Boundary (Topology)/Examples/Reciprocals in Real Numbers

Examples of Boundaries in the context of Topology
Let $\struct {\R, \tau_d}$ be the real number line with the usual (Euclidean) topology. Let $S$ be the set defined as:
 * $S = \set {\dfrac 1 n: n \in \Z_{>0} }$

Then the boundary of $S$ in $\struct {\R, \tau_d}$ is $S \cup \set 0$.