585 research outputs found
Conical Existence of Closed Curves on Convex Polyhedra
Let C be a simple, closed, directed curve on the surface of a convex
polyhedron P. We identify several classes of curves C that "live on a cone," in
the sense that C and a neighborhood to one side may be isometrically embedded
on the surface of a cone Lambda, with the apex a of Lambda enclosed inside (the
image of) C; we also prove that each point of C is "visible to" a. In
particular, we obtain that these curves have non-self-intersecting developments
in the plane. Moreover, the curves we identify that live on cones to both sides
support a new type of "source unfolding" of the entire surface of P to one
non-overlapping piece, as reported in a companion paper.Comment: 24 pages, 15 figures, 6 references. Version 2 includes a solution to
one of the open problems posed in Version 1, concerning quasigeodesic loop
On organizing principles of Discrete Differential Geometry. Geometry of spheres
Discrete differential geometry aims to develop discrete equivalents of the
geometric notions and methods of classical differential geometry. In this
survey we discuss the following two fundamental Discretization Principles: the
transformation group principle (smooth geometric objects and their
discretizations are invariant with respect to the same transformation group)
and the consistency principle (discretizations of smooth parametrized
geometries can be extended to multidimensional consistent nets). The main
concrete geometric problem discussed in this survey is a discretization of
curvature line parametrized surfaces in Lie geometry. We find a discretization
of curvature line parametrization which unifies the circular and conical nets
by systematically applying the Discretization Principles.Comment: 57 pages, 18 figures; In the second version the terminology is
slightly changed and umbilic points are discusse
Recommended from our members
Discrete Differential Geometry
Discrete Differential Geometry is a broad new area where differential geometry (studying smooth curves, surfaces and other manifolds) interacts with discrete geometry (studying polyhedral manifolds), using tools and ideas from all parts of mathematics. This report documents the 29 lectures at the first Oberwolfach workshop in this subject, with topics ranging from discrete integrable systems, polyhedra, circle packings and tilings to applications in computer graphics and geometry processing. It also includes a list of open problems posed at the problem session
Gauss images of hyperbolic cusps with convex polyhedral boundary
We prove that a 3--dimensional hyperbolic cusp with convex polyhedral
boundary is uniquely determined by its Gauss image. Furthermore, any spherical
metric on the torus with cone singularities of negative curvature and all
closed contractible geodesics of length greater than is the metric of
the Gauss image of some convex polyhedral cusp. This result is an analog of the
Rivin-Hodgson theorem characterizing compact convex hyperbolic polyhedra in
terms of their Gauss images.
The proof uses a variational method. Namely, a cusp with a given Gauss image
is identified with a critical point of a functional on the space of cusps with
cone-type singularities along a family of half-lines. The functional is shown
to be concave and to attain maximum at an interior point of its domain. As a
byproduct, we prove rigidity statements with respect to the Gauss image for
cusps with or without cone-type singularities.
In a special case, our theorem is equivalent to existence of a circle pattern
on the torus, with prescribed combinatorics and intersection angles. This is
the genus one case of a theorem by Thurston. In fact, our theorem extends
Thurston's theorem in the same way as Rivin-Hodgson's theorem extends Andreev's
theorem on compact convex polyhedra with non-obtuse dihedral angles.
The functional used in the proof is the sum of a volume term and curvature
term. We show that, in the situation of Thurston's theorem, it is the potential
for the combinatorial Ricci flow considered by Chow and Luo.
Our theorem represents the last special case of a general statement about
isometric immersions of compact surfaces.Comment: 55 pages, 17 figure
Recommended from our members
Discrete Differential Geometry
This is the collection of extended abstracts for the 24 lectures and the open problems session at the third Oberwolfach workshop on Discrete Differential Geometry
The closure constraint for the hyperbolic tetrahedron as a Bianchi identity
The closure constraint is a central piece of the mathematics of loop quantum
gravity. It encodes the gauge invariance of the spin network states of quantum
geometry and provides them with a geometrical interpretation: each decorated
vertex of a spin network is dual to a quantized polyhedron in .
For instance, a 4-valent vertex is interpreted as a tetrahedron determined by
the four normal vectors of its faces. We develop a framework where the closure
constraint is re-interpreted as a Bianchi identity, with the normals defined as
holonomies around the polyhedron faces of a connection (constructed from the
spinning geometry interpretation of twisted geometries). This allows us to
define closure constraints for hyperbolic tetrahedra (living in the
3-hyperboloid of unit future-oriented spacelike vectors in )
in terms of normals living all in or in . The latter
fits perfectly with the classical phase space developed for -deformed loop
quantum gravity supposed to account for a non-vanishing cosmological constant
. This is the first step towards interpreting -deformed twisted
geometries as actual discrete hyperbolic triangulations.Comment: 31 page
A Generalization of the Source Unfolding of Convex Polyhedra
We present a new method for unfolding a convex polyhedron into one piece without overlap, based on shortest paths to a convex curve on the polyhedron. Our “sun unfoldings” encompass source unfolding from a point, source unfolding from an open geodesic curve, and a variant of a recent method of Itoh, O’Rourke, and Vîlcu
- …