138 research outputs found
Unbounded Orbits for Outer Billiards
Outer billiards is a basic dynamical system, defined relative to a planar
convex shape. This system was introduced in the 1950's by B.H. Neumann and
later popularized in the 1970's by J. Moser. All along, one of the central
questions has been: is there an outer billiards system with an unbounded orbit.
We answer this question by proving that outer billiards defined relative to the
Penrose Kite has an unbounded orbit. The Penrose kite is the quadrilateral that
appears in the famous Penrose tiling. We also analyze some of the finer orbit
structure of outer billiards on the penrose kite. This analysis shows that
there is an uncountable set of unbounded orbits. Our method of proof relates
the problem to self-similar tilings, polygon exchange maps, and arithmetic
dynamics.Comment: 65 pages, computer-aided proof. Auxilliary program, Billiard King,
available from author's website. Latest version is essentially the same as
earlier versions, but with minor improvements and many typos fixe
Notes on Shapes of Polyhedra
These are course notes I wrote for my Fall 2013 graduate topics course on
geometric structures, taught at ICERM. The notes rework many of proofs in
William P. Thurston's beautiful but hard-to-understand paper, "Shapes of
Polyhedra". A number of people, both in and out of the class, found these notes
very useful and so I decided to put them on the arXiv.Comment: This is a 21 page expository pape
Outer Billiards, Arithmetic Graphs, and the Octagon
Outer Billiards is a geometrically inspired dynamical system based on a
convex shape in the plane.
When the shape is a polygon, the system has a combinatorial flavor. In the
polygonal case, there is a natural acceleration of the map, a first return map
to a certain strip in the plane. The arithmetic graph is a geometric encoding
of the symbolic dynamics of this first return map.
In the case of the regular octagon, the case we study, the arithmetic graphs
associated to periodic orbits are polygonal paths in R^8. We are interested in
the asymptotic shapes of these polygonal paths, as the period tends to
infinity. We show that the rescaled limit of essentially any sequence of these
graphs converges to a fractal curve that simultaneously projects one way onto a
variant of the Koch snowflake and another way onto a variant of the Sierpinski
carpet. In a sense, this gives a complete description of the asymptotic
behavior of the symbolic dynamics of the first return map.
What makes all our proofs work is an efficient (and basically well known)
renormalization scheme for the dynamics.Comment: 86 pages, mildly computer-aided proof. My java program
http://www.math.brown.edu/~res/Java/OctoMap2/Main.html illustrates
essentially all the ideas in the paper in an interactive and well-documented
way. This is the second version. The only difference from the first version
is that I simplified the proof of Main Theorem, Statement 2, at the end of
Ch.
Complex hyperbolic triangle groups
The theory of complex hyperbolic discrete groups is still in its childhood
but promises to grow into a rich subfield of geometry. In this paper I will
discuss some recent progress that has been made on complex hyperbolic
deformations of the modular group and, more generally, triangle groups. These
are some of the simplest nontrivial complex hyperbolic discrete groups. In
particular, I will talk about my recent discovery of a closed real hyperbolic
3-manifold which appears as the manifold at infinity for a complex hyperbolic
discrete group
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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