18,352 research outputs found
Rossby waves and -effect
Rossby waves drifting in the azimuthal direction are a common feature at the
onset of thermal convective instability in a rapidly rotating spherical shell.
They can also result from the destabilization of a Stewartson shear layer
produced by differential rotation as expected in the liquid sodium experiment
(DTS) working in Grenoble, France. A usual way to explain why Rossby waves can
participate to the dynamo process goes back to Busse (1975). In his picture,
the flow geometry is a cylindrical array of parallel rolls aligned with the
rotation axis. The axial flow component (the component parallel to the rotation
axis) is (i) maximum in the middle of each roll and changes its sign from one
roll to the next. It is produced by the Ekman pumping at the fluid containing
shell boundary. The corresponding dynamo mechanism can be explained in terms of
an -tensor with non-zero coefficients on the diagonal. In rapidly
rotating objects like the Earth's core (or in a fast rotating experiment),
Rossby waves occur in the limit of small Ekman number (). In
that case, the main source of the axial flow component is not the Ekman pumping
but rather the ``geometrical slope effect'' due to the spherical shape of the
fluid containing shell. This implies that the axial flow component is (ii)
maximum at the borders of the rolls and not at the centers. If assumed to be
stationary, such rolls would lead to zero coefficients on the diagonal of the
-tensor, making the dynamo probably less efficient if possible at all.
Actually, the rolls are drifting as a wave, and we show that this drift implies
non--zero coefficients on the diagonal of the -tensor. These new
coefficients are in essence very different from the ones obtained in case (i)
and cannot be interpreted in terms of the heuristic picture of Busse (1975)
Black Extended Objects, Naked Singularities and P-Branes
We treat the horizons of charged, dilaton black extended objects as quantum
mechanical objects. We show that the S matrix for such an object can be written
in terms of a p-brane-like action. The requirements of unitarity of the S
matrix and positivity of the p-brane tension equivalent severely restrict the
number of space-time dimensions and the allowed values of the dilaton parameter
a. Generally, black objects transform at the extremal limit into p-branes.Comment: 9 pages, REVTE
Weyl Geometry as Characterization of Space-Time
Motivated by an axiomatic approach to characterize space-time it is
investigated a reformulation of Einstein's gravity where the pseudo-riemannian
geometry is substituted by a Weyl one. It is presented the main properties of
the Weyl geometry and it is shown that it gives extra contributions to the
trajectories of test particles, serving as one more motivation to study general
relativity in Weyl geometry. It is introduced its variational formalism and it
is established the coupling with other physical fields in such a way that the
theory acquires a gauge symmetry for the geometrical fields. It is shown that
this symmetry is still present for the red-shift and it is concluded that for
cosmological models it opens the possibility that observations can be fully
described by the new geometrical scalar field. It is concluded then that this
reformulation, although representing a theoretical advance, still needs a
complete description of their objects.Comment: 12 page
Constant Rank Bimatrix Games are PPAD-hard
The rank of a bimatrix game (A,B) is defined as rank(A+B). Computing a Nash
equilibrium (NE) of a rank-, i.e., zero-sum game is equivalent to linear
programming (von Neumann'28, Dantzig'51). In 2005, Kannan and Theobald gave an
FPTAS for constant rank games, and asked if there exists a polynomial time
algorithm to compute an exact NE. Adsul et al. (2011) answered this question
affirmatively for rank- games, leaving rank-2 and beyond unresolved.
In this paper we show that NE computation in games with rank , is
PPAD-hard, settling a decade long open problem. Interestingly, this is the
first instance that a problem with an FPTAS turns out to be PPAD-hard. Our
reduction bypasses graphical games and game gadgets, and provides a simpler
proof of PPAD-hardness for NE computation in bimatrix games. In addition, we
get:
* An equivalence between 2D-Linear-FIXP and PPAD, improving a result by
Etessami and Yannakakis (2007) on equivalence between Linear-FIXP and PPAD.
* NE computation in a bimatrix game with convex set of Nash equilibria is as
hard as solving a simple stochastic game.
* Computing a symmetric NE of a symmetric bimatrix game with rank is
PPAD-hard.
* Computing a (1/poly(n))-approximate fixed-point of a (Linear-FIXP)
piecewise-linear function is PPAD-hard.
The status of rank- games remains unresolved
Interhemispheric comparison of atmospheric circulation features as evaluated from Nimbus satellite data
A relationship is established between relative geostrophic vorticity on an isobaric surface and the Laplacian of the underlying layer-mean temperature. This relationship is used to investigate the distribution of vorticity and baroclinicity in a jet-stream model which is constantly recurrent in the winter troposphere. The investigation shows that the baroclinic and vorticity fields of the extratropical troposphere must be bifurcated with two extrema in the middle and subpolar latitudes. This pattern is present in daily tropospheric meridional cross-sections. The reasons for the disappearance of bifurcation in the time-and-longitude averaged distributions are discussed
High Temperature Expansion Study of the Nishimori multicritical Point in Two and Four Dimensions
We study the two and four dimensional Nishimori multicritical point via high
temperature expansions for the distribution, random-bond, Ising model.
In we estimate the the critical exponents along the Nishimori line to be
, . These, and earlier estimates
, are remarkably close to the critical
exponents for percolation, which are known to be , in
and and in . However, the
estimated Nishimori exponents , , are
quite distinct from the percolation results ,
.Comment: 5 pages, RevTex, 3 postscript files; To appear in Physical Review
Interhemispheric comparison of atmospheric circulation features as evaluated from NIMBUS satellite data
Findings are presented for IRIS data from NIMBUS 3 in mapping the global ozone distribution. The seasonal and regional variations of ozone, especially in the Southern Hemisphere, reveal features that were not evident from the sparse ground-based ozone observation network in this hemisphere. A regression analysis was undertaken for temperature and height fields on radiance data. Spectrum analyses of upper wind data from the North American section and Australia were completed
Emergence of a Teicoplanin-Resistant Small Colony Variant of Staphylococcus epidermidis During Vancomycin Therapy
Small colony variants of Staphylococcus aureus can cause persistent and recurrent infections. There are only a few reports of small colony variants of coagulase-negative staphylococci. Herein a case of infection with a teicoplanin-resistant small colony variant of Staphylococcus epidermidis is presented. The small colony variant was isolated from blood cultures of a patient with acute leukaemia and therapy-induced neutropenia who was treated with vancomycin for catheter-associated bloodstream infection. Despite removal of the catheter and adequate antibiotic therapy, the infection did not clear and the patient died 20days after continuous antibiotic therap
On Di\'osi-Penrose criterion of gravity-induced quantum collapse
It is shown that the Di\'osi-Penrose criterion of gravity-induced quantum
collapse may be inconsistent with the discreteness of space-time, which is
generally considered as an indispensable element in a complete theory of
quantum gravity. Moreover, the analysis also suggests that the discreteness of
space-time may result in rapider collapse of the superposition of energy
eigenstates than required by the Di\'osi-Penrose criterion.Comment: 5 pages, no figure
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