9.166 and 9.169. The dispersion relation is given by
\[
\omega = c k \sqrt{1 + \frac{n^2 q^2}{2 \epsilon_0} \sum_{j} \frac{f_j}{\omega_j^2 - \omega^2}}
\]
where $n$ is the refractive index, $q$ is the charge, $\epsilon_0$ is the permittivity of free space, $f_j$
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