I'm a beginner of gauge theory and I find that most materials state the theorem(without proof) that:
Given a principal bundle $P$, for a connection $A$ over $P$, a isotopy group $G_A$ of $A$ consist of the gauge transformation $g$ such that $g^*A=A$. Then there exist a neighborhood $U$ of $A$ and a $\epsilon>0$ (in moduli space $\mathfrak{A}/\mathfrak{G}$, here $\mathfrak{A}$ a space of connection, $\mathfrak{G}$ the gauge transformation group) s.t. $U\cong T_{A,\epsilon}/G_A$, here $T_{A,\epsilon}$ is the coloumb gauge slice of $A$.
The definition of coloumb gauge silce is given as follow: for a connection $A$ and $\epsilon>0$, we call all $a\in\Omega^1({ad(P)})$ satisfies $d^*_A a=0$ and $|a|<\epsilon$ in the $L_2$ norm of bundle $ad(P)$ the coloumb gauge silce $T_{A,\epsilon}$ w.r.t. $A$ and $\epsilon$.
Do there any material contain a proof of the theorem? Thanks!