25 research outputs found
Induction of Hamiltonian Poisson actions
We propose a Poisson-Lie analog of the symplectic induction procedure, using
an appropriate Poisson generalization of the reduction of symplectic manifolds
with symmetry. Having as basic tools the equivariant momentum maps of Poisson
actions, the double group of a Poisson-Lie group and the reduction of Poisson
manifolds with symmetry, we show how one can induce a Poisson action admitting
an equivariant momentum map. We prove that, under certain conditions, the
dressing orbits of a Poisson-Lie group can be obtained by Poisson induction
from the dressing orbits of a Poisson-Lie subgroup.Comment: 17 pages, Plain TeX file, to appear in Diff. Geom. App
A construction of symplectic connections through reduction
We give an elementary construction of symplectic connections through
reduction. This provides an elegant description of a class of symmetric spaces
and gives examples of symplectic connections with Ricci type curvature, which
are not locally symmetric; the existence of such symplectic connections was
unknown.Comment: 16 pages, Plain TeX fil
Semidirect products and the Pukanszky condition
We study the general geometrical structure of the coadjoint orbits of a
semidirect product formed by a Lie group and a representation of this group on
a vector space. The use of symplectic induction methods gives new insight into
the structure of these orbits. In fact, each coadjoint orbit of such a group is
obtained by symplectic induction on some coadjoint orbit of a "smaller" Lie
group. We study also a special class of polarizations related to a semidirect
product and the validity of Pukanszky's condition for these polarizations. Some
examples of physical interest are discussed using the previous methods.Comment: 33 pages, including special macros and fonts (JGPpaper.tex is the
source TeX file), to appear in J. Geom. Phys., also available via anonymous
ftp or via gopher gopher://cpt.univ-mrs.fr
The validation service of the hydrological SAF geostationary and polar satellite precipitation products
Abstract. The development phase (DP) of the EUMETSAT Satellite Application Facility for Support to Operational Hydrology and Water Management (H-SAF) led to the design and implementation of several precipitation products, after 5 yr (2005–2010) of activity. Presently, five precipitation estimation algorithms based on data from passive microwave and infrared sensors, on board geostationary and sun-synchronous platforms, function in operational mode at the H-SAF hosting institute to provide near real-time precipitation products at different spatial and temporal resolutions. In order to evaluate the precipitation product accuracy, a validation activity has been established since the beginning of the project. A Precipitation Product Validation Group (PPVG) works in parallel with the development of the estimation algorithms with two aims: to provide the algorithm developers with indications to refine algorithms and products, and to evaluate the error structure to be associated with the operational products. In this paper, the framework of the PPVG is presented: (a) the characteristics of the ground reference data available to H-SAF (i.e. radar and rain gauge networks), (b) the agreed upon validation strategy settled among the eight European countries participating in the PPVG, and (c) the steps of the validation procedures. The quality of the reference data is discussed, and the efforts for its improvement are outlined, with special emphasis on the definition of a ground radar quality map and on the implementation of a suitable rain gauge interpolation algorithm. The work done during the H-SAF development phase has led the PPVG to converge into a common validation procedure among the members, taking advantage of the experience acquired by each one of them in the validation of H-SAF products. The methodology is presented here, indicating the main steps of the validation procedure (ground data quality control, spatial interpolation, up-scaling of radar data vs. satellite grid, statistical score evaluation, case study analysis). Finally, an overview of the results is presented, focusing on the monthly statistical indicators, referred to the satellite product performances over different seasons and areas
Superization of Homogeneous Spin Manifolds and Geometry of Homogeneous Supermanifolds
Let M_0=G_0/H be a (pseudo)-Riemannian homogeneous spin manifold, with
reductive decomposition g_0=h+m and let S(M_0) be the spin bundle defined by
the spin representation Ad:H->\GL_R(S) of the stabilizer H. This article
studies the superizations of M_0, i.e. its extensions to a homogeneous
supermanifold M=G/H whose sheaf of superfunctions is isomorphic to
Lambda(S^*(M_0)). Here G is the Lie supergroup associated with a certain
extension of the Lie algebra of symmetry g_0 to an algebra of supersymmetry
g=g_0+g_1=g_0+S via the Kostant-Koszul construction. Each algebra of
supersymmetry naturally determines a flat connection nabla^{S} in the spin
bundle S(M_0). Killing vectors together with generalized Killing spinors (i.e.
nabla^{S}-parallel spinors) are interpreted as the values of appropriate
geometric symmetries of M, namely even and odd Killing fields. An explicit
formula for the Killing representation of the algebra of supersymmetry is
obtained, generalizing some results of Koszul. The generalized spin connection
nabla^{S} defines a superconnection on M, via the super-version of a theorem of
Wang.Comment: 50 page