\[\frac{1}{2\pi}\int_{-\infty}^{\infty}{\mathrm{e}}^{-\mathrm{i}at}\left(\int_{-%
\infty}^{\infty}\phi(x){\mathrm{e}}^{\mathrm{i}tx}\,\mathrm{d}x\right)\,%
\mathrm{d}t=\phi(a)\]