mean a closed subgroup of $G$ containing $B$. There exists a natural bijection of the set of subsets of $\Delta$ with the set of parabolic subgroups of $G$ containing $B$ under which for a subset $I \subseteq \Delta$, we assign the parabolic $P_I$ to be the closed subgroup of $G$ generated by $B$ and $X_{^+_- \alpha}$ for all roots $\alpha \in \Delta \backslash I$. Let $W=N(T)/T$ be the Weyl group. Fix a $W$-invariant inner product $\langle$ , $\rangle$ on $X_*(T)
\otimes 
\mathbb {Q}$. Using this inner product we can define norm of any 1-PS $\lambda(t) \in T$ as $\mid \mid \lambda(t) \mid \mid  = \langle \lambda, \lambda \rangle$. For a arbitrary 1-PS in $G$ we can conjugate it into the fixed maximal torus and then define its norm.\
We begin by recalling the definitions of semistability of vector and principal bundles with respect to the fixed polarisation $H$.

For a vector bundle $E$ on $X$, define its slope to be the rational number: $$\mu(E)= deg(E)/rk(E).$$ $~$A vector bundle $E$ on $X$ is said to be [**$\mu$-semistable**]{} (w.r.t. the polarization $H$) if for any proper subbundle $F \subset E$, we have the inequality $\mu(F) \leq \mu(E) $, where $\mu$ denotes the 