10,575 research outputs found
Linear and convex aggregation of density estimators
We study the problem of linear and convex aggregation of estimators of a
density with respect to the mean squared risk. We provide procedures for linear
and convex aggregation and we prove oracle inequalities for their risks. We
also obtain lower bounds showing that these procedures are rate optimal in a
minimax sense. As an example, we apply general results to aggregation of
multivariate kernel density estimators with different bandwidths. We show that
linear and convex aggregates mimic the kernel oracles in asymptotically exact
sense for a large class of kernels including Gaussian, Silverman's and
Pinsker's ones. We prove that, for Pinsker's kernel, the proposed aggregates
are sharp asymptotically minimax simultaneously over a large scale of Sobolev
classes of densities. Finally, we provide simulations demonstrating performance
of the convex aggregation procedure.Comment: 22 page
Exponential Screening and optimal rates of sparse estimation
In high-dimensional linear regression, the goal pursued here is to estimate
an unknown regression function using linear combinations of a suitable set of
covariates. One of the key assumptions for the success of any statistical
procedure in this setup is to assume that the linear combination is sparse in
some sense, for example, that it involves only few covariates. We consider a
general, non necessarily linear, regression with Gaussian noise and study a
related question that is to find a linear combination of approximating
functions, which is at the same time sparse and has small mean squared error
(MSE). We introduce a new estimation procedure, called Exponential Screening
that shows remarkable adaptation properties. It adapts to the linear
combination that optimally balances MSE and sparsity, whether the latter is
measured in terms of the number of non-zero entries in the combination
( norm) or in terms of the global weight of the combination (
norm). The power of this adaptation result is illustrated by showing that
Exponential Screening solves optimally and simultaneously all the problems of
aggregation in Gaussian regression that have been discussed in the literature.
Moreover, we show that the performance of the Exponential Screening estimator
cannot be improved in a minimax sense, even if the optimal sparsity is known in
advance. The theoretical and numerical superiority of Exponential Screening
compared to state-of-the-art sparse procedures is also discussed
Isotropic Huygens dipoles and multipoles with colloidal particles
Huygens sources are elements that scatter light in the forward direction as
used in the Huygens-Fresnel principle. They have remained fictitious until
recently where experimental systems have been fabricated. In this letter, we
propose isotropic meta-atoms that act as Huygens sources. Using clusters of
plasmonic or dielectric colloidal particles, Huygens dipoles that resonate at
visible frequencies can be achieved with scattering cross-sections as high as 5
times the geometric cross-section of the particle surpassing anything
achievable with a hypothetical simple spherical particle. Examples are given
that predict extremely broadband scattering in the forward direction over a
1000 nm wavelength range at optical frequencies. These systems are important to
the fields of nanoantennas, metamaterials and wave physics in general as well
as any application that requires local control over the radiation properties of
a system as in solar cells or bio-sensing
On the co-orbital motion in the planar restricted three-body problem: the quasi-satellite motion revisited
In the framework of the planar and circular restricted three-body problem, we
consider an asteroid that orbits the Sun in quasi-satellite motion with a
planet. A quasi-satellite trajectory is a heliocentric orbit in co-orbital
resonance with the planet, characterized by a non zero eccentricity and a
resonant angle that librates around zero. Likewise, in the rotating frame with
the planet it describes the same trajectory as the one of a retrograde
satellite even though the planet acts as a perturbator. In the last few years,
the discoveries of asteroids in this type of motion made the term
"quasi-satellite" more and more present in the literature. However, some
authors rather use the term "retrograde satellite" when referring to this kind
of motion in the studies of the restricted problem in the rotating frame. In
this paper we intend to clarify the terminology to use, in order to bridge the
gap between the perturbative co-orbital point of view and the more general
approach in the rotating frame. Through a numerical exploration of the
co-orbital phase space, we describe the quasi-satellite domain and highlight
that it is not reachable by low eccentricities by averaging process. We will
show that the quasi-satellite domain is effectively included in the domain of
the retrograde satellites and neatly defined in terms of frequencies.
