Given 2k −1 white segments and 2k −1 black ones
Source:
November 3, 2010
pigeonhole principlesymmetrycombinatorics proposedcombinatorics
Problem Statement
On a line are given white segments and black ones. Assume that each white segment intersects at least black segments, and each black segment intersects at least white ones. Prove that there are a black segment intersecting all the white ones, and a white segment intersecting all the black ones.