[math-fun] packings on the surface of a sphere
Neil Sloane's web page has tables of packings. http://www.research.att.com/~njas/packings/index.html#I surface of sphere http://www.research.att.com/~njas/packings/dim3/ coordinates The first URL has (among many other things) his best results for the maximum minimum angle for K points on the surface of an ordinary sphere. The second URL points to numerical coordinates of the points. They seem to have been obtained numerically -- 20 digit floating point values, including some numbers that should probably be 0 but instead are small values like 10^-20. I didn't see any formulas, so these coordinates are good candidates for algebraic number recognition. More digits will be needed, but this shouldn't be a big problem. The tables don't mention proofs, although there are lots of references in other parts of the web page. One curiosity: There are two instances where K and K+1 points have the same angle, indicating that removing a point from the K+1 packing doesn't make room to expand the remaining K points. These are 5,6 and 11,12. I didn't see any more cases up to 130 points, the table limit. The higher dmensions had some longer runs, where K...K+5 all had the same minimal angle. Is there a general "packings" web page? I had no idea Neil's tables existed -- they're pretty well hidden under the code phrase "spherical codes". Rich rcs@cs.arizona.edu
On Mon, 29 Sep 2003, Richard Schroeppel wrote:
One curiosity: There are two instances where K and K+1 points have the same angle, indicating that removing a point from the K+1 packing doesn't make room to expand the remaining K points. These are 5,6 and 11,12. I didn't see any more cases up to 130 points, the table limit.
In the case 5,6 the equality is provable - it follows from a theorem of Rankin. I once wrote out most of a proof for the 11,12 case, and think I could complete it for sufficient payment. I don't believe there will be any more such N,N+1 cases for the 3D problem. Here's a nice theorem of mine along these lines if 6 non-overlapping pennies are in some circle, then they can continuously be moved around inside that circle so as to make room for a seventh penny. The 12,13 3D analog of this fails, but I think the 24,25 version might be valid in 4D.
The higher dmensions had some longer runs, where K...K+5 all had the same minimal angle.
The same theorem of Rankin will account for some of these. Regards, JHC
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Richard Schroeppel