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The following, stated as e.g. Theorem 8.2.13 in Hörmander’s The Analysis of Linear Partial Differential Operators I, is standard: Theorem 0. If $ X \subseteq \mathbb{R}^n$ , $ Y \subseteq \mathbb{R}^m$ , $ Z \subseteq \mathbb{R}^p$ are open, $ K_1 \in \mathscr{D}'(X \times Y)$ , $ K_2 \in \mathscr{D}'(Y \times Z)$ , the projection $Read more