8,730 research outputs found
A minimal-variable symplectic integrator on spheres
We construct a symplectic, globally defined, minimal-coordinate, equivariant
integrator on products of 2-spheres. Examples of corresponding Hamiltonian
systems, called spin systems, include the reduced free rigid body, the motion
of point vortices on a sphere, and the classical Heisenberg spin chain, a
spatial discretisation of the Landau-Lifschitz equation. The existence of such
an integrator is remarkable, as the sphere is neither a vector space, nor a
cotangent bundle, has no global coordinate chart, and its symplectic form is
not even exact. Moreover, the formulation of the integrator is very simple, and
resembles the geodesic midpoint method, although the latter is not symplectic
Collective symplectic integrators
We construct symplectic integrators for Lie-Poisson systems. The integrators
are standard symplectic (partitioned) Runge--Kutta methods. Their phase space
is a symplectic vector space with a Hamiltonian action with momentum map
whose range is the target Lie--Poisson manifold, and their Hamiltonian is
collective, that is, it is the target Hamiltonian pulled back by . The
method yields, for example, a symplectic midpoint rule expressed in 4 variables
for arbitrary Hamiltonians on . The method specializes in
the case that a sufficiently large symmetry group acts on the fibres of ,
and generalizes to the case that the vector space carries a bifoliation.
Examples involving many classical groups are presented
Symplectic integrators for spin systems
We present a symplectic integrator, based on the canonical midpoint rule, for
classical spin systems in which each spin is a unit vector in .
Unlike splitting methods, it is defined for all Hamiltonians, and is
-equivariant. It is a rare example of a generating function for
symplectic maps of a noncanonical phase space. It yields an integrable
discretization of the reduced motion of a free rigid body
Geometry of discrete-time spin systems
Classical Hamiltonian spin systems are continuous dynamical systems on the
symplectic phase space . In this paper we investigate the underlying
geometry of a time discretization scheme for classical Hamiltonian spin systems
called the spherical midpoint method. As it turns out, this method displays a
range of interesting geometrical features, that yield insights and sets out
general strategies for geometric time discretizations of Hamiltonian systems on
non-canonical symplectic manifolds. In particular, our study provides two new,
completely geometric proofs that the discrete-time spin systems obtained by the
spherical midpoint method preserve symplecticity.
The study follows two paths. First, we introduce an extended version of the
Hopf fibration to show that the spherical midpoint method can be seen as
originating from the classical midpoint method on for a
collective Hamiltonian. Symplecticity is then a direct, geometric consequence.
Second, we propose a new discretization scheme on Riemannian manifolds called
the Riemannian midpoint method. We determine its properties with respect to
isometries and Riemannian submersions and, as a special case, we show that the
spherical midpoint method is of this type for a non-Euclidean metric. In
combination with K\"ahler geometry, this provides another geometric proof of
symplecticity.Comment: 17 pages, 2 figures. arXiv admin note: substantial text overlap with
arXiv:1402.333
Hausdorff dimension of some groups acting on the binary tree
Based on the work of Abercrombie, Barnea and Shalev gave an explicit formula
for the Hausdorff dimension of a group acting on a rooted tree. We focus here
on the binary tree T. Abert and Virag showed that there exist finitely
generated (but not necessarily level-transitive) subgroups of AutT of arbitrary
dimension in [0,1].
In this article we explicitly compute the Hausdorff dimension of the
level-transitive spinal groups. We then show examples of 3-generated spinal
groups which have transcendental Hausdroff dimension, and exhibit a
construction of 2-generated groups whose Hausdorff dimension is 1.Comment: 10 pages; full revision; simplified some proof
Equilibrium composition between liquid and clathrate reservoirs on Titan
Hundreds of lakes and a few seas of liquid hydrocarbons have been observed by
the Cassini spacecraft to cover the polar regions of Titan. A significant
fraction of these lakes or seas could possibly be interconnected with
subsurface liquid reservoirs of alkanes. In this paper, we investigate the
interplay that would happen between a reservoir of liquid hydrocarbons located
in Titan's subsurface and a hypothetical clathrate reservoir that progressively
forms if the liquid mixture diffuses throughout a preexisting porous icy layer.
To do so, we use a statistical-thermodynamic model in order to compute the
composition of the clathrate reservoir that forms as a result of the
progressive entrapping of the liquid mixture. This study shows that clathrate
formation strongly fractionates the molecules between the liquid and the solid
phases. Depending on whether the structure I or structure II clathrate forms,
the present model predicts that the liquid reservoirs would be mainly composed
of either propane or ethane, respectively. The other molecules present in the
liquid are trapped in clathrates. Any river or lake emanating from subsurface
liquid reservoirs that significantly interacted with clathrate reservoirs
should present such composition. On the other hand, lakes and rivers sourced by
precipitation should contain higher fractions of methane and nitrogen, as well
as minor traces of argon and carbon monoxide.Comment: Accepted for publication in Icaru
Parabolic Higgs bundles and representations of the fundamental group of a punctured surface into a real group
We study parabolic G-Higgs bundles over a compact Riemann surface with fixed
punctures, when G is a real reductive Lie group, and establish a correspondence
between these objects and representations of the fundamental group of the
punctured surface in G with arbitrary holonomy around the punctures. Three
interesting features are the relation between the parabolic degree and the
geometry of the Tits boundary, the treatment of the case when the logarithm of
the monodromy is on the boundary of a Weyl alcove, and the correspondence of
the orbits encoding the singularity via the Kostant-Sekiguchi correspondence.
We also describe some special features of the moduli spaces when G is a split
real form or a group of Hermitian type.Comment: v2: references added, a few typos corrected; v3: substantial
revision, added three sections: (3) relation to parahoric bundles, (7) moduli
spaces, (8) examples, 60 pages; v4: several corrections, more details
included on the Hitchin-Kobayashi correspondence, some proofs shortened, 57
page
On the volatile enrichments and composition of Jupiter
Using the clathrate hydrates trapping theory, we discuss the enrichments in
volatiles in the atmosphere of Jupiter measured by the \textit{Galileo} probe
in the framework of new extended core-accretion planet formation models
including migration and disk evolution. We construct a self-consistent model in
which the volatile content of planetesimals accreted during the formation of
Jupiter is calculated from the thermodynamical evolution of the disk. Assuming
CO2:CO:CH4 = 30:10:1 (ratios compatible with ISM measurements), we show that we
can explain the enrichments in volatiles in a way compatible with the recent
constraints set from internal structure modeling on the total amount of heavy
elements present in the planet.Comment: Accepted in ApJLetter
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