72,847 research outputs found
Class two 1-planar graphs with maximum degree six or seven
A graph is 1-planar if it can be drawn on the plane so that each edge is
crossed by at most one other edge. In this note we give examples of class two
1-planar graphs with maximum degree six or seven.Comment: 3 pages, 2 figure
Probing the interaction between dark energy and dark matter with the parametrized post-Friedmann approach
There possibly exists some direct, non-gravitational coupling between dark
energy and dark matter. This possibility should be seriously tested by using
observations, which requires us to understand such a scenario from the aspects
of both expansion history and growth of structure. It is found that once
calculating the perturbations in the interacting dark energy (IDE) scenario,
for most cases the curvature perturbation on superhorizon scales is divergent,
which is a catastrophe for the IDE cosmology. We found a solution to this
issue, which is to establish an effective theory to treat the dark energy
perturbations totally based on the basic facts of dark energy. This scheme
generalizes the parametrized post-Friedmann framework of uncoupled dark energy
and can be used to cure the instability of the IDE cosmology. The whole
parameter space of IDE models can henceforward be explored by observational
data. The IDE scenario can thus be tested or falsified with current and future
observational data by using the PPF approach. We expect that the future highly
accurate observational data would offer the certain answer to the question
whether there is a direct coupling between dark energy and dark matter.Comment: 3 pages, 2 figure
Weighing neutrinos in dynamical dark energy models
We briefly review the recent results of constraining neutrino mass in
dynamical dark energy models using cosmological observations and summarize the
findings. (i) In dynamical dark energy models, compared to CDM, the
upper limit of can become larger and can also become smaller. In
the cases of phantom and early phantom (i.e., the quintom evolving from
to ), the constraint on becomes looser; but in the cases of
quintessence and early quintessence (i.e., the quintom evolving from to
), the constraint on becomes tighter. (ii) In the
holographic dark energy (HDE) model, the tightest constraint on ,
i.e., eV, is obtained, which is almost equal to the lower
limit of of IH case. Thus, it is of great interest to find that
the future neutrino oscillation experiments would potentially offer a possible
falsifying scheme for the HDE model. (iii) The mass splitting of neutrinos can
influence the cosmological fits. We find that the NH case fits the current
observations slightly better than the IH case, although the difference of
of the two cases is still not significant enough to definitely
distinguish the neutrino mass hierarchy.Comment: 3 page
The edge chromatic number of outer-1-planar graphs
A graph is outer-1-planar if it can be drawn in the plane so that all
vertices are on the outer face and each edge is crossed at most once. In this
paper, we completely determine the edge chromatic number of outer 1-planar
graphs
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