Part of the Grundlehren der mathematischen Wissenschaften book series (GL, volume 307)
We say that a set E is a topological space if we have in it the notion of open sets, a class of subsets of E satisfying the following conditions:
The intersection of a finite number of open sets is an open set.
The union of any number of open sets is an open set.
© Springer-Verlag Berlin Heidelberg 1993