61,512 research outputs found
Generation of cubic graphs and snarks with large girth
We describe two new algorithms for the generation of all non-isomorphic cubic
graphs with girth at least which are very efficient for
and show how these algorithms can be efficiently restricted to generate snarks
with girth at least .
Our implementation of these algorithms is more than 30, respectively 40 times
faster than the previously fastest generator for cubic graphs with girth at
least 6 and 7, respectively.
Using these generators we have also generated all non-isomorphic snarks with
girth at least 6 up to 38 vertices and show that there are no snarks with girth
at least 7 up to 42 vertices. We present and analyse the new list of snarks
with girth 6.Comment: 27 pages (including appendix
Polyhedra with few 3-cuts are hamiltonian
In 1956, Tutte showed that every planar 4-connected graph is hamiltonian. In
this article, we will generalize this result and prove that polyhedra with at
most three 3-cuts are hamiltonian. In 2002 Jackson and Yu have shown this
result for the subclass of triangulations. We also prove that polyhedra with at
most four 3-cuts have a hamiltonian path. It is well known that for each non-hamiltonian polyhedra with 3-cuts exist. We give computational
results on lower bounds on the order of a possible non-hamiltonian polyhedron
for the remaining open cases of polyhedra with four or five 3-cuts.Comment: 21 pages; changed titl
A possible feature of thermal matter in relativistic jets of radio-loud quasars
It has been suggested that relativistic jets in quasars may contain a
considerable amount of thermal matter. In this paper, we explore the
possibility that the K-alpha line from the thermal matter may appear at tens of
keV due to a high Doppler blue-shift. In the jet comoving frame, the energy
density of photons originally emitted by the accretion disk and reflected off
the broad line region clouds dominates over that of photons of other origin. We
discuss the photoionization states of the thermal matter and find that the
irons elements are neutral. The high metallicity in quasars enhances the
possibility to detect the thermal matter in the relativistic jet in some
radio-loud quasars. A highly Doppler blue-shifted K-alpha line may be detected.
We make a rediction for 3C 273, in which the K-alpha line luminosity might be
of the order erg/s with an equivalent width of 2.4 keV.
Such a line could be detected in a future mission.Comment: 4 pages, 1 Figur
Conformally Einstein Products and Nearly K\"ahler Manifolds
In the first part of this note we study compact Riemannian manifolds (M,g)
whose Riemannian product with R is conformally Einstein. We then consider
compact 6--dimensional almost Hermitian manifolds of type W_1+W_4 in the
Gray--Hervella classification admitting a parallel vector field and show that
(under some regularity assumption) they are obtained as mapping tori of
isometries of compact Sasaki-Einstein 5-dimensional manifolds. In particular,
we obtain examples of inhomogeneous locally (non-globally) conformal nearly
K\"ahler compact manifolds
CENTRALITY MEASURES ON BRINKMANN GRAPH: BEFORE AND AFTER NODE DELETION
Brinkmann graph is a 4-regular graph with 21 nodes and 42 edges discovered by Gunnar Brinkmann in 1992. To our knowledge, the research specifically on Brinkmann graph is still hard to find. Therefore, this research was carried out to analyze the Brinkmann graph in term of its centrality. The centrality measures used are degree, betweenness, and closeness centrality. In this paper, we presented the centrality measures not only on the Brinkmann graph but also on the Brinkmann graph after node deletion to see how the impact of node deletion to the centrality of graph. Before deletion, the results showed that according to the betweenness centrality, there exist 7 nodes who act as mediators or bridges in the Brinkmann graph. Therefore, when a node among these nodes has deleted, it affected not only any other mediator nodes and the furthest nodes from the deleted node but also the nodes that are adjacent to the deleted nodes
Active Galactic Nuclei and the Truncation of Star Formation in K+A Galaxies
We have searched for active galactic nuclei (AGNs) in K+A galaxies, using
multiwavelength imaging and spectroscopy in the Bootes field of the NOAO Deep
Wide-Field Survey. The K+A galaxies, which have had their star formation
rapidly truncated, are selected via their strong Balmer absorption lines and
weak H-alpha emission. Our sample consists of 24 K+A galaxies selected from
6594 0.10<z<0.35 galaxies brighter than I=20 with optical spectroscopy from the
AGN and Galaxy Evolution Survey. Two thirds of the K+A galaxies are likely
ongoing galaxy mergers, with nearby companion galaxies or tidal tails. Galaxy
mergers may be responsible for the truncation of star formation, or we are
observing the aftermath of merger triggered starbursts. As expected, the
optical colors of K+A galaxies largely fall between blue galaxies with ongoing
star formation and red passive galaxies. However, only 1% of the galaxies with
colors between the red and blue populations are K+A galaxies, and we conclude
that the truncation of star formation in K+A galaxies must have been unusually
abrupt (<100 Myr). We examined the AGN content of K+A galaxies with both
optical emission-line ratios (BPT diagrams) and Chandra X-ray imaging. At least
half of all K+A galaxies display the optical emission-line ratios of AGNs, and
a third of M_R<-22 K+A galaxies host AGNs with X-ray luminosities of 10^{42}
erg/s. The faintest K+A galaxies do not show clear evidence for hosting AGNs,
having emission-line ratios consistent with photoionization by massive stars
and few X-ray detections. We speculate that two mechanisms may be responsible
for the truncation of star formation in K+A galaxies, with AGN feedback only
playing a role in M_R<-20.5 galaxies.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 8
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