\[\mathscr{F}(\phi)(\mathbf{x})=\mathscr{F}\phi(\mathbf{x})=\frac{1}{(2\pi)^{n/2%
}}\int_{{\mathbb{R}}^{n}}\phi(\mathbf{t}){\mathrm{e}}^{\mathrm{i}\mathbf{x}%
\cdot\mathbf{t}}\,\mathrm{d}\mathbf{t},\]