Open set in R

Open sets are among the most important subsets of R. A collection of open sets is called a topolo...

Let U be an open set in R. Then U is a countable union of disjoint intervals. This question has p...

In mathematics, open sets are a generalization of open intervals in the real line. In a metric sp...

In complex analysis, open sets are of key importance when investigating analytic functions: see G...