We study Hamiltonian actions of compact Lie groups K on Kähler manifolds which extend to a holomorphic action of the complexified group K C . For a closed normal subgroup L of K we show that the Kählerian reduction with respect to L is a stratified Hamiltonian Kähler K C /L C -space whose Kählerian reduction with respect to K/L is naturally isomorphic to the Kählerian reduction of the original manifold with respect to K. * The first author is supported by a Promotionsstipendium of the Studienstiftung des deutschen Volkes and by SFB/TR 12 of the DFG.† The second author is partly supported by SFB/TR 12 of the DFG.by the flow of the vector field V H associated to the Hamiltonian H via the equation dH = ı V H ω.A Hamiltonian system with symmetries is a system (X, ω, K, µ, H) where H is invariant with respect to the K-action. In this case it follows from a) thatholds for all ξ ∈ k. This implies Noether's principle in the following geometric formulation: every component µ ξ of the momentum map is a constant of motion for the system described by H.The previous considerations imply that level sets of µ are invariant under the flow of V H . In many cases questions can be organised so that µ −1 (0) is the momentum fibre of interest. By b), the group K acts on the level set µ −1 (0). Let us first consider this action on an infinitesimal level. If we fixThe optimal situation appears if µ has maximal rank at x 0 . In this case, M is smooth at x 0 and T x 0 M = ker(dµ(x 0 )) = (k • x 0 ) ⊥ω holds. It follows that M is a coisotropic K-stable submanifold of X and the symplectic formThese observations imply that once the space M/K is smooth, it will be a symplectic manifold. This is the content of the Marsden-Weinstein-Theorem (see [MW74]):If K acts freely and properly on M, the quotient M/K is a symplectic manifold whose symplectic formω is characterised via the equationHere, π : M → M/K denotes the quotient map and i M : M → X is the inclusion. Furthermore, the restriction of the K-invariant Hamiltonian H to M induces a smooth functionH on M/K. The Hamiltonian system on (M/K,ω) associated toH captures the essential (symmetryindependent) properties of the original K-invariant system that was given by H.The Marsden-Weinstein construction is natural in the sense that it can be done in steps. This means that for a normal closed subgroup L of K, the restricted momentum map µ L : X → l * is K-equivariant, the induced K-action on µ −1 L (0)/L is Hamiltonian with momentum mapμ induced by µ and the symplectic reductionμ −1 (0)/K is symplectomorphic to M/K.
Removing the restrictive regularity assumptions of [MW74], it is proven in [SL91] that symplectic reduction can be carried out for general group actions of compact Lie groups yielding stratified symplectic quotient spaces M/K, i.e. stratified spaces where all strata are symplectic manifolds. The paper [HHL94] proposed an approach to this singular symplectic reduction based on embedding symplectic manifolds into Kähler manifolds and a Kähler reduction theory for Kähler manifolds. Rough...