MathDB
2p pairwise distinct subsets s.t. intersection non-empty

Source: IMO Shortlist 2007, C7

July 13, 2008
combinatoricsSet systemsExtremal combinatoricsIMO Shortlist

Problem Statement

Let \alpha < \frac {3 \minus{} \sqrt {5}}{2} be a positive real number. Prove that there exist positive integers n n and p>α2n p > \alpha \cdot 2^n for which one can select 2p 2 \cdot p pairwise distinct subsets S1,,Sp,T1,,Tp S_1, \ldots, S_p, T_1, \ldots, T_p of the set {1,2,,n} \{1,2, \ldots, n\} such that SiTj S_i \cap T_j \neq \emptyset for all 1i,jp 1 \leq i,j \leq p Author: Gerhard Wöginger, Austria