The geometric meaning of Zhelobenko operators

Let g be the complex semisimple Lie algebra associated to a complex semisimple algebraic group G, b a Borel subalgebra of g , h⊂b the Cartan sublagebra, and N ⊂ G the unipotent subgroup corresponding to the nilradical n⊂b . We show that the explicit formula for the extremal projection operator for g obtained by Asherova, Smirnov, and Tolstoy and similar formulas for Zhelobenko operators are related to the existence of a birational equivalence N×h→b given by the restriction of the adjoint action. Simple geometric proofs of formulas for the “classical” counterparts of the extremal projection operator and of Zhelobenko operators are also obtained.