= Counter-rotating weak-transition amplitude
{title2=$c_2=-\Omega\sum_{\pm}(e^{i(\omega_0\pm\omega)t}-1)/[2(\omega_0\pm\omega)]$}
The first-order response to a cosine drive contains both resonant and counter-rotating terms. The <rotating-wave approximation> removes the rapidly oscillating contribution only under near-resonant, weak-drive and appropriate time-scale assumptions. It does not remove the separate requirement of weak excitation when the initial amplitude is held fixed.
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