A randomized strategy in the mirror game

Abstract

Alice and Bob take turns (with Alice playing first) in declaring numbers from the set $[1,2N]$. If a player declares a number that was previously declared, that player looses and the other player wins. If all numbers are declared without repetition, the outcome is a tie. If both players have unbounded memory and play optimally, then the game will be tied. Garg and Schneider [ITCS 2019] showed that if Alice has unbounded memory, then Bob can secure a tie with $\log N$ memory, whereas if Bob has unbounded memory, then Alice needs memory linear in $N$ in order to secure a tie. Garg and Schneider also considered an {\em auxiliary matching} model in which Alice gets as an additional input a random matching $M$ over the numbers $[1,2N]$, and storing this input does not count towards the memory used by Alice. They showed that is this model there is a strategy for Alice that ties with probability at least $1 - \frac{1}{N}$, and uses only $O(\sqrt{N} (\log N)^2)$ memory. We show how to modify Alice's strategy so that it uses only $O((\log N)^3)$ space.