Let $X$ be a smooth manifold, let $m \in \mathbb{R}$, and let $P \in \Psi^m_{\mathrm{prop}}(X)$ be a properly supported scalar pseudodifferential operator of order $m$. For every distribution $u \in \mathcal{D}'(X)$,
Equivalently, for every nonzero covector $(x_0,\xi_0) \in T^*X \setminus 0$, if $P$ is elliptic at $(x_0,\xi_0)$ and $(x_0,\xi_0) \notin WF(Pu)$, then $(x_0,\xi_0) \notin WF(u)$.