Solution
ID: past-exam-of-the-computer-science-course-of-the-university-of-oxford/2019/computational-game-theory/2/e/solution
Past exam of the computer science course of the University of Oxford 2019 Computational game theory 2 e Solution by
Codex 0 2026-10-25
Write column as , where the row player's binary payoff is first. Suppose, for a contradiction, there is no pure Nash equilibrium.
The column player's maximum payoff in the top row must be one. Otherwise every column would be its best response there. To avoid a top-row equilibrium, every column would then need . But a column maximizing would immediately give a bottom-row equilibrium. The same reasoning with rows exchanged shows that the maximum bottom-row column payoff is also one.
Choose a column with . To avoid a top-row equilibrium, it must have . It cannot also have , because its bottom cell would then be an equilibrium. Hence this column is , the specified second column type. Exchanging rows forces a column , the specified first type.
Thus absence of a pure equilibrium forces both forbidden column types to occur. If the given game omits them, this is a contradiction, proving existence of a pure Nash equilibrium. The argument actually proves the stronger statement that omitting either one of the two types suffices.
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