Eventually, we highlight a remarkable high eccentric quasi-satellite orbit
corresponding to a frozen ellipse in the heliocentric frame. We extend this
result to the eccentric case (planet on an eccentric motion) and show that two
families of frozen ellipses originate from this remarkable orbit.Comment: 30 pages, 13 figures, 1 tabl
Proportionality of components, Liouville theorems and a priori estimates for noncooperative elliptic systems
We study qualitative properties of positive solutions of noncooperative,
possibly nonvariational, elliptic systems. We obtain new classification and
Liouville type theorems in the whole Euclidean space, as well as in
half-spaces, and deduce a priori estimates and existence of positive solutions
for related Dirichlet problems. We significantly improve the known results for
a large class of systems involving a balance between repulsive and attractive
terms. This class contains systems arising in biological models of
Lotka-Volterra type, in physical models of Bose-Einstein condensates and in
models of chemical reactions.Comment: 35 pages, to appear in Archive Rational Mech. Ana
Rigorous treatment of the averaging process for co-orbital motions in the planetary problem
We develop a rigorous analytical Hamiltonian formalism adapted to the study
of the motion of two planets in co-orbital resonance. By constructing a complex
domain of holomorphy for the planetary Hamilto-nian, we estimate the size of
the transformation that maps this Hamil-tonian to its first order averaged over
one of the fast angles. After having derived an integrable approximation of the
averaged problem, we bound the distance between this integrable approximation
and the averaged Hamiltonian. This finally allows to prove rigorous theorems on
the behavior of co-orbital motions over a finite but large timescale
The family of Quasi-satellite periodic orbits in the circular co-planar RTBP
In the circular case of the coplanar Restricted Three-body Problem, we
studied how the family of quasi-satellite (QS) periodic orbits allows to define
an associated libration center. Using the averaged problem, we highlighted a
validity limit of this one: for QS orbits with low eccentricities, the averaged
problem does not correspond to the real problem. We do the same procedure to L
3 , L 4 and L 5 emerging periodic orbits families and remarked that for very
high eccentricities F L4 and F L5 merge with F L3 which bifurcates to a stable
family
Spin-orbit coupling and chaotic rotation for coorbital bodies in quasi-circular orbits
Coorbital bodies are observed around the Sun sharing their orbits with the
planets, but also in some pairs of satellites around Saturn. The existence of
coorbital planets around other stars has also been proposed. For close-in
planets and satellites, the rotation slowly evolves due to dissipative tidal
effects until some kind of equilibrium is reached. When the orbits are nearly
circular, the rotation period is believed to always end synchronous with the
orbital period. Here we demonstrate that for coorbital bodies in quasi-circular
orbits, stable non-synchronous rotation is possible for a wide range of mass
ratios and body shapes. We show the existence of an entirely new family of
spin-orbit resonances at the frequencies , where is the
orbital mean motion, the orbital libration frequency, and an integer.
In addition, when the natural rotational libration frequency due to the axial
asymmetry, , has the same magnitude as , the rotation becomes
chaotic. Saturn coorbital satellites are synchronous since , but
coorbital exoplanets may present non-synchronous or chaotic rotation. Our
results prove that the spin dynamics of a body cannot be dissociated from its
orbital environment. We further anticipate that a similar mechanism may affect
the rotation of bodies in any mean-motion resonance.Comment: 6 pages. Astrophysical Journal (2013) 6p
Background subtraction based on Local Shape
We present a novel approach to background subtraction that is based on the
local shape of small image regions. In our approach, an image region centered
on a pixel is mod-eled using the local self-similarity descriptor. We aim at
obtaining a reliable change detection based on local shape change in an image
when foreground objects are moving. The method first builds a background model
and compares the local self-similarities between the background model and the
subsequent frames to distinguish background and foreground objects.
Post-processing is then used to refine the boundaries of moving objects.
Results show that this approach is promising as the foregrounds obtained are
com-plete, although they often include shadows.Comment: 4 pages, 5 figures, 3 tabl
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