1. A. Borel, Linear Algebraic Groups, W. A. Benjamin 1969.
2. A. Borel and J. Tits, Elements unipotents et sous-groupes paraboliques de groupes reductifs I, Inv. Math. 12 (1971), 95–104.
3. B. Kostant, Lie group representations on polynomial rings, Am. J. Math. 85 (1963), 327–404.
4. B. Kostant, Quantization and unitary representations, Lectures in Modern Analysis and Applications III, Lecture Notes in Math., No. 170, Springer-Verlag, 1970, 87–208
5. R.W. Richardson, “Derivatives of invariant polynomials on a semisimple Lie algebra,” Harmonic analysis and operator theory, Proc. Cent. Math. Anal. Aust. Natl. Univ. 15 (1987), 228–241.