223 research outputs found
Tracing KAM tori in presymplectic dynamical systems
We present a KAM theorem for presymplectic dynamical systems. The theorem has
a " a posteriori " format. We show that given a Diophantine frequency
and a family of presymplectic mappings, if we find an embedded torus which is
approximately invariant with rotation such that the torus and the
family of mappings satisfy some explicit non-degeneracy condition, then we can
find an embedded torus and a value of the parameter close to to the original
ones so that the torus is invariant under the map associated to the value of
the parameter. Furthermore, we show that the dimension of the parameter space
is reduced if we assume that the systems are exact.Comment: 33 pages and one figur
A Renormalization Group for Hamiltonians: Numerical Results
We describe a renormalization group transformation that is related to the
breakup of golden invariant tori in Hamiltonian systems with two degrees of
freedom. This transformation applies to a large class of Hamiltonians, is
conceptually simple, and allows for accurate numerical computations. In a
numerical implementation, we find a nontrivial fixed point and determine the
corresponding critical index and scaling. Our computed values for various
universal constants are in good agreement with existing data for
area-preserving maps. We also discuss the flow associated with the nontrivial
fixed point.Comment: 11 Pages, 2 Figures. For future updates, check
ftp://ftp.ma.utexas.edu/pub/papers/koch
Boundaries of Siegel Disks: Numerical Studies of their Dynamics and Regularity
Siegel disks are domains around fixed points of holomorphic maps in which the maps are locally linearizable (i.e., become a rotation under an appropriate change of coordinates which is analytic in a neighborhood of the origin). The dynamical behavior of the iterates of the map on the boundary of the Siegel disk exhibits strong scaling properties which have been intensively studied in the physical and mathematical literature. In the cases we study, the boundary of the Siegel disk is a Jordan curve containing a critical point of the map (we consider critical maps of different orders), and there exists a natural parametrization which transforms the dynamics on the boundary into a rotation. We compute numerically this parameterization and use methods of harmonic analysis to compute the global Holder regularity of the parametrization for different maps and rotation numbers. We obtain that the regularity of the boundaries and the scaling exponents are universal numbers in the sense of renormalization theory (i.e., they do not depend on the map when the map ranges in an open set), and only depend on the order of the critical point of the map in the boundary of the Siegel disk and the tail of the continued function expansion of the rotation number. We also discuss some possible relations between the regularity of the parametrization of the boundaries and the corresponding scaling exponents. (C) 2008 American Institute of Physics.NSFMathematic
On the continuum limit for discrete NLS with long-range lattice interactions
We consider a general class of discrete nonlinear Schroedinger equations
(DNLS) on the lattice with mesh size . In the continuum
limit when , we prove that the limiting dynamics are given by a
nonlinear Schroedinger equation (NLS) on with the fractional
Laplacian as dispersive symbol. In particular, we obtain
that fractional powers arise from long-range lattice
interactions when passing to the continuum limit, whereas NLS with the
non-fractional Laplacian describes the dispersion in the continuum
limit for short-range lattice interactions (e.g., nearest-neighbor
interactions).
Our results rigorously justify certain NLS model equations with fractional
Laplacians proposed in the physics literature. Moreover, the arguments given in
our paper can be also applied to discuss the continuum limit for other lattice
systems with long-range interactions.Comment: 26 pages; no figures. Some minor revisions. To appear in Comm. Math.
Phy
Regularity of critical invariant circles of the standard nontwist map
We study critical invariant circles of several noble rotation numbers at the edge of break-up for an area-preserving map of the cylinder, which violates the twist condition.These circles admit essentially unique parametrizations by rotational coordinates. We present a high accuracy computation of about 107 Fourier coefficients. This allows us to compute the regularity of the conjugating maps and to show that, to the extent of numerical precision, it only depends on the tail of the continued fraction expansion.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/49075/2/non5_3_013.pd
Canonical Melnikov theory for diffeomorphisms
We study perturbations of diffeomorphisms that have a saddle connection
between a pair of normally hyperbolic invariant manifolds. We develop a
first-order deformation calculus for invariant manifolds and show that a
generalized Melnikov function or Melnikov displacement can be written in a
canonical way. This function is defined to be a section of the normal bundle of
the saddle connection.
We show how our definition reproduces the classical methods of Poincar\'{e}
and Melnikov and specializes to methods previously used for exact symplectic
and volume-preserving maps. We use the method to detect the transverse
intersection of stable and unstable manifolds and relate this intersection to
the set of zeros of the Melnikov displacement.Comment: laTeX, 31 pages, 3 figure
Linearization of Cohomology-free Vector Fields
We study the cohomological equation for a smooth vector field on a compact
manifold. We show that if the vector field is cohomology free, then it can be
embedded continuously in a linear flow on an Abelian group
Entropy production in Gaussian thermostats
We show that an arbitrary Anosov Gaussian thermostat on a surface is
dissipative unless the external field has a global potential
Local structure of the set of steady-state solutions to the 2D incompressible Euler equations
It is well known that the incompressible Euler equations can be formulated in
a very geometric language. The geometric structures provide very valuable
insights into the properties of the solutions. Analogies with the
finite-dimensional model of geodesics on a Lie group with left-invariant metric
can be very instructive, but it is often difficult to prove analogues of
finite-dimensional results in the infinite-dimensional setting of Euler's
equations. In this paper we establish a result in this direction in the simple
case of steady-state solutions in two dimensions, under some non-degeneracy
assumptions. In particular, we establish, in a non-degenerate situation, a
local one-to-one correspondence between steady-states and co-adjoint orbits.Comment: 81 page
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