Let $U \subset \mathbb{R}^n$ be open, and let $u \in \mathcal{D}'(U)$ be a distribution. Then the wave front set $\operatorname{WF}(u)$ is a closed conic subset of $U \times \mathbb{R}^n_0$, where closedness is understood relative to the [subspace topology](/page/Subspace%20Topology) on $U \times \mathbb{R}^n_0$.