An abstract setting for hamiltonian actions Karl-Hermann Neeb and Cornelia Vizman October 31, 2018 Abstract In this paper we develop an abstract setup for hamiltonian group actions as follows: Starting with a continuous 2-cochain ω on a Lie algebra h with values in an h-module V , we associate subalgebras sp(h,ω) ⊇ ham(h,ω) of symplectic, resp., hamiltonian elements. Then ham(h,ω) has a natural central extension which in turn is contained in a larger abelian extension of sp(h,ω). In this setting, we study linear actions of a Lie group G on V which are compatible with a ho- momorphism g → ham(h,ω), i.e., abstract hamiltonian actions, cor- responding central and abelian extensions of G and momentum maps J : g → V . Keywords: central extension, momentum map, hamiltonian action, abelian extension, infinite dimensional Lie group MSC: 17B56, 35Q53 Introduction. arXiv:0802.3360v1 [math.SG] 22 Feb 2008 In [Br93] Brylinski describes how to associate to a connected, not necessarily finite-dimensional, smooth manifold M, endowed with a closed 2-form ω ∈ Ω2(M, R), a central extension of the Lie algebra ham(M,ω) of hamiltonian vector fields on M. If there exists an associated pre-quantum bundle P with connection 1-form θ and curvature ω (which is the case if ω is integral and M is smoothly paracompact), then Kostant’s central extension ([Ko70]) is given by the short exact sequence (T ֒→ Aut(P, θ)0 →→ Ham(M,ω) → 1 (1 → 1 1 of groups, where Aut(P, θ)0 is the group of those connection preserving auto- morphisms of P isotopic to the identity and Ham(M,ω) is the group of hamil- tonian diffeomorphisms of M. In general, neither Ham(M,ω) nor Aut(P, θ)0 carries a Lie group structure if M is not assumed to be compact. However, we have shown in [NV03] that central Lie group extensions can be obtained as follows, even if M is infinite dimensional. Let Z be an abelian Lie group of the form Z = z/ΓZ, where ΓZ is a discrete subgroup of the Mackey complete space z, ω ∈ Ω2(M, z) a closed 2-form, and (P, θ) a corresponding Z-pre-quantum bundle, i.e., P is a Z-principal bundle and 1 d ∗ θ ∈ Ω (P, z) a connection 1-form with θ = qP ω, where qP : P → M is the bundle projection. We call a smooth action of a (possibly infinite dimen- sional) connected Lie group G on a (possibly infinite dimensional) manifold M hamiltonian if the derived homomorphism maps into hamiltonian vector fields: ∞ ζ : g → ham(M,ω) := {X ∈V(M): (∃f ∈ C (M, R)) iX ω = df}. Then the pullback of Aut(P, θ) defines a central Lie group extension (Z ֒→ Gcen →→ G → 1, (2 → 1 b i.e., Gcen carries a Lie group structure for which it is a principal Z-bundle over G. A Lie algebra 2-cocycle for the associated central Lie algebra ex- b tension g of g by R is given by (X,Y ) 7→ −ω(ζ(X),ζ(Y ))(m0) for any fixed element m0 ∈ M. Theb main point of the present paper is to provide an abstract setting for this kind of hamiltonian group actions, momentum maps, and the associated central Lie group actions. As we shall see in the examples in Section 1, our setting is sufficiently general to cover various kinds of examples of different nature described below. Starting with a continuous 2-cochain ω on a Lie algebra h with values in a topological h-module V , we associate the subalgebras sp(h,ω) := {ξ ∈ h: Lξω =0, iξdhω =0} = {ξ ∈ h: dh(iξω)= iξdhω =0} ⊇ham(h,ω) := {ξ ∈ h: iξdhω =0, (∃v ∈ V ) iξω = dhv}. of symplectic, resp., hamiltonian elements of h. Then ham(h,ω) has a natural central extension ham(h,ω) := {(v,ξ) ∈ V × ham(h,ω): dhv = iξω} d 2 by the trivial module V h which in turn is contained in the larger abelian ex- tension of sp(h,ω) by V defined by ω. To obtain the Lie bracket on ham(h,ω), ham h d we observe that the space Vω := {v ∈ V : (∃ξ ∈ ( ,ω)) hv d= iξω} of so-called admissible elements carries a Lie bracket analogous to the Poisson bracket: d {v1, v2} := ω(ξ2,ξ1) for hvj = iξj ω, and ham(h,ω) is a subalgebra of the Lie algebra direct sum Vω ⊕ ham(h,ω). The classical example is given by h = V(M), V = C∞(M, R) and a closed d 2-form ω. In a similar spirit is the example arising from a