5,003 research outputs found
The geometry of nondegeneracy conditions in completely integrable systems
Nondegeneracy conditions need to be imposed in K.A.M. theorems to insure that
the set of diophantine tori has a large measure. Although they are usually
expressed in action coordinates, it is possible to give a geometrical
formulation using the notion of regular completely integrable systems defined
by a fibration of a symplectic manifold by lagrangian tori together with a
Hamiltonian function constant on the fibers. In this paper, we give a
geometrical definition of different nondegeneracy conditions, we show the
implication relations that exist between them, and we show the uniqueness of
the fibration for non-degenerate Hamiltonians
Semi-classical approach for Anosov diffeomorphisms and Ruelle resonances
In this paper, we show that some spectral properties of Anosov
diffeomorphisms can be obtained by semi-classical analysis. In particular the
Ruelle resonances which are eigenvalues of the Ruelle transfer operator acting
in suitable anisotropic Sobolev spaces and which govern the decay of dynamical
correlations, can be treated as the quantum resonances of open quantum systems
in the Aguilar-Baslev-Combes theory or the more recent Helffer-Sjostrand
phase-space theory.Comment: 48 page
Heterogeneous multireference alignment: a single pass approach
Multireference alignment (MRA) is the problem of estimating a signal from
many noisy and cyclically shifted copies of itself. In this paper, we consider
an extension called heterogeneous MRA, where signals must be estimated, and
each observation comes from one of those signals, unknown to us. This is a
simplified model for the heterogeneity problem notably arising in cryo-electron
microscopy. We propose an algorithm which estimates the signals without
estimating either the shifts or the classes of the observations. It requires
only one pass over the data and is based on low-order moments that are
invariant under cyclic shifts. Given sufficiently many measurements, one can
estimate these invariant features averaged over the signals. We then design
a smooth, non-convex optimization problem to compute a set of signals which are
consistent with the estimated averaged features. We find that, in many cases,
the proposed approach estimates the set of signals accurately despite
non-convexity, and conjecture the number of signals that can be resolved as
a function of the signal length is on the order of .Comment: 6 pages, 3 figure
Ligand-dependent opening of the multiple AMPA receptor conductance states: a concerted model
Modulation of the properties of AMPA receptors at the post-synaptic membrane
is one of the main suggested mechanisms behind synaptic plasticity in the
central nervous system of vertebrates. Electrophysiological recordings of
single channels stimulated with agonists showed that both recombinant and
native AMPA receptors visit multiple conductance states in an agonist
concentration dependent manner. We propose an allosteric model of the multiple
conductance states based on concerted conformational transitions of the four
subunits, as an iris diaphragm. Our model predicts that the thermodynamic
behaviour of the conductance states upon full and partial agonist stimulations
can be described with increased affinity of receptors as they progress to
higher conductance states. The model also predicts existence of AMPA receptors
in non-liganded conductive substates. However, spontaneous openings probability
decreases with increasing conductances. Finally, we predict that the large
conductance states are stabilized within the rise phase of a whole-cell EPSC in
glutamatergic hippocampal neurons. Our model provides a mechanistic link
between ligand concentration and conductance states that can explain
thermodynamic and kinetic features of AMPA receptor gating.Comment: 4 figures, models available on demand. They will be published by
BioModels Database upon publication of the articl
Heterogeneity in Technical Efficiency of the French Urban Transport: 1995 to 2002
In this paper, we analyze the heterogeneity in the technical efficiency of a sample of French urban transport companies with a translog production frontier model. The model generates efficiency disentangling homogenous and heterogeneous variables. Our study concluded that outputs and inputs play a major role in transport efficiency and we find that the efficiency scores vary along the sample. Policy implication is derived.Urban Transport; France, Translog random Frontier Model and Decision-Making Unit.
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