"How to pack table tennis balls so that they can hold the most table tennis balls?" this is known as the "sphere packing problem", also known as "Kepler conjecture". On the surface, this does not seem to be a problem, but it is really not easy to think clearly. If the volume of the box is l, the radius of the ball is r, and the number of balls is n, then there is (nx4π R^3/3)/L<1, and the formula on the left can be regarded as the density. Of course, the above formula is very rough. The sphere filling problem is to find the supremum of this density, and if possible, we hope to find out the method of packing.

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