220 research outputs found
On the stability of Hamiltonian relative equilibria with non-trivial isotropy
We consider Hamiltonian systems with symmetry, and relative equilibria with
isotropy subgroup of positive dimension. The stability of such relative
equilibria has been studied by Ortega and Ratiu and by Lerman and Singer. In
both papers the authors give sufficient conditions for stability which require
first determining a splitting of a subspace of the Lie algebra of the symmetry
group, with different splittings giving different criteria. In this note we
remove this splitting construction and so provide a more general and more
easily computed criterion for stability. The result is also extended to apply
to systems whose momentum map is not coadjoint equivariant
Some Integrals Involving Bessel Functions
AbstractA number of new definite integrals involving Bessel functions are presented. These have been derived by finding new integral representations for the product of two Bessel functions of different order and argument in terms of the generalized hypergeometric function with subsequent reduction to special cases. Connection is made with Weber′s second exponential integral and Laplace transforms of products of three Bessel functions
Point vortices on the hyperbolic plane
We investigate some properties of the dynamical system of point vortices on
the hyperboloid. This system has noncompact symmetry SL(2, R) and a coadjoint
equivariant momentum map J. The relative equilibrium conditions are found and
the trajectories of relative equilibria with non-zero momentum value are
described. We also provide the classification of relative equilibria and the
stability criteria for a number of cases, focusing on N=2, 3. Contrary to the
system on the sphere, relative equilibria with non-compact momentum isotropy
subgroup are found, and are used to illustrate the different stability types of
relative equilibria.Comment: To appear in J. Mathematical Physic
Spatial Autocorrelation Analysis for New FUA Inner Strategic Asset: A Case Study of the Metropolitan City of Milan, Italy
Functional urban areas represent integrated urban contexts whose territories are economically interconnected. They, therefore, include a central city and all the municipalities that make up the commuting area for work reasons. The economic energies and settlement transformations that characterize these territories have been consolidated over time. The current geographic conformation, as defined today, does not provide information on each municipality's rank (role) in the overall functioning. In this perspective, the work presented examines the demographic and urban dynamics that have affected the FUA of Milan in the last 60 years and then evaluates the presence of possible homogeneous geographic clusters (hot and cold spots) through spatial correlation techniques. Statistic validation was performed through the ANOVA and subsequent posthoc analysis (Tukey-Kramer method). Results show a new configurational asset within the FUA of Milan, which could provide a new key to interpreting the territory, aimed at identifying homogeneous areas to adopt new and more effective forms of strategic planning
Critical points of higher order for the normal map of immersions in R^d
We study the critical points of the normal map v : NM -> Rk+n, where M is an immersed k-dimensional submanifold of Rk+n, NM is the normal bundle of M and v(m, u) = m + u if u is an element of NmM. Usually, the image of these critical points is called the focal set. However, in that set there is a subset where the focusing is highest, as happens in the case of curves in R-3 with the curve of the centers of spheres with contact of third order with the curve. We give a definition of r-critical points of a smooth map between manifolds, and apply it to study the 2 and 3-critical points of the normal map in general and the 2-critical points for the case k = n = 2 in detail. In the later case we analyze the relation with the strong principal directions of Montaldi (1986) [2]. (C) 2011 Elsevier B.V. All rights reserved.Work partially supported by CAPES (BEX 4533/06-2).Monera, M.; Montesinos-Amilibia, A.; Moraes, S.; Sanabria Codesal, E. (2012). Critical points of higher order for the normal map of immersions in R^d. Topology and its Applications. 159:537-544. https://doi.org/10.1016/j.topol.2011.09.029S53754415
Point vortices on the sphere: a case with opposite vorticities
We study systems formed of 2N point vortices on a sphere with N vortices of
strength +1 and N vortices of strength -1. In this case, the Hamiltonian is
conserved by the symmetry which exchanges the positive vortices with the
negative vortices. We prove the existence of some fixed and relative
equilibria, and then study their stability with the ``Energy Momentum Method''.
Most of the results obtained are nonlinear stability results. To end, some
bifurcations are described.Comment: 35 pages, 9 figure
Stability of relative equilibria with singular momentum values in simple mechanical systems
A method for testing -stability of relative equilibria in Hamiltonian
systems of the form "kinetic + potential energy" is presented. This method
extends the Reduced Energy-Momentum Method of Simo et al. to the case of
non-free group actions and singular momentum values. A normal form for the
symplectic matrix at a relative equilibrium is also obtained.Comment: Partially rewritten. Some mistakes fixed. Exposition improve
Deformation of geometry and bifurcation of vortex rings
We construct a smooth family of Hamiltonian systems, together with a family
of group symmetries and momentum maps, for the dynamics of point vortices on
surfaces parametrized by the curvature of the surface. Equivariant bifurcations
in this family are characterized, whence the stability of the Thomson heptagon
is deduced without recourse to the Birkhoff normal form, which has hitherto
been a necessary tool.Comment: 26 page
Memory for single items, word pairs, and temporal order of different kinds in a patient with selective hippocampal lesions
One kind of between-list and two kinds of within-list temporal order memory were examined in a patient with selective bilateral hippocampal lesions. This damage disrupted memory for all three kinds of temporal order memory, but left item and word pair recognition relatively intact. These findings are inconsistent with claims that (1) hippocampal lesions, like those of the medial temporal lobe (MTL) cortex, disrupt item and word pair recognition, and that (2) hippocampal lesions disrupt temporal order memory and item recognition to the same degree. Not only was word pair recognition intact in the patient, but further evidence indicates that her recognition of other associations between items of the same kind is also spared so retrieval of such associations cannot be sufficient to support within-list temporal order recognition. Rather, as other evidence indicates that the patient is impaired at recogni-tion of associations between different kinds of information, within-list (and possibly between-list) temporal order memory may be impaired by hippocampal lesions because it critically depends on re-trieving associations between different kinds of information
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