= Degenerate ion-acoustic wave
{title2=$c_s^2\simeq m_ev_{Fe}^2/(3m_i)$}
For equal densities of singly charged ions and electrons, a completely degenerate electron equation of state gives $dP_e/dn_e=m_ev_{Fe}^2/3$. Coupling this restoring force to ion inertia yields the sound speed in the title. If both species are degenerate and $m_e\ll m_i$, then $v_{Fi}\ll c_s\ll v_{Fe}$. Ion resonances are absent, while the electron parallel <marginal distribution> has a small slope at the <phase velocity>, producing relative <Landau damping> of order $\sqrt{m_e/m_i}$.
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