogenerator Q/Z is quite useful in the study of modules over general rings. One forms the (algebraic) character module H* of homomorphisms from H to Q/Z. Being a cogenerator says precisely that H* is 0 if and only if H is zero. Even more is true: The * operation takes a homomorphism f:H->K to a homomorphism f* : K* -> H*, and f* is 0 if and only if f is zero. Similarly for the topological character module of continuous homomorphisms from H to R/Z (the circle group).
Every H* is very special in structure : it is pure-injective (also called algebraically compact), which says more or less that solving equations in H* is relatively straightforward. One can often consider a problem after applying the * to simplify matters.
The Tietze extension theorem can be used to show that an interval is an injective cogenerator in a category of topological spaces subject to separation axioms.The abelian group case
General theory
In general topology