### A convexity theorem for Poisson actions of compact Lie groups

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Let ℳ be a type II₁ von Neumann algebra, τ a trace in ℳ, and L²(ℳ,τ) the GNS Hilbert space of τ. If L²(ℳ,τ)₊ is the completion of the set ${\mathcal{M}}_{sa}$ of selfadjoint elements, then each element ξ ∈ L²(ℳ,τ)₊ gives rise to a selfadjoint unbounded operator ${L}_{\xi}$ on L²(ℳ,τ). In this note we show that the exponential exp: L²(ℳ,τ)₊ → L²(ℳ,τ), $exp\left(\xi \right)={e}^{i{L}_{\xi}}$, is continuous but not differentiable. The same holds for the Cayley transform $C\left(\xi \right)=({L}_{\xi}-i){({L}_{\xi}+i)}^{-1}$. We also show that the unitary group ${U}_{\mathcal{M}}\subset L\xb2(\mathcal{M},\tau )$ with the strong operator topology is not an embedded submanifold...

The study of diffeomorphism group actions requires methods of infinite dimensional analysis. Really convenient tools can be found in the Frölicher - Kriegl - Michor differentiation theory and its geometrical aspects. In terms of it we develop the calculus of various types of one parameter diffeomorphism groups in infinite dimensional spaces with smooth structure. Some spectral properties of the derivative of exponential mapping for manifolds are given.

In the present paper, we consider the class of control systems which are induced by the action of a semi-simple Lie group on a manifold, and we give a sufficient condition which insures that such a system can be steered from any initial state to any final state by an admissible control. The class of systems considered contains, in particular, essentially all the bilinear systems. Our condition is semi-algebraic but unlike the celebrated Kalman criterion for linear systems, it is not necessary. In...

Let A be a unital C*-algebra. Denote by P the space of selfadjoint projections of A. We study the relationship between P and the spaces of projections ${P}_{a}$ determined by the different involutions ${}_{a}$ induced by positive invertible elements a ∈ A. The maps $\phi :P\to {P}_{a}$ sending p to the unique $q\in {P}_{a}$ with the same range as p and ${\Omega}_{a}:{P}_{a}\to {P}_{a}$ sending q to the unitary part of the polar decomposition of the symmetry 2q-1 are shown to be diffeomorphisms. We characterize the pairs of idempotents q,r ∈ A with ||q-r|| < 1 such that...

On the unit sphere $\mathbb{S}$ in a real Hilbert space $\mathbf{H}$, we derive a binary operation $\odot $ such that $(\mathbb{S},\odot )$ is a power-associative Kikkawa left loop with two-sided identity ${\mathbf{e}}_{0}$, i.e., it has the left inverse, automorphic inverse, and ${A}_{l}$ properties. The operation $\odot $ is compatible with the symmetric space structure of $\mathbb{S}$. $(\mathbb{S},\odot )$ is not a loop, and the right translations which fail to be injective are easily characterized. $(\mathbb{S},\odot )$ satisfies the left power alternative and left Bol identities “almost everywhere” but not everywhere....

In this paper, we construct a hyperkähler structure on the complexification ${\mathcal{O}}^{\u2102}$ of any Hermitian symmetric affine coadjoint orbit $\mathcal{O}$ of a semi-simple ${L}^{*}$-group of compact type, which is compatible with the complex symplectic form of Kirillov-Kostant-Souriau and restricts to the Kähler structure of $\mathcal{O}$. By a relevant identification of the complex orbit ${\mathcal{O}}^{\u2102}$ with the cotangent space $T\mathcal{O}$ of $\mathcal{O}$ induced by Mostow’s decomposition theorem, this leads to the existence of a hyperkähler structure on $T\mathcal{O}$ compatible with...

The purpose of this paper is to show how central extensions of (possibly infinite-dimensional) Lie algebras integrate to central extensions of étale Lie 2-groups in the sense of [Get09, Hen08]. In finite dimensions, central extensions of Lie algebras integrate to central extensions of Lie groups, a fact which is due to the vanishing of ${\pi}_{2}$ for each finite-dimensional Lie group. This fact was used by Cartan (in a slightly other guise) to construct the simply connected Lie group associated to each finite-dimensional...