Cho $\alpha,\beta\in \mathbb{N}^n$ là các đa chỉ số. Chứng minh rằng
\[\dfrac{1}{(2\pi)^n}\int_{\mathbb{R}^n\times \mathbb{R}^n} e^{-i\langle \nu,\eta\rangle}\nu^{\alpha}\eta^{\beta}d\nu d\eta=\delta_{\alpha=\beta}\left(\frac{1}{i}\right)^{|\alpha|}\alpha!.\]