Definition:Open Set/Real Analysis

Also see

 * By Open Sets in Real Number Line every open set $I \subseteq \R$ is a countable union of pairwise disjoint open intervals:
 * $\ds I = \bigcup_{n \mathop \in \N} \openint {a_n} {b_n} \subseteq \R$


 * Definition:Closed Set (Real Analysis)