90,444 research outputs found
Sequential order under CH
Revisiting and completing a work due to A. I. Ba\v{s}kirov, we construct
compact sequential spaces of any sequential order up to and including
as quotient spaces of under CH
The efficient certification of knottedness and Thurston norm
We show that the problem of determining whether a knot in the 3-sphere is
non-trivial lies in NP. This is a consequence of the following more general
result. The problem of determining whether the Thurston norm of a second
homology class in a compact orientable 3-manifold is equal to a given integer
is in NP. As a corollary, the problem of determining the genus of a knot in the
3-sphere is in NP. We also show that the problem of determining whether a
compact orientable 3-manifold has incompressible boundary is in NP.Comment: 101 pages, 24 figures; v2 contains some improvements suggested by the
referee, which have strengthened the main theorem
Safe Routes to School Improves the Built Environment
This report focuses on case studies describing how ten states (California, District of Columbia, Georgia, Illinois, Kentucky, Louisiana, New York, Oklahoma, Texas and Virginia) are awarding their SRTS federal funds to support improved infrastructure such as sidewalks, bike lanes, pathways, improved intersections, traffic calming, and more. Safe Routes to School: Improves the Built Environment shares information on local level implementation challenges, best practices, and securing more improvements to the built environment in your community
The Computational Complexity of Knot and Link Problems
We consider the problem of deciding whether a polygonal knot in 3-dimensional
Euclidean space is unknotted, capable of being continuously deformed without
self-intersection so that it lies in a plane. We show that this problem, {\sc
unknotting problem} is in {\bf NP}. We also consider the problem, {\sc
unknotting problem} of determining whether two or more such polygons can be
split, or continuously deformed without self-intersection so that they occupy
both sides of a plane without intersecting it. We show that it also is in NP.
Finally, we show that the problem of determining the genus of a polygonal knot
(a generalization of the problem of determining whether it is unknotted) is in
{\bf PSPACE}. We also give exponential worst-case running time bounds for
deterministic algorithms to solve each of these problems. These algorithms are
based on the use of normal surfaces and decision procedures due to W. Haken,
with recent extensions by W. Jaco and J. L. Tollefson.Comment: 32 pages, 1 figur
The number of Reidemeister Moves Needed for Unknotting
There is a positive constant such that for any diagram representing
the unknot, there is a sequence of at most Reidemeister moves that
will convert it to a trivial knot diagram, is the number of crossings in
. A similar result holds for elementary moves on a polygonal knot
embedded in the 1-skeleton of the interior of a compact, orientable,
triangulated 3-manifold . There is a positive constant such that
for each , if consists of tetrahedra, and is unknotted,
then there is a sequence of at most elementary moves in which
transforms to a triangle contained inside one tetrahedron of . We obtain
explicit values for and .Comment: 48 pages, 14 figure
Optics: general-purpose scintillator light response simulation code
We present the program optics that simulates the light response of an
arbitrarily shaped scintillation particle detector. Predicted light responses
of pure CsI polygonal detectors, plastic scintillator staves, cylindrical
plastic target scintillators and a Plexiglas light-distribution plate are
illustrated. We demonstrate how different bulk and surface optical properties
of a scintillator lead to specific volume and temporal light collection
probability distributions. High-statistics optics simulations are calibrated
against the detector responses measured in a custom-made cosmic muon tomography
apparatus. The presented code can also be used to track particles intersecting
complex geometrical objects.Comment: RevTeX LaTeX, 37 pages in e-print format, 12 Postscript Figures and 1
Table, also available at
http://pibeta.phys.virginia.edu/public_html/preprints/optics.p
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