# Nested interval topology

In mathematics, more specifically general topology, the nested interval topology is an example of a topology given to the open interval (0,1), i.e. the set of all real numbers x such that 0 < x < 1. The open interval (0,1) is the set of all real numbers between 0 and 1; but not including either 0 or 1.

To give the set (0,1) a topology means to say which subsets of (0,1) are "open", and to do so in a way that the following axioms are met:[1]

1. The union of open sets is an open set.
2. The finite intersection of open sets is an open set.
3. The set (0,1) and the empty set ∅ are open sets.

## Construction

The set (0,1) and the empty set ∅ are required to be open sets, and so we define (0,1) and ∅ to be open sets in this topology. The other open sets in this topology are all of the form (0,1 − 1/n) where n is a positive whole number greater than or equal to two i.e. n = 2, 3, 4, 5, ….[1]

## References

1. Steen, L. A.; Seebach, J. A. (1995), Counterexamples in Topology, Dover, ISBN 0-486-68735-X