Alice and Bob play a game on a complete graph with 2014 vertices. They take moves in turn with Alice beginning. At each move Alice directs one undirected edge of . At each move Bob chooses a positive integer number , and after that directs undirected edges of . The game ends when all edges are directed. If there is some directed cycle in 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 after her -th move. The case is obvious. Suppose that prior to Alice's -th move there is a directed path of length and there is no directed path of length . If Alice can not construct a cycle at her -th move it follows that all edges connecting are already directed and there is a vertex in the complement of . Consider the edge , if it is not directed Alice directs it as and enlarges the path. Consider the edge ; if it is not directed Alice directs it as and enlarges the path. Otherwise is directed as and is directed as . All edges , can not be directed, otherwise there exists a minimal index (well defined) with and and there is a longer directed path of length , a contradiction. Now Alice can take an undirected path with minimal index and direct it as and get a path of length . Done.
Suppose that Alice has no winning strategy, then Alice makes at least 2016 moves since and since prior to Alice's -th move there is a directed path of length , in order to avoid a cycle all edges connecting the vertices of this path should be directed by Bob: . Thus Alice should move at most times. Contradiction.