Is it possible to draw circles on the plane so that every line intersects at least one of them but no more than of them?
Solution
To determine whether it is possible to draw circles on the plane such that every line intersects at least one of them but no more than 100 of them, we will argue by contradiction.
1. Assume such circles exist:
Suppose we have a set of circles such that every line intersects at least one of them but no more than 100 of them. Let be a point outside all these circles.
2. **Apply inversion with center **:
Under inversion with center , each circle (with center and radius ) is mapped to another circle that does not pass through . Denote these images by .
3. Properties of the new family of circles:
The new family of circles has the property that any line or circle through meets at least one of them, but no more than 100 of them.
4. Define distances and angles:
Let . The angle at which the circle is seen from satisfies:
This means there exist constants and such that:
5. Convergence of the series:
The series converges. If it did not, we could find a line through that intersects more than 100 circles, which contradicts our assumption. Therefore, the series must also converge.
6. **Fix a small **:
There exists a distance such that the family of circles that intersect the disk centered at with radius satisfies:
7. Rotate and shrink a circle:
Consider a circle with diameter that passes through . Rotate around and denote the rotated circle by . The range of when the circle is swept by the circumference of is:
According to the inequality above, when runs through , there will exist a that does not intersect any , leading to a contradiction.
8. Addressing the edge case:
The claim may not hold when is close to . In such cases, it holds that , which is not useful. To fix this, we rotate and shrink it simultaneously. Let such that is a diameter of , and be inside with obtuse. For any with , denote by the circle with diameter . Now, holds, and taking sufficiently small , the argument shows there exists some that does not intersect any circle, leading to a contradiction.
Therefore, it is impossible to draw such circles.