= Normal space to a quiver orbit
{title2=$N_x\mathcal O_X\cong\operatorname{Ext}^1_Q(X,X)$}
The infinitesimal <base change action on quiver representations> is $\xi_x(u)_\rho=u_{t(\rho)}x_\rho-x_\rho u_{s(\rho)}$. Its image is the <Zariski tangent space> to the orbit, because the stabilizer is a smooth open subset of the <endomorphism ring>. The <extension complex of quiver representations> identifies the quotient of the ambient tangent space by this image with $\operatorname{Ext}^1_Q(X,X)$. Consequently <rigid quiver representations have open orbits>.
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