Do there exist 100 lines in the plane, no three of them concurrent, such that they intersect exactly in 2002 points?
Solution
Solution:
Any set of 100 lines in the plane can be partitioned into a finite number of disjoint sets, say A1,A2,A3,…,Ak, such that
(i) Any two lines in each Aj are parallel to each other, for 1≤j≤k (provided, of course, ∣Aj∣≥2);
(ii) for j=l, the lines in Aj and Al are not parallel.
If ∣Aj∣=mj, 1≤j≤k, then the total number of points of intersection is given by ∑1≤j<l≤kmjml, as no three lines are concurrent. Thus we have to find positive integers m1,m2,…,mk such that