Ann wins the nonrepetitive game over four letters and the erase-repetition game over six letters

Abstract

We consider two games between two players Ann and Ben who build a word together by adding alternatively a letter at the end of a shared word. In the nonrepetitive game, Ben wins the game if he can create a square of length at least 4, and Ann wins if she can build an arbitrarily long word without Ben winning. In the erase-repetition game, whenever a square occurs the second part of the square is erased and the goal of Ann is still to build an arbitrarily long word (Ben simply wants to limit the size of the word in this game).Grytczuk, Kozik, and Micek showed that Ann has a winning strategy for the nonrepetitive game if the alphabet is of size at least 6 and for the erase-repetition game if the alphabet is of size at least 8. In this article, we lower these bounds to respectively 4 and 6. The bound obtained by Grytczuk et al. relied on the so-called entropy compression and the previous bound by Pegden relied on some particular version of the Lovász Local Lemma. We recently introduced a counting argument that can be applied to the same set of problems as entropy compression or the Lovász Local Lemma and we use our method here.For these two games, we know that Ben has a winning strategy when the alphabet is of size at most 3, so our result for the nonrepetitive game is optimal, but we are not able to close the gap for the erase-repetition game.