9,965 research outputs found
Constant angle surfaces in the Lorentzian Heisenberg group
In this paper, we define and, then, we characterize constant angle spacelike
and timelike surfaces in the three-dimensional Heisenberg group, equipped with
a 1-parameter family of Lorentzian metrics. In particular, we give an explicit
local parametrization of these surfaces and we produce some examples.Comment: 13 pages, 8 figure
DBI models for the unification of dark matter and dark energy
We propose a model based on a DBI action for the unification of dark matter
and dark energy. This is supported by the results of the study of its
background behavior at early and late times, and reinforced by the analysis of
the evolution of perturbations. We also perform a Bayesian analysis to set
observational constraints on the parameters of the model using type Ia SN, CMB
shift and BAO data. Finally, to complete the study we investigate its
kinematics aspects, such as the effective equation of state parameter,
acceleration parameter and transition redshift. Particularizing those
parameters for the best fit one appreciates that an effective phantom is
preferred.Comment: 11 pages, 8 figures, revtex, new reference
Maximum size binary matroids with no AG(3,2)-minor are graphic
We prove that the maximum size of a simple binary matroid of rank
with no AG(3,2)-minor is and characterise those matroids
achieving this bound. When , the graphic matroid is the
unique matroid meeting the bound, but there are a handful of smaller examples.
In addition, we determine the size function for non-regular simple binary
matroids with no AG(3,2)-minor and characterise the matroids of maximum size
for each rank
Rotating turbulence under "precession-like" perturbation
The effects of changing the orientation of the rotation axis on homogeneous
turbulence is considered. We perform direct numerical simulations on a periodic
box of grid points, where the orientation of the rotation axis is
changed (a) at a fixed time instant (b) regularly at time intervals
commensurate with the rotation time scale. The former is characterized by a
dominant inverse energy cascade whereas in the latter, the inverse cascade is
stymied due to the recurrent changes in the rotation axis resulting in a strong
forward energy transfer and large scale structures that resemble those of
isotropic turbulence.Comment: 7 pages, 8 figures, The European Physical Journal E (EPJ E
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