n. The closure property exhibited by an operation which is not allowed to apply to its own output. For example, in a GPSG-type grammar, the (finite) set of immediate dominance rules is closed under the application of...

In topology, a CW complex is a type of topological space introduced by J. H. C. Whitehead to meet the needs of homotopy theory. The idea was to have a class of spaces that was broader than simplicial complexes (in modern language, which had better...