Problem:
In the plane there are closed disks with equal radius . Every point of the plane is contained in at most of these disks. Prove that every disk intersects at most other disks.
Solution
Solution:
We give a proof by contradiction. In addition to the hypothesis, we assume that a disk (without loss of generality let this be ) intersects at least other disks. The centers of these disks then obviously all lie in a disk of radius about the center of . If or more of the centers of these other disks were to lie in or on the boundary of , then all these disks would contain . Then would be contained in more than disks, since is also contained in . Hence at least centers of the other disks must lie in an annulus with inner radius and outer radius about . If we divide this annulus into equally sized sectors with interior angle , then by the pigeonhole principle at least one sector contains at least of these centers.
In the diagram this sector is shown with the midpoint of the segment . From and it follows that is equilateral. Hence and are its midlines, and it follows that . Therefore the sector lies entirely within a circle about with radius . Each of the at least disks with centers in this sector therefore contains the point . Thus is contained in
at least disks — a contradiction to the assumption.