Maths Olympiad Prep

Library / KöMaL problem A /136 of 136

KöMaL problem A, 2022

Combinatorics Difficulty 9.0 IMO level Prove it Hungary

For every i∈Ni \in \mathbb{N} let AiA_i, BiB_i and CiC_i be three finite and pairwise disjoint subsets of N\mathbb{N}. Suppose that for every partition of N\mathbb{N} consisting of sets AA, BB and CC there exists i∈Ni\in \mathbb{N} such that Ai⊂AA_i \subset A, Bi⊂BB_i \subset B and Ci⊂CC_i \subset C. Prove that there also exists a finite S⊂NS\subset \mathbb{N} such that for every partition of N\mathbb{N} consisting of sets AA, BB and CC there exists i∈Si\in S such that Ai⊂AA_i \subset A, Bi⊂BB_i \subset B and Ci⊂CC_i \subset C.

We don't reproduce this publisher's solutions. Their own solution is here — work the problem first.
Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: KöMaL, licensed Rights held by KöMaL and the MATFUND Foundation. Statement reproduced verbatim; metadata (topic, difficulty) added by this project. Solutions are the publisher's, linked not copied.