\item $\sigma_{\perp} = \alpha \rho_0$, $\alpha \ll 1$\\
$\sqrt{\rho_0^2+\sigma_{\perp}^2}=
\sqrt{\rho_0^2+\alpha^2\rho_0^2}=
\item $\sigma_{\perp} = \alpha \rho_0$, $\alpha \ll 1$\\
$\sqrt{\rho_0^2+\sigma_{\perp}^2}=
\sqrt{\rho_0^2+\alpha^2\rho_0^2}=