8. C2 (GEO) Let be closed disks in the plane. (A closed disk is a region bounded by a circle, taken jointly with this circle.) Suppose that every point in the plane is contained in at most 2003 disks . Prove that there exists disk that intersects at most other disks .
Solution
8. Let be the disk with the smallest radius, say , and the center of that disk. Divide the plane into 7 regions: one bounded by disk and 6 regions shown in the figure. Any of the disks different from , say , has its center in one of the seven regions. If its center is inside then contains point . Hence the number of disks different from having their centers in is at most 2002. Consider a disk that intersects and whose center is in the region . Let be the point such that bisects the region and ! . We claim that contains . Divide the region by a line through perpendicular to into two regions and , where and are on the same side of . Let be the center of . Consider two cases: (i) . Since the disk with the center and radius contains , we see that . Hence contains . (ii) . Denote by the intersection point of the segment with the circle . We want to prove that . It is enough to prove that . However, it is obvious that and , hence , while . This implies that ( is the point on the edge of as shown in the figure). Our claim is thus proved. Now we see that the number of disks with centers in that intersect is less than or equal to 2003, and the total number of disks that intersect is not greater than .