3,875 research outputs found
Line transversals to disjoint balls
We prove that the set of directions of lines intersecting three disjoint
balls in in a given order is a strictly convex subset of . We then
generalize this result to disjoint balls in . As a consequence, we can
improve upon several old and new results on line transversals to disjoint balls
in arbitrary dimension, such as bounds on the number of connected components
and Helly-type theorems.Comment: 21 pages, includes figure
On tangents to quadric surfaces
We study the variety of common tangents for up to four quadric surfaces in
projective three-space, with particular regard to configurations of four
quadrics admitting a continuum of common tangents.
We formulate geometrical conditions in the projective space defined by all
complex quadric surfaces which express the fact that several quadrics are
tangent along a curve to one and the same quadric of rank at least three, and
called, for intuitive reasons: a basket. Lines in any ruling of the latter will
be common tangents.
These considerations are then restricted to spheres in Euclidean three-space,
and result in a complete answer to the question over the reals: ``When do four
spheres allow infinitely many common tangents?''.Comment: 50 page
Ray and Wave Aberrations Revisited: A Huygens-Like Construction yields Exact Relations
The optical aberrations of a system can be described in terms of the wave
aberrations, defined as the departure from the ideal spherical wavefront; or
the ray aberrations, which are in turn the deviations from the paraxial ray
intersection measured in the image plane. The classical connection between the
two descriptions is an approximation, the error of which has, so far, not been
quantified analytically. We derive exact analytical equations for computing the
wavefront surface, the aberrated ray directions, and the transverse ray
aberrations in terms of the wave aberrations (OPD) and the reference sphere. We
introduce precise conditions for a function to be an OPD function, show that
every such function has an associated wavefront, and study the error arising
from the classical approximation. We establish strict conditions for the error
to be small. We illustrate our results with numerical simulations. Our results
show that large numerical apertures and high-frequency OPD functions yield
larger approximation errors.Comment: 12 pages, 10 Figures, JOSA A, vol. 33, no.
Contact graphs of ball packings
A contact graph of a packing of closed balls is a graph with balls as
vertices and pairs of tangent balls as edges. We prove that the average degree
of the contact graph of a packing of balls (with possibly different radii) in
is not greater than . We also find new upper bounds for
the average degree of contact graphs in and
A better proof of the Goldman-Parker conjecture
The Goldman-Parker Conjecture classifies the complex hyperbolic C-reflection
ideal triangle groups up to discreteness. We proved the Goldman-Parker
Conjecture in [Ann. of Math. 153 (2001) 533--598] using a rigorous
computer-assisted proof. In this paper we give a new and improved proof of the
Goldman-Parker Conjecture. While the proof relies on the computer for extensive
guidance, the proof itself is traditional.Comment: Published by Geometry and Topology at
http://www.maths.warwick.ac.uk/gt/GTVol9/paper35.abs.htm
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