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v2009.01.01 - Convex Optimization

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362 CHAPTER 5. EUCLIDEAN DISTANCE MATRIX<br />

Figure 96: Sphere-packing illustration from [326, kissing number].<br />

Translucent balls illustrated all have the same diameter.<br />

the kissing number as 12 (Figure 96) while Gregory argued for 13. Their<br />

dispute was finally resolved in 1953 by Schütte & van der Waerden. [258] In<br />

2003, Oleg Musin tightened the upper bound on kissing number K in four<br />

dimensions from 25 to K = 24 by refining a method by Philippe Delsarte<br />

from 1973 providing an infinite number [15] of linear inequalities necessary<br />

for converting a rank-constrained semidefinite program 5.9 to a linear<br />

program. 5.10 [236]<br />

There are no proofs known for kissing number in higher dimension<br />

excepting dimensions eight and twenty four.<br />

Translating this problem to an EDM graph realization (Figure 93,<br />

Figure 97) is suggested by Pfender & Ziegler. Imagine the centers of each<br />

sphere are connected by line segments. Then the distance between centers<br />

must obey simple criteria: Each sphere touching the central sphere has a line<br />

segment of length exactly 1 joining its center to the central sphere’s center.<br />

All spheres, excepting the central sphere, must have centers separated by a<br />

distance of at least 1.<br />

From this perspective, the kissing problem can be posed as a semidefinite<br />

program. Assign index 1 to the central sphere, and assume a total of N<br />

5.9 whose feasible set belongs to that subset of an elliptope (5.9.1.0.1) bounded above<br />

by some desired rank.<br />

5.10 Simplex-method solvers for linear programs produce numerically better results than<br />

contemporary log-barrier (interior-point method) solvers for semidefinite programs by<br />

about 7 orders of magnitude.

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