556 research outputs found
Minkowski Sum Construction and other Applications of Arrangements of Geodesic Arcs on the Sphere
We present two exact implementations of efficient output-sensitive algorithms
that compute Minkowski sums of two convex polyhedra in 3D. We do not assume
general position. Namely, we handle degenerate input, and produce exact
results. We provide a tight bound on the exact maximum complexity of Minkowski
sums of polytopes in 3D in terms of the number of facets of the summand
polytopes. The algorithms employ variants of a data structure that represents
arrangements embedded on two-dimensional parametric surfaces in 3D, and they
make use of many operations applied to arrangements in these representations.
We have developed software components that support the arrangement
data-structure variants and the operations applied to them. These software
components are generic, as they can be instantiated with any number type.
However, our algorithms require only (exact) rational arithmetic. These
software components together with exact rational-arithmetic enable a robust,
efficient, and elegant implementation of the Minkowski-sum constructions and
the related applications. These software components are provided through a
package of the Computational Geometry Algorithm Library (CGAL) called
Arrangement_on_surface_2. We also present exact implementations of other
applications that exploit arrangements of arcs of great circles embedded on the
sphere. We use them as basic blocks in an exact implementation of an efficient
algorithm that partitions an assembly of polyhedra in 3D with two hands using
infinite translations. This application distinctly shows the importance of
exact computation, as imprecise computation might result with dismissal of
valid partitioning-motions.Comment: A Ph.D. thesis carried out at the Tel-Aviv university. 134 pages
long. The advisor was Prof. Dan Halperi
Panoramic optical and near-infrared SETI instrument: optical and structural design concepts
We propose a novel instrument design to greatly expand the current optical
and near-infrared SETI search parameter space by monitoring the entire
observable sky during all observable time. This instrument is aimed to search
for technosignatures by means of detecting nano- to micro-second light pulses
that could have been emitted, for instance, for the purpose of interstellar
communications or energy transfer. We present an instrument conceptual design
based upon an assembly of 198 refracting 0.5-m telescopes tessellating two
geodesic domes. This design produces a regular layout of hexagonal collecting
apertures that optimizes the instrument footprint, aperture diameter,
instrument sensitivity and total field-of-view coverage. We also present the
optical performance of some Fresnel lenses envisaged to develop a dedicated
panoramic SETI (PANOSETI) observatory that will dramatically increase sky-area
searched (pi steradians per dome), wavelength range covered, number of stellar
systems observed, interstellar space examined and duration of time monitored
with respect to previous optical and near-infrared technosignature finders.Comment: 14 pages, 5 figures, 3 table
Ramification conjecture and Hirzebruch's property of line arrangements
The ramification of a polyhedral space is defined as the metric completion of
the universal cover of its regular locus.
We consider mainly polyhedral spaces of two origins: quotients of Euclidean
space by a discrete group of isometries and polyhedral metrics on the complex
projective plane with singularities at a collection of complex lines.
In the former case we conjecture that quotient spaces always have a CAT[0]
ramification and prove this in several cases. In the latter case we prove that
the ramification is CAT[0] if the metric is non-negatively curved. We deduce
that complex line arrangements in the complex projective plane studied by
Hirzebruch have aspherical complement.Comment: 19 pages 1 figur
Attaching handlebodies to 3-manifolds
The main theorem of this paper is a generalisation of well known results
about Dehn surgery to the case of attaching handlebodies to a simple
3-manifold. The existence of a finite set of `exceptional' curves on the
boundary of the 3-manifold is established. Provided none of these curves is
attached to the boundary of a disc in a handlebody, the resulting manifold is
shown to be word hyperbolic and `hyperbolike'. We then give constructions of
gluing maps satisfying this condition. These take the form of an arbitrary
gluing map composed with powers of a suitable homeomorphism of the boundary of
the handlebodies.Comment: Published by Geometry and Topology at
http://www.maths.warwick.ac.uk/gt/GTVol6/paper26.abs.htm
Reappraising the distribution of the number of edge crossings of graphs on a sphere
Many real transportation and mobility networks have their vertices placed on
the surface of the Earth. In such embeddings, the edges laid on that surface
may cross. In his pioneering research, Moon analyzed the distribution of the
number of crossings on complete graphs and complete bipartite graphs whose
vertices are located uniformly at random on the surface of a sphere assuming
that vertex placements are independent from each other. Here we revise his
derivation of that variance in the light of recent theoretical developments on
the variance of crossings and computer simulations. We show that Moon's
formulae are inaccurate in predicting the true variance and provide exact
formulae.Comment: Corrected mistakes in equation 31. Added new figure (7). Added
acknowledgements to J. W. Moon. Other minor changes. Updated figures. Minor
changes in the last updat
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