This article is a stub. Help us out by expanding it.

Let $X$ be a topological space and $S$ be a subspace of $X$. Then $S$ is dense in $X$ if, for any $x\in X$ and any open neighborhood $U\ni x$, $U\cap S\neq\varnothing$. For example, the rational numbers are dense in the real numbers.

Invalid username
Login to AoPS