most recent 30 from http://mathoverflow.net 2013-05-23T14:12:37Z http://mathoverflow.net/feeds/question/106120 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/106120/inter-kissing-number-for-spheres-of-different-sizes bobuhito 2012-09-01T17:47:22Z 2012-09-01T20:22:50Z

<p>What is the maximum number of spheres that can be placed in 3D such that all inter-touch?</p> <p>One can of course place four unit spheres tetrahedrally and then add a smaller sphere in the middle, so this number must be at least 5.</p> <p>[By the way, I was trying to extend the "five points in 2D cannot be inter-connected without a crossing" limitation to 3D with a simple statement, but this was sadly the best I could do. If anyone knows a better simple extension, please comment.]</p>

http://mathoverflow.net/questions/106120/inter-kissing-number-for-spheres-of-different-sizes/106124#106124 Anton Petrunin 2012-09-01T18:15:29Z 2012-09-01T20:22:50Z

<p>In $\mathbb R^n$, the answer is $n+2$.</p> <p>You can apply an inversion which sends two of the spheres in to two parallel hyperplanes. The rest of the spheres will have the same radii and their centers lie in a hyperplane. Hence everything follows.</p>