59 research outputs found
On comparing the writhe of a smooth curve to the writhe of an inscribed polygon
We find bounds on the difference between the writhing number of a smooth
curve, and the writhing number of a polygon inscribed within. The proof is
based on an extension of Fuller's difference of writhe formula to the case of
polygonal curves. The results establish error bounds useful in the computation
of writhe.Comment: 16 pages, 5 figure
On the Minimum Ropelength of Knots and Links
The ropelength of a knot is the quotient of its length and its thickness, the
radius of the largest embedded normal tube around the knot. We prove existence
and regularity for ropelength minimizers in any knot or link type; these are
curves, but need not be smoother. We improve the lower bound for the
ropelength of a nontrivial knot, and establish new ropelength bounds for small
knots and links, including some which are sharp.Comment: 29 pages, 14 figures; New version has minor additions and
corrections; new section on asymptotic growth of ropelength; several new
reference
Knot Tightening By Constrained Gradient Descent
We present new computations of approximately length-minimizing polygons with
fixed thickness. These curves model the centerlines of "tight" knotted tubes
with minimal length and fixed circular cross-section. Our curves approximately
minimize the ropelength (or quotient of length and thickness) for polygons in
their knot types. While previous authors have minimized ropelength for polygons
using simulated annealing, the new idea in our code is to minimize length over
the set of polygons of thickness at least one using a version of constrained
gradient descent.
We rewrite the problem in terms of minimizing the length of the polygon
subject to an infinite family of differentiable constraint functions. We prove
that the polyhedral cone of variations of a polygon of thickness one which do
not decrease thickness to first order is finitely generated, and give an
explicit set of generators. Using this cone we give a first-order minimization
procedure and a Karush-Kuhn-Tucker criterion for polygonal ropelength
criticality.
Our main numerical contribution is a set of 379 almost-critical prime knots
and links, covering all prime knots with no more than 10 crossings and all
prime links with no more than 9 crossings. For links, these are the first
published ropelength figures, and for knots they improve on existing figures.
We give new maps of the self-contacts of these knots and links, and discover
some highly symmetric tight knots with particularly simple looking self-contact
maps.Comment: 45 pages, 16 figures, includes table of data with upper bounds on
ropelength for all prime knots with no more than 10 crossings and all prime
links with no more than 9 crossing
A Fast Direct Sampling Algorithm for Equilateral Closed Polygons
Sampling equilateral closed polygons is of interest in the statistical study
of ring polymers. Over the past 30 years, previous authors have proposed a
variety of simple Markov chain algorithms (but have not been able to show that
they converge to the correct probability distribution) and complicated direct
samplers (which require extended-precision arithmetic to evaluate numerically
unstable polynomials). We present a simple direct sampler which is fast and
numerically stable, and analyze its runtime using a new formula for the volume
of equilateral polygon space as a Dirichlet-type integral.Comment: 10 pages, 2 figures. Added Duplantier as coauthor; we now give the
precise asymptotic complexity of the algorith
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