Principal Orbit Types for Reductive Groups Acting on Stein Manifolds. R.W. jr. Richardson — 1974 Mathematische Annalen
The Variation of Isotropy Subalgebras for Analytic Transformation Goups. R.W. jr. Richardson — 1973 Mathematische Annalen
Principal Orbit Types for Algebraic Transformation Spaces in Characteristic Zero. R.W. Jr. Richardson — 1972 Inventiones mathematicae