6,276 research outputs found
Spectral properties of a class of random walks on locally finite groups
We study some spectral properties of random walks on infinite countable
amenable groups with an emphasis on locally finite groups, e.g. the infinite
symmetric group. On locally finite groups, the random walks under consideration
are driven by infinite divisible distributions. This allows us to embed our
random walks into continuous time L\'evy processes whose heat kernels have
shapes similar to the ones of alpha-stable processes. We obtain examples of
fast/slow decays of return probabilities, a recurrence criterion, exact values
and estimates of isospectral profiles and spectral distributions, formulae and
estimates for the escape rates and for heat kernels.Comment: 62 pages, 1 figure, 2 table
Guidelines for a Dynamic Ontology - Integrating Tools of Evolution and Versioning in Ontology
Ontologies are built on systems that conceptually evolve over time. In
addition, techniques and languages for building ontologies evolve too. This has
led to numerous studies in the field of ontology versioning and ontology
evolution. This paper presents a new way to manage the lifecycle of an ontology
incorporating both versioning tools and evolution process. This solution,
called VersionGraph, is integrated in the source ontology since its creation in
order to make it possible to evolve and to be versioned. Change management is
strongly related to the model in which the ontology is represented. Therefore,
we focus on the OWL language in order to take into account the impact of the
changes on the logical consistency of the ontology like specified in OWL DL
Connected Lie groups and property RD
For a locally compact group, property RD gives a control on the convolution
norm of any compactly supported measure in terms of the -norm of its
density and the diameter of its support. We give a complete classification of
those Lie groups with property RD.Comment: 29 page
Bounded characteristic classes and flat bundles
Let G be a connected Lie group, G^d the underlying discrete group, and BG,
BG^d their classifying spaces. Let R denote the radical of G. We show that all
classes in the image of the canonical map in cohomology H^*(BG,R)->H^*(BG^d,R)
are bounded if and only if the derived group [R,R] is simply connected. We also
give equivalent conditions in terms of stable commutator length and distortion.Comment: 12 pages, no figur
Six-axis decentralized control design for spacecraft formation flying mission
This contribution addresses the control design for
the three-spacecraft formation flying interferometry mission Pegase. The operational mode considered is the high-precision nulling phase. The control design has as major objective the minimization of the variance of the controlled outputs, e.g. the optical path difference. The payload performance demands are shown to be fulfilled in spite of orbital disturbances, solar radiation pressure as well as sensor and actuator noise. Furthermore, a novel iterative algorithm is proposed, capable of designing decentralized H2-suboptimal controllers. These controllers consist of a set of individual closed loops on board the different spacecraft which only use locally available
measurements, forces and torques. This approach reduces
communication bandwidth and enhances robustness concerning
faulty communication links. Finally, the performance loss due to decentralization is investigated
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