1. N. R. Wallach, “Principal orbit type theorems for reductive algebraic group actions and the Kempf-Ness Theorem,” arXiv:1811.07195v1 (17 Nov 2018).
2. Graduate Texts in Math.;A Borel,1991
3. V. L. Popov, E. B. Vinberg, “Invariant theory,” in Algebraic Geometry IV, Encyclopaedia of Mathematical Sciences (Springer-Verlag, Berlin, 1994), Vol. 55, pp. 123–284.
4. R. W. Richardson, “Principal orbit types for algebraic transformation spaces in characteristic zero,” Invent. math. 16, 6–14 (1972).
5. D. Luna, “Slices étales,” Bull. Soc. Math. de France 33, 81–105 (1973).