## The Cat's Cradle

The surface shown in figure above is the lower boundary of the
convex hull of the curve traced in red around the edge of the surface.
The curve lies on the unit sphere, so that its stereographic projection from
the point (0,0,1) (the North Pole in the figure) is a plane
curve. The figure shown here is the same as in Figure 3.24 in
*Pattern Theory* by David Mumford and Agnès Desolneux. As
explained in the caption to that figure, the lower boundary of the convex hull
is a developable surface. The large triangular faces correspond to tritangent
circles of the plane curve.

Conversely, the curve on the unit sphere is obtained from the
plane curve by inverting the stereographic projection map. **. . . **

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*I made this page by substituting my own data in a *Geometry Center *webpage.*

Prof. Joel Roberts

School of Mathematics

University of Minnesota

Minneapolis, MN 55455

USA

Office: 351 Vincent Hall

Phone: (612) 625-1076

Dept. FAX: (612) 626-2017

e-mail: `roberts@math.umn.edu
`

`http://www.math.umn.edu/~roberts`