_0$ and $p=0$. We have $G=\langle x_\alpha(t)\mid \alpha\in\Phi,t\in k\rangle$ where $\Phi$ is the root system of $G$ with respect to $T$. Let $\{\alpha_0,\alpha_0, \alpha_0,\alpha_0\}$ be the set of simple roots with respect to $B$, where 