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\begin{align}
 M_\mathrm{r,t+\Delta t}^\mathrm{d} = \left\lbrace \begin{array}{cl}
 -C_\mathrm{r} \dot{\theta}_\mathrm{r} & \mathrm{if } |M_\mathrm{r,t+\Delta t}^\mathrm{k}| < M_\mathrm{r}^\mathrm{m} \\
 -f C_\mathrm{r} \dot{\theta}_\mathrm{r} & \mathrm{if } |M_\mathrm{r,t+\Delta t}^\mathrm{k}| = M_\mathrm{r}^\mathrm{m} \end{array} \right.
\end{align}

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