Mobius transformations of polygons and partitions of 3-space

Richard Randell, Jonathan Simon, Joshua Tokle

Published 2006-02-21Version 1

The image of a polygonal knot K under a spherical inversion of R^3 (union infinity) is a simple closed curve made of arcs of circles, having the same knot type as the mirror image of K. Suppose we reconnect the vertices of the inverted polygon with straight lines, making a new polygon. This may be a different knot type. For example, a certain 7-segment figure-8 knot can be transformed to a figure-8 knot, right and left handed trefoils, or an unknot, by selecting different inverting spheres. Which knot types can be obtained from a given original polygon K under this process? We show that for large n, almost all n-segment knot types cannot be reached from one initial n-segment polygon, using a single inversion or even the whole Mobius group. The number of knot types arising from an n-vertex polygon is bounded by the number of complementary domains of a certain system of n(n-3)/2 round 2-spheres in R^3. We show the number of domains is at most polynomial in the number of spheres. In the analysis, we obtain an exact formula for the number of complementary domains. On the other hand, the number of knot types that can be represented by n-segment polygons is exponential in n.