Let \(X\) be a topological space. A subset \(Q\subseteq X\) is dense in \(X\) if every neighborhood of every point \(x\in X\) intersects \(Q\).
A subset \(Q\subseteq X\) is dense in \(X\) if and only if its closure is \(\cl_X(Q) = X\).
See [@Mun00].
In terms of closure, when is $A \subset X$ dense in $X$?