38 research outputs found
Poisson-Lie group of pseudodifferential symbols
We introduce a Lie bialgebra structure on the central extension of the Lie
algebra of differential operators on the line and the circle (with scalar or
matrix coefficients). This defines a Poisson--Lie structure on the dual group
of pseudodifferential symbols of an arbitrary real (or complex) order. We show
that the usual (second) Benney, KdV (or GL_n--Adler--Gelfand--Dickey) and KP
Poisson structures are naturally realized as restrictions of this Poisson
structure to submanifolds of this ``universal'' Poisson--Lie group.
Moreover, the reduced (=SL_n) versions of these manifolds (W_n-algebras in
physical terminology) can be viewed as subspaces of the quotient (or Poisson
reduction) of this Poisson--Lie group by the dressing action of the group of
functions.
Finally, we define an infinite set of functions in involution on the
Poisson--Lie group that give the standard families of Hamiltonians when
restricted to the submanifolds mentioned above. The Poisson structure and
Hamiltonians on the whole group interpolate between the Poisson structures and
Hamiltonians of Benney, KP and KdV flows. We also discuss the geometrical
meaning of W_\infty as a limit of Poisson algebras W_\epsilon as \epsilon goes
to 0.Comment: 64 pages, no figure
Non-Local Matrix Generalizations of W-Algebras
There is a standard way to define two symplectic (hamiltonian) structures,
the first and second Gelfand-Dikii brackets, on the space of ordinary linear
differential operators of order , . In this paper, I consider in detail the case where the are
-matrix-valued functions, with particular emphasis on the (more
interesting) second Gelfand-Dikii bracket. Of particular interest is the
reduction to the symplectic submanifold . This reduction gives rise to
matrix generalizations of (the classical version of) the {\it non-linear}
-algebras, called -algebras. The non-commutativity of the
matrices leads to {\it non-local} terms in these -algebras. I show
that these algebras contain a conformal Virasoro subalgebra and that
combinations of the can be formed that are -matrices of
conformally primary fields of spin , in analogy with the scalar case .
In general however, the -algebras have a much richer structure than
the -algebras as can be seen on the examples of the {\it non-linear} and
{\it non-local} Poisson brackets of any two matrix elements of or
which I work out explicitly for all and . A matrix Miura transformation
is derived, mapping these complicated second Gelfand-Dikii brackets of the
to a set of much simpler Poisson brackets, providing the analogue of the
free-field realization of the -algebras.Comment: 43 pages, a reference and a remark on the conformal properties for
adde
Exotic clouds in the local interstellar medium
The neutral interstellar medium (ISM) inside the Local Bubble (LB) has been
known to have properties typical of the warm neutral medium (WNM). However,
several recent neutral hydrogen (HI) absorption experiments show evidence for
the existence of at least several cold diffuse clouds inside or at the boundary
of the LB, with properties highly unusual relative to the traditional cold
neutral medium. These cold clouds have a low HI column density, and AU-scale
sizes. As the kinematics of cold and warm gas inside the LB are similar, this
suggests a possibility of all these different flavors of the local ISM
belonging to the same interstellar flow. The co-existence of warm and cold
phases inside the LB is exciting as it can be used to probe the thermal
pressure inside the LB. In addition to cold clouds, several discrete screens of
ionized scattering material are clearly located inside the LB.
The cold exotic clouds inside the LB are most likely long-lived, and we
expect many more clouds with similar properties to be discovered in the future
with more sensitive radio observations. While physical mechanisms responsible
for the production of such clouds are still poorly understood, dynamical
triggering of phase conversion and/or interstellar turbulence are likely to
play an important role.Comment: 10 pages, refereed, accepted for publication in the proceedings of
the "From the Outer Heliosphere to the Local Bubble: Comparisons of New
Observations with Theory" conference, Space Science Review
Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites
Reconciling Bayesian and frequentist evidence in the point null testing problem
Posterior probability, p-value, point null hypothesis, 62F15, 62A15,