 Solution

Retain $K=k^2{\lambda }_{\mathrm{De}}^2$ while making the same narrow-velocity plateau expansion as in d. With $u=v_1+\delta a$, the factor that varies rapidly across the plateau is $(v_2-u)/(u-v_1)=(1-a)/a$. The other factors vary slowly and may be evaluated at $u=v_1$, givingThereforeAt $K\ll 1$, the denominator tends to $v_0^2/v_1^2$, recovering d. At $K\gg 1$,recovering the exponentially close approach to the plateau edge in c. The phase velocity remains strictly inside the velocity plateau for every finite $k$.

As an asymptotic expansion, the formula takes $\delta \to 0$ at fixed $0<v_1/v_0<1$. If the plateau also approaches the outer edge, so that $v_0-v_1$ is comparable to $\delta$, the factors treated as slowly varying need not be slowly varying. In that additional limiting regime, use the exact dispersion relation in b instead.