 $$\begin{aligned}     \label{eq:I0*}     I^*_0=\left\{\begin{array}{ll} I^* & \ 0\leq \theta < \theta_0 \\ M & \ \theta_0\leq\theta\leq\theta_0 \\ I^* & \ \theta_0 < \theta < \theta_0 \\ -M & \ \theta_0\leq\theta\leq\theta_0\\ I^* & \ \theta_0 < \theta \leq 0\pi.     \end{array}\right.\end{aligned}$$ The 