15,990 research outputs found
A Bayesian framework for optimal motion planning with uncertainty
Modeling robot motion planning with uncertainty in a Bayesian framework leads to a computationally intractable stochastic control problem. We seek hypotheses that can justify a separate implementation of control, localization and planning. In the end, we reduce the stochastic control problem to path- planning in the extended space of poses x covariances; the transitions between states are modeled through the use of the Fisher information matrix. In this framework, we consider two problems: minimizing the execution time, and minimizing the final covariance, with an upper bound on the execution time. Two correct and complete algorithms are presented. The first is the direct extension of classical graph-search algorithms in the extended space. The second one is a back-projection algorithm: uncertainty constraints are propagated backward from the goal towards the start state
Variational principle for the Wheeler-Feynman electrodynamics
We adapt the formally-defined Fokker action into a variational principle for
the electromagnetic two-body problem. We introduce properly defined boundary
conditions to construct a Poincare-invariant-action-functional of a finite
orbital segment into the reals. The boundary conditions for the variational
principle are an endpoint along each trajectory plus the respective segment of
trajectory for the other particle inside the lightcone of each endpoint. We
show that the conditions for an extremum of our functional are the
mixed-type-neutral-equations with implicit state-dependent-delay of the
electromagnetic-two-body problem. We put the functional on a natural Banach
space and show that the functional is Frechet-differentiable. We develop a
method to calculate the second variation for C2 orbital perturbations in
general and in particular about circular orbits of large enough radii. We prove
that our functional has a local minimum at circular orbits of large enough
radii, at variance with the limiting Kepler action that has a minimum at
circular orbits of arbitrary radii. Our results suggest a bifurcation at some
radius below which the circular orbits become saddle-point extrema. We give a
precise definition for the distributional-like integrals of the Fokker action
and discuss a generalization to a Sobolev space of trajectories where the
equations of motion are satisfied almost everywhere. Last, we discuss the
existence of solutions for the state-dependent delay equations with slightly
perturbated arcs of circle as the boundary conditions and the possibility of
nontrivial solenoidal orbits
Minimizers with discontinuous velocities for the electromagnetic variational method
The electromagnetic two-body problem has \emph{neutral differential delay}
equations of motion that, for generic boundary data, can have solutions with
\emph{discontinuous} derivatives. If one wants to use these neutral
differential delay equations with \emph{arbitrary} boundary data, solutions
with discontinuous derivatives must be expected and allowed. Surprisingly,
Wheeler-Feynman electrodynamics has a boundary value variational method for
which minimizer trajectories with discontinuous derivatives are also expected,
as we show here. The variational method defines continuous trajectories with
piecewise defined velocities and accelerations, and electromagnetic fields
defined \emph{by} the Euler-Lagrange equations \emph{% on} trajectory points.
Here we use the piecewise defined minimizers with the Li{\'{e}}nard-Wierchert
formulas to define generalized electromagnetic fields almost everywhere (but on
sets of points of zero measure where the advanced/retarded velocities and/or
accelerations are discontinuous). Along with this generalization we formulate
the \emph{generalized absorber hypothesis} that the far fields vanish
asymptotically \emph{almost everywhere%} and show that localized orbits with
far fields vanishing almost everywhere \emph{must} have discontinuous
velocities on sewing chains of breaking points. We give the general solution
for localized orbits with vanishing far fields by solving a (linear) neutral
differential delay equation for these far fields. We discuss the physics of
orbits with discontinuous derivatives stressing the differences to the
variational methods of classical mechanics and the existence of a spinorial
four-current associated with the generalized variational electrodynamics.Comment: corrected minor typo: piecewise differentiable on closed instead of
open interval
Testing Primordial Black Holes as Dark Matter through LISA
The idea that primordial black holes (PBHs) can comprise most of the dark
matter of the universe has recently reacquired a lot of momentum. Observational
constraints, however, rule out this possibility for most of the PBH masses,
with a notable exception around . These light PBHs may be
originated when a sizeable comoving curvature perturbation generated during
inflation re-enters the horizon during the radiation phase. During such a
stage, it is unavoidable that gravitational waves (GWs) are generated. Since
their source is quadratic in the curvature perturbations, these GWs are
generated fully non-Gaussian. Their frequency today is about the mHz, which is
exactly the range where the LISA mission has the maximum of its sensitivity.
This is certainly an impressive coincidence. We show that this scenario of PBHs
as dark matter can be tested by LISA by measuring the GW two-point correlator.