Poisson manifold (M, Λ), where we put h = Ω1(M, R), endowed with the natural Lie bracket (cf. Example 1.6), V = C∞(M, R) and ω := Λ, considered as a V -valued 2-cocycle on h. Of a different character are the examples obtained with h = p p p−1 V(M), V := Ω (M, R) := Ω (M, R)/dΩ (M, R) and ω(X,Y ) := [iY iX ω], where ω is a differential (p +2)-form on M. There are more examples arising from associative algebras in the spirit of non-commutative geometry. e In Sectione 2 we turn to abstract hamiltonian actions of a Lie group G on V , i.e., actions which are compatible with a homomorphism ζ : g → ham(h,ω) ∗ and the h-action on V . Then the pullback extension gcen := ζ ham(M,ω) g h momentum map defines a central extension of by V , and a is a continuousd linear map J : g → V satisfying b dh(J(X)) = iζ(X)ω for X ∈ g, i.e., J defines a continuous linear section σ := (J, ζ, idg): g → gcen of the asso- ciated central extension. The obstruction to the existence of an equivariant momentum map is measured by the 1-cocycle κ: G → C1(bg,V h), κ(g) := g.J − J which in turn can be used to describe the adjoint action of the group G on the extended Lie algebra gcen. In Section 3 we briefly discuss the existence of a central Lie group ex- b tension Gcen with Lie algebra gcen. Once a momentum map J is given for a hamiltonian G-action, the only condition for the existence of such an b b extension is the discreteness of the image Πω of a period homomorphism h ∗ perωg : π2(G) → V , associated to the Lie algebra cocycle ωg := ζ ω. If Πω is discrete, there also is an abelian Lie group extension Gab of G integrating the Lie algebra extension g := V ⋊ g. ab ωg b In many situations the period map perωg is quite hard to evaluate, but one may nevertheless showb that its image is discrete. To illustrate this point, 3 we take in Section 4 a closer look at a smooth action of a connected Lie group G on a manifold M, leaving a closed z-valued 2-form ω invariant. If the group S = ω of spherical periods of ω is discrete in V , then Z := z/S is an ω π2(M) ω abelianR Lie group, and there exists a Lie group extension 1 → Z → Gcen → G → 1 b e integrating the Lie algebra gcen. Here qG : G → G denotes a simply connected covering group of G, so that G can also be viewed as an extension of G by cen e a 2-step nilpotent Lie groupbπ (G) which is a central extension of the discrete b1 group π1(G) = ker qG by Z. Here we use that the universal covering manifold b ∗ qM : M → M, endowed with ω := qM ω, permits us to embed sp(M,ω) into sp(M, ω) = ham(M, ω), so that we actually obtain an abstract hamiltonian f e action of the universal covering group G for the module V := C∞(M, z) and f e f e the central extension Gcen acts by quantomorphismse on a Z-principalf bundle P over M. This further leads to an abelian Lie group extension b f ∞ 1 → C (M,Z)0 → Gab → G → 1, integrating the cocycle ζ∗ω ∈ Z2(g,C∞(bM, z)).e In the short Section 5 we formulate the algebraic essence of the Noether Theorem in our context and the paper concludes with an appendix recalling some of the results from [Ne04] on the integrability of abelian Lie algebra extensions to Lie group extensions. 1 A Lie algebraic hamiltonian setup Let V be a topological module of the topological Lie algebra h, i.e., the mod- • ule operation h × V → V, (ξ, v) 7→ ξ.v, is continuous. We write (C (h,V ), dh) for the Chevalley–Eilenberg complex of continuous Lie algebra cochains with values in V , Zp(h,V ) denotes the space of p-cocycles, Hp(h,V ) the cohomol- ogy space etc. We further write Lξ for the operators defining the natural action of h on the spaces Cp(h,V ). We refer to [FF01] for the basic notation concerning continuous Lie algebra cohomology. We first recall some results from [Ne06a] which we specialize here for 2-cochains. Fix a continuous 2-cochain ω ∈ C2(h,V ). Then sp(h,ω) := {ξ ∈ h : Lξω =0, iξdhω =0} = {ξ ∈ h: dh(iξω)=0, iξdhω =0} 4 is a closed subspace of h and from [Lξ, Lη]= L[ξ,η] and [Lξ, iη]= i[ξ,η] it follows that sp(h,ω) is a Lie subalgebra of h, called the Lie algebra of symplectic elements of h.