50,835 research outputs found
Integer colorings with forbidden rainbow sums
For a set of positive integers , an -coloring of is
rainbow sum-free if it contains no rainbow Schur triple. In this paper we
initiate the study of the rainbow Erd\H{o}s-Rothchild problem in the context of
sum-free sets, which asks for the subsets of with the maximum number of
rainbow sum-free -colorings. We show that for , the interval is
optimal, while for , the set is optimal. We
also prove a stability theorem for . The proofs rely on the hypergraph
container method, and some ad-hoc stability analysis.Comment: 20 page
Obscured Binary Quasar Cores in SDSS J104807.74+005543.5?
We report the discovery of a possible close binary system of quasars in SDSS
J1048+0055. The [OIII]4959,5007 emission lines are clearly double-peaked, and
two discrete radio sources with a projected physical separation of ~20 pc are
found in the VLBA milliarcsec resolution image at 8.4 GHz. Each of the [O
III]4959,5007 doublets and Hbeta can be well modelled by two Gaussians and the
line ratio, [O III]5007/Hbeta ~7, is typical of Seyfert 2 galaxies. No broad
component of Hbeta was detected and its [O III]5007 luminosity, L_[OIII] ~ 9.2
times 10^42 erg s^-1, is comparable to luminous quasars and is a few ten times
more luminous than typical Seyfert galaxies. One natural interpretation is that
SDSS J1048+0055 contains two close quasar-like nuclei and the BLR around them
are obscured. Other possible models are also discussed. We suggest that
double-peaked narrow emission line profile may be an effective way of selecting
candidates of binary black holes with intermediate separation
The multiple effects of gradient coupling on network synchronization
Recent studies have shown that synchronizability of complex networks can be
significantly improved by asymmetric couplings, and increase of coupling
gradient is always in favor of network synchronization. Here we argue and
demonstrate that, for typical complex networks, there usually exists an optimal
coupling gradient under which the maximum network synchronizability is
achieved. After this optimal value, increase of coupling gradient could
deteriorate synchronization. We attribute the suppression of network
synchronization at large gradient to the phenomenon of network breaking, and
find that, in comparing with sparsely connected homogeneous networks, densely
connected heterogeneous networks have the superiority of adopting large
gradient. The findings are supported by indirect simulations of eigenvalue
analysis and direct simulations of coupled nonidentical oscillator networks.Comment: 4 pages, 4 figure
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