A Lower Bound on the Area of a 3-Coloured Disk Packing (2008)
Brass, Peter, Hurtado, Ferran, Lafreniere, Benjamin, Lubiw, Anna
Given a set of unit-disks in the plane with union area $A$, what fraction of $A$ can be covered by selecting a pairwise disjoint subset of the disks? Rado conjectured 1/4 and proved $1/4.41$....