138 research outputs found
Singularities of equidistants and global centre symmetry sets of Lagrangian submanifolds
We define the Global Centre Symmetry set (GCS) of a smooth closed
m-dimensional submanifold M of R^n, , which is an affinely invariant
generalization of the centre of a k-sphere in R^{k+1}. The GCS includes both
the centre symmetry set defined by Janeczko and the Wigner caustic defined by
Berry. We develop a new method for studying generic singularities of the GCS
which is suited to the case when M is lagrangian in R^{2m} with canonical
symplectic form. The definition of the GCS, which slightly generalizes one by
Giblin and Zakalyukin, is based on the notion of affine equidistants, so, we
first study singularities of affine equidistants of Lagrangian submanifolds,
classifying all the stable ones. Then, we classify the affine-Lagrangian stable
singularities of the GCS of Lagrangian submanifolds and show that, already for
smooth closed convex curves in R^2, many singularities of the GCS which are
affine stable are not affine-Lagrangian stable.Comment: 26 pages, 2 figure
Chaos in the Gauge/Gravity Correspondence
We study the motion of a string in the background of the Schwarzschild black
hole in AdS_5 by applying the standard arsenal of dynamical systems. Our
description of the phase space includes: the power spectrum, the largest
Lyapunov exponent, Poincare sections and basins of attractions. We find
convincing evidence that the motion is chaotic. We discuss the implications of
some of the quantities associated with chaotic systems for aspects of the
gauge/gravity correspondence. In particular, we suggest some potential
relevance for the information loss paradox.Comment: 29 pages, 11 figure
Vanishing cycles and mutation
This is the writeup of a talk given at the European Congress of Mathematics,
Barcelona. It considers Picard-Lefschetz theory from the Floer cohomology
viewpoint.Comment: 20 pages, LaTeX2e. TeXnical problem should now be fixed, so that the
images will appear even if you download the .ps fil
Singularities and Topology of Meromorphic Functions
We present several aspects of the "topology of meromorphic functions", which
we conceive as a general theory which includes the topology of holomorphic
functions, the topology of pencils on quasi-projective spaces and the topology
of polynomial functions.Comment: 21 pages, 1 figur
Analytical method for perturbed frozen orbit around an Asteroid in highly inhomogeneous gravitational fields : A first approach
This article provides a method for nding initial conditions for perturbed frozen orbits around inhomogeneous fast rotating asteroids. These orbits can be used as reference trajectories in missions that require close inspection of any rigid body. The generalized perturbative procedure followed exploits the analytical methods of relegation of the argument of node and Delaunay normalisation to arbitrary order. These analytical methods are extremely powerful but highly computational. The gravitational potential of the heterogeneous body is rstly stated, in polar-nodal coordinates, which takes into account the coecients of the spherical harmonics up to an arbitrary order. Through the relegation of the argument of node and the Delaunay normalization, a series of canonical transformations of coordinates is found, which reduces the Hamiltonian describing the system to a integrable, two degrees of freedom Hamiltonian plus a truncated reminder of higher order. Setting eccentricity, argument of pericenter and inclination of the orbit of the truncated system to be constant, initial conditions are found, which evolve into frozen orbits for the truncated system. Using the same initial conditions yields perturbed frozen orbits for the full system, whose perturbation decreases with the consideration of arbitrary homologic equations in the relegation and normalization procedures. Such procedure can be automated for the first homologic equation up to the consideration of any arbitrary number of spherical harmonics coefficients. The project has been developed in collaboration with the European Space Agency (ESA)
Shear sum rules at finite chemical potential
We derive sum rules which constrain the spectral density corresponding to the
retarded propagator of the T_{xy} component of the stress tensor for three
gravitational duals. The shear sum rule is obtained for the gravitational dual
of the N=4 Yang-Mills, theory of the M2-branes and M5-branes all at finite
chemical potential. We show that at finite chemical potential there are
additional terms in the sum rule which involve the chemical potential. These
modifications are shown to be due to the presence of scalars in the operator
product expansion of the stress tensor which have non-trivial vacuum
expectation values at finite chemical potential.Comment: The proof for the absence of branch cuts is corrected.Results
unchange
Snap evaporation of droplets on smooth topographies
Droplet evaporation on solid surfaces is important in many applications including printing, micro-patterning and cooling. While seemingly simple, the configuration of evaporating droplets on solids is difficult to predict and control. This is because evaporation typically proceeds as a “stick-slip” sequence—a combination of pinning and de-pinning events dominated by static friction or “pinning”, caused by microscopic surface roughness. Here we show how smooth, pinning-free, solid surfaces of non-planar topography promote a different process called snap evaporation. During snap evaporation a droplet follows a reproducible sequence of configurations, consisting of a quasi-static phase-change controlled by mass diffusion interrupted by out-of-equilibrium snaps. Snaps are triggered by bifurcations of the equilibrium droplet shape mediated by the underlying non-planar solid. Because the evolution of droplets during snap evaporation is controlled by a smooth topography, and not by surface roughness, our ideas can inspire programmable surfaces that manage liquids in heat- and mass-transfer applications
The use of normal forms for analysing nonlinear mechanical vibrations.
A historical introduction is given of the theory of normal forms for simplifying nonlinear dynamical systems close to resonances or bifurcation points. The specific focus is on mechanical vibration problems, described by finite degree-of-freedom second-order-in-time differential equations. A recent variant of the normal form method, that respects the specific structure of such models, is recalled. It is shown how this method can be placed within the context of the general theory of normal forms provided the damping and forcing terms are treated as unfolding parameters. The approach is contrasted to the alternative theory of nonlinear normal modes (NNMs) which is argued to be problematic in the presence of damping. The efficacy of the normal form method is illustrated on a model of the vibration of a taut cable, which is geometrically nonlinear. It is shown how the method is able to accurately predict NNM shapes and their bifurcations
Complexity and integrability in 4D bi-rational maps with two invariants
In this letter we give fourth-order autonomous recurrence relations with two
invariants, whose degree growth is cubic or exponential. These examples
contradict the common belief that maps with sufficiently many invariants can
have at most quadratic growth. Cubic growth may reflect the existence of
non-elliptic fibrations of invariants, whereas we conjecture that the
exponentially growing cases lack the necessary conditions for the applicability
of the discrete Liouville theorem.Comment: 16 pages, 2 figure
Time optimal control for the deployment of a tethered satellite allowing for a massive tether
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