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\begin{equation*}
  f'(z_{1e}) f'(z_{2e}) \approx
  f'(\mu) f'(\frac{1}{\mu^{n-1}}) =
    \frac{-\mu n \mu^{n-1}}{(1 + \mu^n)^2} \cdot
    \frac{-\mu n \mu^{-(n-1)^2}}{{\color{blue} (}1 + \mu^{-n(n-1)}{\color{blue} )^2}}
  \approx n^2 \mu^{-n^2 + n}.
\end{equation*}
\end{document}