Before revision:  An optimal distribution for the frequencies (\ell k,\omega'_{\ell k}) of the random Fourier features e^{  i \ell k\bar z_\ell} and e^{  i \omega'_{\ell k}\cdot x} is derived.
After revision:  An optimal distribution for the frequencies (\ell k,\omega'_{\ell k}) of the random Fourier features e^{  i \ell k\bar z_\ell} and e^{  \mathrm{i \omega'_{\ell k}\cdot x} is derived.