\[\frac{1}{2\pi}\sum_{k=-\infty}^{\infty}{\mathrm{e}}^{-\mathrm{i}ka}\left(\int_%
{-\pi}^{\pi}\phi(x){\mathrm{e}}^{\mathrm{i}kx}\,\mathrm{d}x\right)=\phi(a),\]