461 research outputs found
Introducing symplectic billiards
In this article we introduce a simple dynamical system called symplectic
billiards. As opposed to usual/Birkhoff billiards, where length is the
generating function, for symplectic billiards symplectic area is the generating
function. We explore basic properties and exhibit several similarities, but
also differences of symplectic billiards to Birkhoff billiards.Comment: 41 pages, 16 figure
Shape-based invariant features extraction for object recognition
International audienceThe emergence of new technologies enables generating large quantity of digital information including images; this leads to an increasing number of generated digital images. Therefore it appears a necessity for automatic systems for image retrieval. These systems consist of techniques used for query specification and re-trieval of images from an image collection. The most frequent and the most com-mon means for image retrieval is the indexing using textual keywords. But for some special application domains and face to the huge quantity of images, key-words are no more sufficient or unpractical. Moreover, images are rich in content; so in order to overcome these mentioned difficulties, some approaches are pro-posed based on visual features derived directly from the content of the image: these are the content-based image retrieval (CBIR) approaches. They allow users to search the desired image by specifying image queries: a query can be an exam-ple, a sketch or visual features (e.g., colour, texture and shape). Once the features have been defined and extracted, the retrieval becomes a task of measuring simi-larity between image features. An important property of these features is to be in-variant under various deformations that the observed image could undergo. In this chapter, we will present a number of existing methods for CBIR applica-tions. We will also describe some measures that are usually used for similarity measurement. At the end, and as an application example, we present a specific ap-proach, that we are developing, to illustrate the topic by providing experimental results
Maps, immersions and permutations
We consider the problem of counting and of listing topologically inequivalent
"planar" {4-valent} maps with a single component and a given number n of
vertices. This enables us to count and to tabulate immersions of a circle in a
sphere (spherical curves), extending results by Arnold and followers. Different
options where the circle and/or the sphere are/is oriented are considered in
turn, following Arnold's classification of the different types of symmetries.
We also consider the case of bicolourable and bicoloured maps or immersions,
where faces are bicoloured. Our method extends to immersions of a circle in a
higher genus Riemann surface. There the bicolourability is no longer automatic
and has to be assumed. We thus have two separate countings in non zero genus,
that of bicolourable maps and that of general maps. We use a classical method
of encoding maps in terms of permutations, on which the constraints of
"one-componentness" and of a given genus may be applied. Depending on the
orientation issue and on the bicolourability assumption, permutations for a map
with n vertices live in S(4n) or in S(2n). In a nutshell, our method reduces to
the counting (or listing) of orbits of certain subset of S(4n) (resp. S(2n))
under the action of the centralizer of a certain element of S(4n) (resp.
S(2n)). This is achieved either by appealing to a formula by Frobenius or by a
direct enumeration of these orbits. Applications to knot theory are briefly
mentioned.Comment: 46 pages, 18 figures, 9 tables. Version 2: added precisions on the
notion used for the equivalence of immersed curves, new references. Version
3: Corrected typos, one array in Appendix B1 was duplicated by mistake, the
position of tables and the order of the final sections have been modified,
results unchanged. To be published in the Journal of Knot Theory and Its
Ramification
A finite difference method for the solution of the transonic flow around harmonically oscillating wings
A finite difference method for the solution of the transonic flow about a harmonically oscillating wing is presented. The partial differential equation for the unsteady transonic flow was linearized by dividing the flow into separate steady and unsteady perturbation velocity potentials and by assuming small amplitudes of harmonic oscillation. The resulting linear differential equation is of mixed type, being elliptic or hyperbolic whereever the steady flow equation is elliptic or hyperbolic. Central differences were used for all derivatives except at supersonic points where backward differencing was used for the streamwise direction. Detailed formulas and procedures are described in sufficient detail for programming on high speed computers. To test the method, the problem of the oscillating flap on a NACA 64A006 airfoil was programmed. The numerical procedure was found to be stable and convergent even in regions of local supersonic flow with shocks
Phase retrieval for characteristic functions of convex bodies and reconstruction from covariograms
We propose strongly consistent algorithms for reconstructing the
characteristic function 1_K of an unknown convex body K in R^n from possibly
noisy measurements of the modulus of its Fourier transform \hat{1_K}. This
represents a complete theoretical solution to the Phase Retrieval Problem for
characteristic functions of convex bodies. The approach is via the closely
related problem of reconstructing K from noisy measurements of its covariogram,
the function giving the volume of the intersection of K with its translates. In
the many known situations in which the covariogram determines a convex body, up
to reflection in the origin and when the position of the body is fixed, our
algorithms use O(k^n) noisy covariogram measurements to construct a convex
polytope P_k that approximates K or its reflection -K in the origin. (By recent
uniqueness results, this applies to all planar convex bodies, all
three-dimensional convex polytopes, and all symmetric and most (in the sense of
Baire category) arbitrary convex bodies in all dimensions.) Two methods are
provided, and both are shown to be strongly consistent, in the sense that,
almost surely, the minimum of the Hausdorff distance between P_k and K or -K
tends to zero as k tends to infinity.Comment: Version accepted on the Journal of the American Mathematical Society.
With respect to version 1 the noise model has been greatly extended and an
appendix has been added, with a discussion of rates of convergence and
implementation issues. 56 pages, 4 figure
Looking backward: From Euler to Riemann
We survey the main ideas in the early history of the subjects on which
Riemann worked and that led to some of his most important discoveries. The
subjects discussed include the theory of functions of a complex variable,
elliptic and Abelian integrals, the hypergeometric series, the zeta function,
topology, differential geometry, integration, and the notion of space. We shall
see that among Riemann's predecessors in all these fields, one name occupies a
prominent place, this is Leonhard Euler. The final version of this paper will
appear in the book \emph{From Riemann to differential geometry and relativity}
(L. Ji, A. Papadopoulos and S. Yamada, ed.) Berlin: Springer, 2017
Higher Order Corrections in Perturbative Quantum Field Theory via Sector Decomposition
The calculation of higher order corrections in perturbative quantum field theories is
a particularly important subject. Our current model for particle physics is the stan-
dard model; a quantum field theory which has served to describe a huge amount
of observed data very well. As the Large Hadron Collider is collecting more and
more high energy data with smaller and smaller experimental errors, the accuracy
of theoretical calculations must keep up with experiment in order to discriminate be-
tween physics arising from our current standard model, and beyond standard model
physics.
In chapter 2 we give a brief introduction to the fundamentals of perturbative quan-
tum field theories, with particular emphasis on Quantum ChromoDynamics, where
higher order calculations are particularly important due to the fact that
αs (M_Z) >> α. In chapter 3 we present a review of methods for calculations within
perturbative quantum field theories, both for real and virtual corrections. In chap-
ter 4 we give a detailed explanation of the method of sector decomposition, and
highlight how it can be applied to the calculation of multi-parameter polynomial
integrals, which appear widely in high energy physics, and in particular within the
higher order calculations of perturbative quantum field theories. In chapter 5 we
present SecDec - a publicly available computer code which implements sector de-
composition. We give a range of examples to demonstrate its power in calculating
various integrals appearing in higher order calculations in perturbative quantum
field theories
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