Boundary (Topology)/Examples/Half-Open Real Interval

Examples of Boundaries in the context of Topology
Let $\struct {\R, \tau_d}$ be the real number line with the usual (Euclidean) topology. Let $\hointl a b$ be a half-open interval of $\R$.

Then the boundary of $\hointl a b$ is the set of its endpoints $\set {a, b}$.