467 research outputs found
Phase transitions as topology changes in configuration space: an exact result
The phase transition in the mean-field XY model is shown analytically to be
related to a topological change in its configuration space. Such a topology
change is completely described by means of Morse theory allowing a computation
of the Euler characteristic--of suitable submanifolds of configuration
space--which shows a sharp discontinuity at the phase transition point, also at
finite N. The present analytic result provides, with previous work, a new key
to a possible connection of topological changes in configuration space as the
origin of phase transitions in a variety of systems.Comment: REVTeX file, 5 pages, 1 PostScript figur
Geometric Approach to Lyapunov Analysis in Hamiltonian Dynamics
As is widely recognized in Lyapunov analysis, linearized Hamilton's equations
of motion have two marginal directions for which the Lyapunov exponents vanish.
Those directions are the tangent one to a Hamiltonian flow and the gradient one
of the Hamiltonian function. To separate out these two directions and to apply
Lyapunov analysis effectively in directions for which Lyapunov exponents are
not trivial, a geometric method is proposed for natural Hamiltonian systems, in
particular. In this geometric method, Hamiltonian flows of a natural
Hamiltonian system are regarded as geodesic flows on the cotangent bundle of a
Riemannian manifold with a suitable metric. Stability/instability of the
geodesic flows is then analyzed by linearized equations of motion which are
related to the Jacobi equations on the Riemannian manifold. On some geometric
setting on the cotangent bundle, it is shown that along a geodesic flow in
question, there exist Lyapunov vectors such that two of them are in the two
marginal directions and the others orthogonal to the marginal directions. It is
also pointed out that Lyapunov vectors with such properties can not be obtained
in general by the usual method which uses linearized Hamilton's equations of
motion. Furthermore, it is observed from numerical calculation for a model
system that Lyapunov exponents calculated in both methods, geometric and usual,
coincide with each other, independently of the choice of the methods.Comment: 22 pages, 14 figures, REVTeX
On the approach to equilibrium of an Hamiltonian chain of anharmonic oscillators
In this note we study the approach to equilibrium of a chain of anharmonic
oscillators. We find indications that a sufficiently large system always
relaxes to the usual equilibrium distribution. There is no sign of an
ergodicity threshold. The time however to arrive to equilibrium diverges when
, being the anharmonicity.Comment: 8 pages, 5 figure
Semi-Analytic Estimates of Lyapunov Exponents in Lower-Dimensional Systems
Recent work has shown that statistical arguments, seemingly well-justified in
higher dimensions, can also be used to derive reasonable, albeit less accurate,
estimates of the largest Lyapunov exponent in lower-dimensional
Hamiltonian systems. This letter explores the detailed assumptions incorporated
into these arguments. The predicted values of are insensitive to most
of these details, which can in any event be relaxed straightforwardly, but {\em
can} depend sensitively on the nongeneric form of the auto-correlation function
characterising the time-dependence of an orbit. This dependence on dynamics
implies a fundamental limitation to the application of thermodynamic arguments
to such lower-dimensional systems.Comment: 6 pages, 3 PostScript figure
TuST: from Raw Data to Vehicular Traffic Simulation in Turin
Traffic simulations are becoming a standard way to study urban mobility patterns, to evaluate new traffic policies and to test modern vehicular technologies. For this reason, in recent years, mobility projects pushed towards an increase in the demand of traffic simulators and towards an extension of their area of investigation, aiming at covering a whole city and its suburbs. In this paper we describe the methodology we followed in the creation of a large-scale traffic simulation of a 400-Km^2 area around the Municipality of Turin. Our preliminary results demonstrate that a complete modeling of such a wide tool is possible at the expense of minor simplifications
The Southern Proper Motion Program IV. The SPM4 Catalog
We present the fourth installment of the Yale/San Juan Southern Proper Motion
Catalog, SPM4. The SPM4 contains absolute proper motions, celestial
coordinates, and (B,V) photometry for over 103 million stars and galaxies
between the south celestial pole and -20 deg declination. The catalog is
roughly complete to V=17.5 and is based on photographic and CCD observations
taken with the Yale Southern Observatory's double-astrograph at Cesco
Observatory in El Leoncito, Argentina. The proper-motion precision, for
well-measured stars, is estimated to be 2 to 3 mas/yr, depending on the type of
second-epoch material. At the bright end, proper motions are on the
International Celestial Reference System by way of Hipparcos Catalog stars,
while the faint end is anchored to the inertial system using external galaxies.
Systematic uncertainties in the absolute proper motions are on the order of 1
mas/yr.Comment: 34 pages, 8 figures, 3 tables; accepted for publication in AJ; note -
modified author list and acknowledgements sectio
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