Waiter-Client Clique-Factor Game

Abstract

Fix two integers $n, k$, with $n$ divisible by $k$, and consider the following game played by two players, Waiter and Client, on the edges of $K_n$. Starting with all the edges marked as unclaimed, in each round, Waiter picks two yet unclaimed edges. Client then chooses one of these edges to be added to Client's graph, while the other edge is added to Waiter's graph. Waiter wins if she eventually forces Client to create a $K_k$-factor in Client's graph. If she does not manage to do that, Client wins. For fixed $k$ and large enough $n$, it can be easily shown that Waiter wins if she plays optimally (in particular, this is an immediate consequence of our result that for such $n$, Waiter can win quite fast). The question posed by Clemens et al. is how long the game will last if Waiter aims to win as fast as she can, Client tries to delay her as much as he can, and they both play optimally. We denote this optimal number of rounds by $\tau_{WC}(\mathcal{F}_{n,K_k-\text{fac}},1 ) $. In the present paper, we obtain the first non-trivial lower bound on this quantity for large $k$. Together with a simple upper bound following the strategy of Clemens et al., this gives $2^{k/3-o(k)}n \leq \tau_{WC}(\mathcal{F}_{n,K_k-\text{fac}},1 ) \leq 2^k\frac{n}{k}+C(k)$, where $C(k)$ is a constant dependent only on $k$ and the $o(k)$ term is independent of $n$ as well.