4,080 research outputs found
The Generation of Fullerenes
We describe an efficient new algorithm for the generation of fullerenes. Our
implementation of this algorithm is more than 3.5 times faster than the
previously fastest generator for fullerenes -- fullgen -- and the first program
since fullgen to be useful for more than 100 vertices. We also note a
programming error in fullgen that caused problems for 136 or more vertices. We
tabulate the numbers of fullerenes and IPR fullerenes up to 400 vertices. We
also check up to 316 vertices a conjecture of Barnette that cubic planar graphs
with maximum face size 6 are hamiltonian and verify that the smallest
counterexample to the spiral conjecture has 380 vertices.Comment: 21 pages; added a not
Four New BL Lac Surveys: Sampling New Populations
The advent of large area deep radio and X-ray surveys is leading to the
creation of many new BL Lac samples. In particular, the ROSAT All-Sky, Green
Bank and FIRST surveys are proving to be rich sources of new BL Lacs. We will
discuss the methods used in four independent BL Lac searches based on these
surveys. Comparison of the broadband spectral energy distributions of these BL
Lacs with those of previously known objects clearly points to the existence of
a large previously unrecognized population of objects with characteristics
intermediate between those exhibited by Low and High energy peaked BL Lacs.Comment: 4 pages, 3 postscript figures, To be published in the Proceedings of
the conference "BL Lac Phenomenon" held in Turku, Finland, June 22-26, 199
The size distribution of galaxies in the Sloan Digital Sky Survey
abridged: We use a complete sample of about 140,000 galaxies from the Sloan
Digital Sky Survey (SDSS) to study the size distribution of galaxies and its
dependence on their luminosity, stellar mass, and morphological type. The large
SDSS database provides statistics of unprecedented accuracy. For each type of
galaxy, the size distribution at given luminosity (or stellar mass) is well
described by a log-normal function, characterized by its median and
dispersion . For late-type galaxies, there is a characteristic
luminosity at (assuming ) corresponding to a stellar
mass M_0\sim 10^{10.6}\Msun. Galaxies more massive than have
and , while less massive
galaxies have and . For
early-type galaxies, the - relation is significantly steeper,
, but the - relation is similar
to that of late-type galaxies. Faint red galaxies have sizes quite independent
of their luminosities.Comment: 42 pages, 18 figures, 2 tables; replaced with the version accepted by
MNRA
S-Duality and Exact Type IIB Superstring Backgrounds
A geometrical approach in the non-symmetric connection framework is employed
to examine the issue of higher order corrections to D=10 type IIB
superstring backgrounds with a covariantly constant null Killing isometry and
non-zero Ramond-Ramond field content. These describe generalized supersymmetric
string waves and were obtained recently by us through the S-duality
transformation of purely NS-NS plane wave backgrounds. We find that the
backgrounds are exact subject to the existence of certain field redefinitions
and provided certain restrictive conditions are satisfied.Comment: 21 Pages, LATEX fil
HST Observations of the Serendipitous X-ray Companion to Mrk 273: Cluster at z=0.46?
We have used HST I-band images to identify Mrk 273X, the very unusual
high-redshift X-ray-luminous Seyfert 2 galaxy found by ROSAT in the same
field-of-view as Mrk 273. We have measured the photometric properties of Mrk
273X and have also analyzed the luminosity distribution of the faint galaxy
population seen in the HST image. The luminosity of the galaxy and the
properties of the surrounding environment suggest that Mrk 273X is the
brightest galaxy in a relatively poor cluster at a redshift near 0.46. Its
off-center location in the cluster and the presence of other galaxy groupings
in the HST image may indicate that this is a dynamically young cluster on the
verge of merging with its neighboring clusters. We find that Mrk 273X is a
bright featureless elliptical galaxy with no evidence for a disk. It follows
the de Vaucouleurs (r^{1/4}) surface brightness law very well over a range of 8
magnitudes. Though the surface brightness profile does not appear to be
dominated by the AGN, the galaxy has very blue colors that do appear to be
produced by the AGN. Mrk 273X is most similar to the IC 5063 class of active
galaxies --- a hybrid Sy 2 / powerful radio galaxy.Comment: Accepted for publication in the Astrophysical Journal. 8 pages,
including 4 postscript figures. Uses emulateapj.sty and psfig.sty. Higher
quality version of Figure 1 is available at
http://rings.gsfc.nasa.gov/~borne/fig1-markgals.gi
Temperature Dependence of the Cu(2) NQR Line Width in YBaCuO
Systematic measurements of the Cu(2) NQR line width were performed in
underdoped YBaCuO samples over the temperature range 4.2 K
K. It was shown that the copper NQR line width monotonically increases
upon lowering temperature in the below-critical region, resembling temperature
behavior of the superconducting gap. The observed dependence is explained by
the fact that the energy of a condensate of sliding charge-current states of
the charge-density-wave type depends on the phase of order parameter.
Calculations show that this dependence appears only at . Quantitative
estimates of the line broadening at agree with the measurement results.Comment: 4 pages, 2 figure
Beam Based Alignment of Interaction Region Magnets
In conventional beam based alignment (BBA) procedures, the relative alignment
of a quadrupole to a nearby beam position monitor is determined by finding a
beam position in the quadrupole at which the closed orbit does not change when
the quadrupole field is varied. The final focus magnets of the interaction
regions (IR) of circular colliders often have some specialized properties that
make it difficult to perform conventional beam based alignment procedures. At
the HERA interaction points, for example, these properties are: (a) The
quadrupoles are quite strong and long. Therefore a thin lens approximation is
quite imprecise. (b) The effects of angular magnet offsets become significant.
(c) The possibilities to steer the beam are limited as long as the alignment is
not within specifications. (d) The beam orbit has design offsets and design
angles with respect to the axis of the low-beta quadrupoles. (e) Often
quadrupoles do not have a beam position monitor in their vicinity. Here we
present a beam based alignment procedure that determines the relative offset of
the closed orbit from a quadrupole center without requiring large orbit changes
or monitors next to the quadrupole. Taking into account the alignment angle
allows us to reduce the sensitivity to optical errors by one to two orders of
magnitude. We also show how the BBA measurements of all IR quadrupoles can be
used to determine the global position of the magnets. The sensitivity to errors
of this method is evaluated and its applicability to HERA is shown
Contact Representations of Graphs in 3D
We study contact representations of graphs in which vertices are represented
by axis-aligned polyhedra in 3D and edges are realized by non-zero area common
boundaries between corresponding polyhedra. We show that for every 3-connected
planar graph, there exists a simultaneous representation of the graph and its
dual with 3D boxes. We give a linear-time algorithm for constructing such a
representation. This result extends the existing primal-dual contact
representations of planar graphs in 2D using circles and triangles. While
contact graphs in 2D directly correspond to planar graphs, we next study
representations of non-planar graphs in 3D. In particular we consider
representations of optimal 1-planar graphs. A graph is 1-planar if there exists
a drawing in the plane where each edge is crossed at most once, and an optimal
n-vertex 1-planar graph has the maximum (4n - 8) number of edges. We describe a
linear-time algorithm for representing optimal 1-planar graphs without
separating 4-cycles with 3D boxes. However, not every optimal 1-planar graph
admits a representation with boxes. Hence, we consider contact representations
with the next simplest axis-aligned 3D object, L-shaped polyhedra. We provide a
quadratic-time algorithm for representing optimal 1-planar graph with L-shaped
polyhedra
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