In topology, a compactly generated space is a topological space X satisfying the following condition: a subspace A is closed in X if and only if A ∩ K is closed in K for all compact subspaces K ⊆ X. Equivalently, one can replace closed with open in this definition.

Most topological spaces commonly studied in mathematics are compactly generated. For instance, every locally compact (and compact) space is compactly generated, as is every first-countable space. Additionally, every CW complex is compactly generated.

One of the primary motivations for studying compactly generated spaces comes from category theory. The category of topological spaces, Top, is defective in the sense that it fails to be a cartesian closed category. There have been various attempts to remedy this situation, one of which is to restrict oneself to the full subcategory of compactly generated Hausdorff spaces, that is, compactly generated spaces which are also Hausdorff. This category is cartesian closed. The category of compactly generated Hausdorff spaces is general enough to include all metric spaces, topological manifolds, and all CW complexes.

Properties

Given any topological space X we can define a (possibly) finer topology on X which is compactly generated. Let {K_α} denote the family of compact subsets of X. We define the new topology on X by declaring a subset A to be closed iff A ∩ K_α is closed in K_α for each α. Denote this new space by X_c. One can show that the compact subsets of X_c and X coincide and the induced topologies are the same. It follows that X_c is compactly generated. If X was compactly generated to start with then X_c = X otherwise the topology on X_c is strictly finer than X (i.e. there are more open sets).

The continuity of a map defined on compactly generated space X can be determined solely by looking at the compact subsets of X. Specifically, a function f : X → Y is continuous iff it is continuous when restricted to each compact subset K ⊆ X.

If X and Y are two compactly generated spaces the product X × Y may not be compactly generated (it will be if at least one of the factors is locally compact). Therefore when working in categories of compactly generated spaces it is necessary to define the product as (X × Y)_c.

See also

• compact-open topology
• CW complex