131 research outputs found
Politique agricole et bien-être des consommateurs
The Swiss agriculture is considered to be one of the more subsidized in the world. Using an applied general equilibrium model, we study the effects of two reforms of the agricultural policy on the welfare of various categories of households. Our framework of analysis allows to take into account the high number of agricultural policy instruments set up by the government. The analysis of the reform AP 2002 shows that, in spite of almost non-existent aggregate welfare gains, great disparities appear between the various households considered. The farmers are by far the losers whereas the pensioners and the wealthy employees profit largely from the reform. With regard to project AP 2007, the effect of an auction of the tariff quotas is positive for most of the households in the sense that it increases their welfare gains compared to the reform AP 2002. However, the farmers, the pensioners as well as some other households do not improve their situation compared to AP 2002. As a whole, the project AP 2007 is nevertheless positive for the population except for the farmers and the some other households.agricultural policy; applied general equilibrium; welfare
Excitation of travelling multibreathers in anharmonic chains
We study the dynamics of the "externally" forced and damped Fermi-Pasta-Ulam
(FPU) 1D lattice. The forcing has the spatial symmetry of the Fourier mode with
wavenumber p and oscillates sinusoidally in time with the frequency omega. When
omega is in the phonon band, the p-mode becomes modulationally unstable above a
critical forcing, which we determine analytically in terms of the parameters of
the system. For omega above the phonon band, the instability of the p-mode
leads to the formation of a travelling multibreather, that, in the
low-amplitude limit could be described in terms of soliton solutions of a
suitable driven-damped nonlinear Schroedinger (NLS) equation. Similar
mechanisms of instability could show up in easy-axis magnetic structures, that
are governed by such NLS equations.Comment: To appear in Physica D (2002
Statistical mechanics of a nonlinear discrete system
Statistical mechanics of the discrete nonlinear Schr\"odinger equation is
studied by means of analytical and numerical techniques. The lower bound of the
Hamiltonian permits the construction of standard Gibbsian equilibrium measures
for positive temperatures. Beyond the line of , we identify a phase
transition, through a discontinuity in the partition function. The phase
transition is demonstrated to manifest itself in the creation of breather-like
localized excitations. Interrelation between the statistical mechanics and the
nonlinear dynamics of the system is explored numerically in both regimes.Comment: 4 pages, 3 figure
Localization and Equipartition of Energy in the beta-FPU Chain : Chaotic Breathers
The evolution towards equipartition in the -FPU chain is studied
considering as initial condition the highest frequency mode. Above an
analytically derived energy threshold, this zone-boundary mode is shown to be
modulationally unstable and to give rise to a striking localization process.
The spontaneously created excitations have strong similarity with moving exact
breathers solutions. But they have a finite lifetime and their dynamics is
chaotic. These chaotic breathers are able to collect very efficiently the
energy in the chain. Therefore their size grows in time and they can transport
a very large quantity of energy. These features can be explained analyzing the
dynamics of perturbed exact breathers of the FPU chain. In particular, a close
connection between the Lyapunov spectrum of the chaotic breathers and the
Floquet spectrum of the exact ones has been found. The emergence of chaotic
breathers is convincingly explained by the absorption of high frequency phonons
whereas a breather's metastability is for the first time identified. The
lifetime of the chaotic breather is related to the time necessary for the
system to reach equipartition. The equipartition time turns out to be dependent
on the system energy density only. Moreover, such time diverges as
in the limit and vanishes as
for .Comment: 20 pages, Revtex - Submitted to Physica
Bushes of vibrational modes for Fermi-Pasta-Ulam chains
Some exact solutions and multi-mode invariant submanifolds were found for the
Fermi-Pasta-Ulam (FPU) beta-model by Poggi and Ruffo in Phys. D 103 (1997) 251.
In the present paper we demonstrate how results of such a type can be obtained
for an arbitrary N-particle chain with periodic boundary conditions with the
aid of our group-theoretical approach [Phys. D 117 (1998) 43] based on the
concept of bushes of normal modes for mechanical systems with discrete
symmetry. The integro-differential equation describing the FPU-alfa dynamics in
the modal space is derived. The loss of stability of the bushes of modes for
the FPU-alfa model, in particular, for the limiting case N >> 1 for the
dynamical regime with displacement pattern having period twice the lattice
spacing (Pi-mode) is studied. Our results for the FPU-alfa chain are compared
with those by Poggi and Ruffo for the FPU-beta chain.Comment: To be published in Physica
On modulational instability and energy localization in anharmonic lattices at finite energy density
The localization of vibrational energy, induced by the modulational
instability of the Brillouin-zone-boundary mode in a chain of classical
anharmonic oscillators with finite initial energy density, is studied within a
continuum theory. We describe the initial localization stage as a gas of
envelope solitons and explain their merging, eventually leading to a single
localized object containing a macroscopic fraction of the total energy of the
lattice. The initial-energy-density dependences of all characteristic time
scales of the soliton formation and merging are described analytically. Spatial
power spectra are computed and used for the quantitative explanation of the
numerical results.Comment: 12 pages, 7 figure
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