2,614 research outputs found
Asymptotic estimates for interpolation and constrained approximation in H2 by diagonalization of Toeplitz operators
Sharp convergence rates are provided for interpolation and approximation schemes in the Hardy space H-2 that use band-limited data. By means of new explicit formulae for the spectral decomposition of certain Toeplitz operators, sharp estimates for Carleman and Krein-Nudel'man approximation schemes are derived. In addition, pointwise convergence results are obtained. An illustrative example based on experimental data from a hyperfrequency filter is provided
Consistency Conditions for Brane Worlds in Arbitrary Dimensions
We consider ``brane world sum rules'' for compactifications involving an
arbitrary number of spacetime dimensions. One of the most striking results
derived from such consistency conditions is the necessity for negative tension
branes to appear in five--dimensional scenarios. We show how this result is
easily evaded for brane world models with more than five dimensions. As an
example, we consider a novel realization of the Randall--Sundrum scenario in
six dimensions involving only positive tension branes.Comment: 18 pages, LaTex, refs. adde
Robust identification from band-limited data
Consider the problem of identifying a scalar bounded-input/bounded-output stable transfer function from pointwise measurements at frequencies within a bandwidth. We propose an algorithm which consists of building a sequence of maps from data to models converging uniformly to the transfer function on the bandwidth when the number of measurements goes to infinity, the noise level to zero, and asymptotically meeting some gauge constraint outside. Error bounds are derived, and the procedure is illustrated by numerical experiment
Anyons, group theory and planar physics
Relativistic and nonrelativistic anyons are described in a unified formalism
by means of the coadjoint orbits of the symmetry groups in the free case as
well as when there is an interaction with a constant electromagnetic field. To
deal with interactions we introduce the extended Poincar\'e and Galilei Maxwell
groups.Comment: 22 pages, journal reference added, bibliography update
A mechanical behavior law for the numerical simulation of the mushy zone in welding
The aim of this work is to propose a mechanical behavior law dedicated to the mushy zone located between the solid phase and the weld pool in welding. The objective is to take into account of the influence of the mushy zone in the simulation of welding in order to improve the computation of induced effects such as residual stresses
Hopf instantons, Chern-Simons vortices, and Heisenberg ferromagnets
The dimensional reduction of the three-dimensional fermion-Chern-Simons model
(related to Hopf maps) of Adam et el. is shown to be equivalent to (i) either
the static, fixed--chirality sector of our non-relativistic spinor-Chern-Simons
model in 2+1 dimensions, (ii) or a particular Heisenberg ferromagnet in the
plane.Comment: 4 pages, Plain Tex, no figure
Galilean Lee Model of the Delta Function Potential
The scattering cross section associated with a two dimensional delta function
has recently been the object of considerable study. It is shown here that this
problem can be put into a field theoretical framework by the construction of an
appropriate Galilean covariant theory. The Lee model with a standard Yukawa
interaction is shown to provide such a realization. The usual results for delta
function scattering are then obtained in the case that a stable particle exists
in the scattering channel provided that a certain limit is taken in the
relevant parameter space. In the more general case in which no such limit is
taken finite corrections to the cross section are obtained which (unlike the
pure delta function case) depend on the coupling constant of the model.Comment: 7 pages, latex, no figure
(In)finite extensions of algebras from their Inonu-Wigner contractions
The way to obtain massive non-relativistic states from the Poincare algebra
is twofold. First, following Inonu and Wigner the Poincare algebra has to be
contracted to the Galilean one. Second, the Galilean algebra is to be extended
to include the central mass operator. We show that the central extension might
be properly encoded in the non-relativistic contraction. In fact, any
Inonu-Wigner contraction of one algebra to another, corresponds to an infinite
tower of abelian extensions of the latter. The proposed method is
straightforward and holds for both central and non-central extensions. Apart
from the Bargmann (non-zero mass) extension of the Galilean algebra, our list
of examples includes the Weyl algebra obtained from an extension of the
contracted SO(3) algebra, the Carrollian (ultra-relativistic) contraction of
the Poincare algebra, the exotic Newton-Hooke algebra and some others. The
paper is dedicated to the memory of Laurent Houart (1967-2011).Comment: 7 pages, revtex style; v2: Minor corrections, references added; v3:
Typos correcte
Performing an Invisibility Spell: Global Models, Food Regimes and Smallholders
The present construction of global representations of food and farming is problematic. For example, how can we ‘know’ the world needs to double food production even though we cannot foresee a food crisis? How can we estimate investment opportunities while failing to quantify their impacts on smallholders? Global models constrain the manner in which we perceive the food regime while producing such representations. We need to identify the causal relations embedded inside models’ equations and why they are arrayed in this fashion. This article combines actor-network theory and structuration theory to analyse a sample of 70 global models. It locates the modules and equations of these black boxes in the sociotechnical and political context of their production. Finally, a bibliometric analysis sketches the overall epistemic community that drove models into success or extinction. Dominant global models recycle equations, modules and databases to effectuate narrow worlds. They make smallholder farming invisible in spite of its prevalence around the world. They do not address food needs and construct pixellated representations of underutilized land. They systematically favour large-scale agricultural trade and investments in production and productivity. This reflects the structure of signification modellers adhere to as well as the structure of domination they are embedded in. Securing clients ensures the success of global models independently from their validation. The article demonstrates the manner in which modelling is a social practice embedded in power relations. Considering simultaneously the structure of domination formalized inside models and surrounding modelling is crucial. Future research should investigate how various actors resort to global models to champion their goals. It should question the policy recommendations drawn from such models and their relevance as decision support tools.ualisms, what leads us to believe that dualistic oppositions are still a part of the agri-food reality and are something to take into account when different actors have to collaborate
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