1,559 research outputs found
On the Sylvester-Gallai and the orchard problem for pseudoline arrangements
We study a non-trivial extreme case of the orchard problem for
pseudolines and we provide a complete classification of pseudoline arrangements
having triple points and double points. We have also classified those
that can be realized with straight lines. They include new examples different
from the known example of B\"or\"oczky. Since Melchior's inequality also holds
for arrangements of pseudolines, we are able to deduce that some combinatorial
point-line configurations cannot be realized using pseudolines. In particular,
this gives a negative answer to one of Gr\"unbaum's problems. We formulate some
open problems which involve our new examples of line arrangements.Comment: 5 figures, 11 pages, to appear in Periodica Mathematica Hungaric
Complex Matrix Models and Statistics of Branched Coverings of 2D Surfaces
We present a complex matrix gauge model defined on an arbitrary
two-dimensional orientable lattice. We rewrite the model's partition function
in terms of a sum over representations of the group U(N). The model solves the
general combinatorial problem of counting branched covers of orientable Riemann
surfaces with any given, fixed branch point structure. We then define an
appropriate continuum limit allowing the branch points to freely float over the
surface. The simplest such limit reproduces two-dimensional chiral U(N)
Yang-Mills theory and its string description due to Gross and Taylor.Comment: 21 pages, 2 figures, TeX, harvmac.tex, epsf.tex, TeX "big
On a new (21_4) polycyclic configuration
When searching for small 4-configurations of points and lines, polycyclic
configurations, in which every symmetry class of points and lines contains the
same number of elements, have proved to be quite useful. In this paper we
construct and prove the existence of a previously unknown (21_4) conguration,
which provides a counterexample to a conjecture of Branko Gr\"unbaum. In
addition, we study some of its most important properties; in particular, we
make a comparison with the well-known Gr\"unbaum-Rigby configuration. We show
that there are exactly two (21_4) geometric polycyclic configurations and
seventeen (21_4) combinatorial polycyclic configurations. We also discuss some
possible generalizations.Comment: 22 pages, 12 figure
Automorphisms of order three on numerical Godeaux surfaces
We prove that a numerical Godeaux surface cannot have an automorphism of
order three
Deletion constructions of symmetric 4-configurations. Part I.
By deletion constructions we mean several methods of generation of new geometric configurations by the judicious deletion of certain points and lines, and introduction of other lines or points. A number of such procedures have recently been developed in a systematic way. We present here one family of such constructions, and will describe other families in the following parts
Measured Radiation and Background Levels During Transmission of Megawatt Electron Beams Through Millimeter Apertures
We report measurements of photon and neutron radiation levels observed while
transmitting a 0.43 MW electron beam through millimeter-sized apertures and
during beam-off, but accelerating gradient RF-on, operation. These measurements
were conducted at the Free-Electron Laser (FEL) facility of the Jefferson
National Accelerator Laboratory (JLab) using a 100 MeV electron beam from an
energy-recovery linear accelerator. The beam was directed successively through
6 mm, 4 mm, and 2 mm diameter apertures of length 127 mm in aluminum at a
maximum current of 4.3 mA (430 kW beam power). This study was conducted to
characterize radiation levels for experiments that need to operate in this
environment, such as the proposed DarkLight Experiment. We find that sustained
transmission of a 430 kW continuous-wave (CW) beam through a 2 mm aperture is
feasible with manageable beam-related backgrounds. We also find that during
beam-off, RF-on operation, multipactoring inside the niobium cavities of the
accelerator cryomodules is the primary source of ambient radiation when the
machine is tuned for 130 MeV operation.Comment: 9 pages, 11 figures, submitted to Nuclear Instruments and Methods in
Physics Research Section
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