Show that the maximum number of spheres of radius that can be placed touching a fixed sphere of radius so that no pair of spheres has an interior point in common is between and .
Solution
We can place spheres with their centers coplanar with the fixed sphere. Then we can place more above and more below as shown above. Thus can be achieved.

Take to be the center of the fixed sphere. Another sphere touching it blocks off a conical solid angle as shown. The angle between the center line of the cone and the surface is . It's not hard to show that for an angle the solid angle is , so for , it is . Thus we can have at most spheres touching the fixed sphere. Hence at most .
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