Reduced invariant sets

Abstract: Let K be a compact Lie group and W a finite-dimensional real K-module. Let X be a K-stable real algebraic subset of W. Let I(X) denote the ideal of X in R[W] and let I_K(X) be the ideal generated by I(X)^K. We find necessary conditions and sufficient conditions for I(X)= I_K(X) and for \sqrt{I_K(X)}=I(X). We consider analogous questions for actions of complex reductive groups.