Question/Answer Games on Towers and Pyramids
MFCS '08 Proceedings of the 33rd international symposium on Mathematical Foundations of Computer Science
Complexity of question/answer games
Theoretical Computer Science
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Let $T_n$ denote a complete binary tree of depth $n$. Each internal node, $v$, of $T_n$ has two children denoted by $\lf(v)$ and $\rt(v)$. Consider the following game between two Players, Paul and Carole. For each internal node, $v$, Carole chooses $X(v) \in \{ \lf(v) , \rt(v) \}$. This naturally defines a path from the root, $\lambda$, of $T_n$ to one of its leaves as follows: $$\lambda, X(\lambda), X^2(\lambda ), \ldots, X^n(\lambda).$$ Paul has to find this path by asking questions of the following form: \smallskip \centerline{$Q_v$:``Is $X(v) = \lf(v)$?''''} \smallskip \noindent The game proceeds in $r$ rounds. In every round Paul can $k$ questions. Carole supplies the answers to these $k$ questions in parallel. We give necessary and sufficient conditions for Paul to win this game.