Maths Olympiad Prep

Library / /62 of 73

Combinatorics Difficulty 8.6 Shortlist Prove it Turkey

Alice and Bob play a game on a complete graph GG with 2014 vertices. They take moves in turn with Alice beginning. At each move Alice directs one undirected edge of GG. At each move Bob chooses a positive integer number mm, 1m10001 \le m \le 1000 and after that directs mm undirected edges of GG. The game ends when all edges are directed. If there is some directed cycle in GG Alice wins. Determine whether Alice has a winning strategy.

Solution

Let us prove by induction that Alice can guarantee the existence of directed path of length kk after her kk-th move. The case k=1k = 1 is obvious. Suppose that prior to Alice's k+1k+1-th move there is a directed path v0,v1,v1,v2,,vk1,vk\overrightarrow{v_0, v_1}, \overrightarrow{v_1, v_2}, \dots, \overrightarrow{v_{k-1}, v_k} of length kk and there is no directed path of length k+1k+1. If Alice can not construct a cycle at her k+1k+1-th move it follows that all edges connecting v0,,vkv_0, \dots, v_k are already directed and there is a vertex vv' in the complement of v0,,vkv_0, \dots, v_k. Consider the edge v,v0v', v_0, if it is not directed Alice directs it as v,v0v', v_0 and enlarges the path. Consider the edge (v,vk)(v', v_k); if it is not directed Alice directs it as vk,vv_k, v' and enlarges the path. Otherwise v,v0v', v_0 is directed as v0,v\overrightarrow{v_0, v'} and v,vkv', v_k is directed as v,vk\overrightarrow{v', v_k}. All edges v,vkv', v_k, 1ik1 \le i \le k can not be directed, otherwise there exists a minimal index (well defined) l,1<l<kl, 1 < l < k with vl1,vv_{l-1}, v' and v,vlv', v_l and there is a longer directed path v0,v1,,vl1,v,vl,,vkv_0, v_1, \dots, v_{l-1}, v', v_l, \dots, v_k of length k+1k+1, a contradiction. Now Alice can take an undirected path (v,vl)(v', v_l) with minimal index 1<l<k1 < l < k and direct it as v,vl\overrightarrow{v', v_l} and get a path of length k+1:v0,v1,,vl1,v,vl,,vkk+1: v_0, v_1, \dots, v_{l-1}, v', v_l, \dots, v_k. Done.

Suppose that Alice has no winning strategy, then Alice makes at least 2016 moves since (2014)21001>2005\frac{(2014)^2}{1001} > 2005 and since prior to Alice's t+1t+1-th move there is a directed path of length tt, in order to avoid a cycle all edges connecting the vertices of this path should be directed by Bob: (t2)t1000t\binom{t}{2} - t \le 1000t. Thus Alice should move at most t2003t \le 2003 times. Contradiction.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.