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$$
   F_{hz} = \sqrt{\delta} \sqrt{\frac{R_i R_j}{R_i + R_j}} F_{hk} = 
     \sqrt{\delta} \sqrt{\frac{R_i R_j}{R_i + R_j}} 
     \Big[ (k_n \delta \mathbf{n}_{ij} -  
       m_{\mbox{\scriptsize{eff}}} \: \gamma_n \mathbf{ v}_n) -
       (k_t \mathbf{ \Delta s}_t +
       m_{\mbox{\scriptsize{eff}}} \: \gamma_t \mathbf{v}_t) \Big]
$$

\end{document}
