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$$
   F = f\left(\delta/d\right)\left(k_n \delta \mathbf{n}_{ij} -  
   m_{\mbox{\scriptsize{eff}}} \gamma_n\mathbf{ v}_n\right) + 
   f\left(\delta/d\right)\left(-k_t \delta\mathbf{ \Delta s}_t - 
   m_{\mbox{\scriptsize{eff}}} \gamma_t \mathbf{v}_t\right)
$$

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