On the other hand, we show that the short observation time (as compared to the
age of the universe) and propagation effects of the GWs across the perturbed
universe from the production point to the LISA detector suppress the bispectrum
to an unobservable level. This suppression is completely general and not
specific to our model.Comment: 22 pages, 12 figures. v3: matching published versio
To work or not to work? Immigrant women and work between constraints and opportunities : a comparison between Lombardy and Cataluña
The aim of this article is to analyse how family constraints, in term of gender roles and child-care responsibilities, affect the behaviour and attitudes of immigrant women towards work, comparing Italy (Lombardy) and Spain (Barcelona). Moreover, personal skills, the economic crisis, and the limited job opportunities for migrants (who access mostly low-qualified jobs) also play a role in the employment status of immigrant women. Therefore, we are interested in understanding the reasons that make women decide to work (or seek a job) or not, and how they feel about their work or their unemployed status. We will analyse 72 interviews (47 in Italy and 25 in Spain) with working and non-working women of different nationalities, taking into account years of staying in the hosting country, family composition, level of education, and kind of job, in order to combine these different aspects with the attitudes and outcomes towards work
Expectations For an Interferometric Sunyaev-Zel'dovich Effect Survey for Galaxy Clusters
Non-targeted surveys for galaxy clusters using the Sunyaev-Zel'dovich effect
(SZE) will yield valuable information on both cosmology and evolution of the
intra-cluster medium (ICM). The redshift distribution of detected clusters will
constrain cosmology, while the properties of the discovered clusters will be
important for studies of the ICM and galaxy formation. Estimating survey yields
requires a detailed model for both cluster properties and the survey strategy.
We address this by making mock observations of galaxy clusters in cosmological
hydrodynamical simulations. The mock observatory consists of an interferometric
array of ten 2.5 m diameter telescopes, operating at a central frequency of 30
GHz with a bandwidth of 8 GHz. We find that clusters with a mass above will be detected at any redshift, with the
exact limit showing a very modest redshift dependence. Using a Press-Schechter
prescription for evolving the number densities of clusters with redshift, we
determine that such a survey should find hundreds of galaxy clusters per year,
many at high redshifts and relatively low mass -- an important regime uniquely
accessible to SZE surveys. Currently favored cosmological models predict
roughly 25 clusters per square degree.Comment: revised to match published versio
Diversity of Rainfall Thresholds for early warning of hydro-geological disasters
For early warning of disasters induced by precipitation
(such as floods and landslides), different kinds of rainfall thresholds are
adopted, which vary from each other, on the basis on adopted hypotheses. In
some cases, they represent the occurrence probability of an event (landslide
or flood), in other cases the exceedance probability of a critical value for
an assigned indicator I (a function of rainfall heights), and in further
cases they only indicate the exceeding of a prefixed percentage a critical
value for I, indicated as Icr. For each scheme, it is usual to define
three different criticality levels (ordinary, moderate and severe), which
are associated to warning levels, according to emergency plans. This work
briefly discusses different schemes of rainfall thresholds, focusing
attention on landslide prediction, with some applications to a real case
study in Calabria region (southern Italy)
Mesoscopic continuous and discrete channels for quantum information transfer
We study the possibility of realizing perfect quantum state transfer in
mesoscopic devices. We discuss the case of the Fano-Anderson model extended to
two impurities. For a channel with an infinite number of degrees of freedom, we
obtain coherent behavior in the case of strong coupling or in weak coupling
off-resonance. For a finite number of degrees of freedom, coherent behavior is
associated to weak coupling and resonance conditions
An overview of the EXTraS project: Exploring the X-ray Transient and Variable Sky
The EXTraS project (Exploring the X-ray Transient and variable Sky) will
harvest the hitherto unexplored temporal domain information buried in the
serendipitous data collected by the European Photon Imaging Camera (EPIC)
instrument onboard the ESA XMM-Newton X-ray observatory since its launch. This
will include a search for fast transients, as well as a search and
characterization of variability (both periodic and aperiodic) in hundreds of
thousands of sources spanning more than nine orders of magnitude in time scale
and six orders of magnitude in flux. X-ray results will be complemented by
multiwavelength characterization of new discoveries. Phenomenological
classification of variable sources will also be performed. All our results will
be made available to the community. A didactic program in selected High Schools
in Italy, Germany and the UK will also be implemented. The EXTraS project
(2014-2016), funded within the EU/FP7 framework, is carried out by a
collaboration including INAF (Italy), IUSS (Italy), CNR/IMATI (Italy),
University of Leicester (UK), MPE (Germany) and ECAP (Germany).Comment: 6 pages, 1 figure. Proceedings of "Swift: 10 years of Discovery", to
appear in Po
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