Let $n \in \mathbb{N}$ and let $m \in \mathbb{N} \cup \{0\}$. Let $\mathbb{N}_0 := \mathbb{N} \cup \{0\}$, and for each $k \in \mathbb{N}_0$ let $\mathcal{L}^k$ denote $k$-dimensional [Lebesgue measure](/page/Lebesgue%20Measure) on $\mathbb{R}^k$, with $\mathbb{R}^0 := \{0\}$. Let
be a polynomial with coefficients $a_\alpha \in \mathbb{C}$. For $u \in \mathcal{S}(\mathbb{R}^n)$, define the constant-coefficient differential operator