A formulation of the Kepler conjecture
  This is the second in a series of papers giving a proof of the Kepler conjecture, which asserts that the density of a packing of congruent spheres in three dimensions is never greater than $\pi/\sqrt{18}\approx 0.74048...$. This is the oldest problem in discrete geometry and is an important part of Hilbert's 18th problem. An example of a packing achieving this density is the face-centered cubic packing.   This paper defines a local formulation of the conjecture which is used in the proof. 
Samuel P. Ferguson, Thomas C. Hales