Rigid quiver representations have open orbits
= Rigid quiver representations have open orbits
The stabilizer is the <automorphism group of a quiver representation>, an open set in its endomorphism algebra. Thus $\dim\mathcal O_X=\sum_i n_i^2-\dim\operatorname{End}_Q(X)$. The <Ringel form> gives orbit codimension $\dim\operatorname{Ext}^1_Q(X,X)$. Since orbits are locally closed and the representation space is irreducible, rigidity is equivalent to an open dense orbit.