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Counting pure equilibria in a sealed coin-chest auction

Question

Ten bidders compete for a chest whose dollar value equals the number of heads in 2020 fair coin tosses made out of sight; everyone knows this rule but nobody sees the tosses. Each bidder submits a bid that is a non-negative whole number of dollars. A highest bidder (chosen uniformly at random if several tie) receives the chest and pays their bid; all other bids are refunded. Bidders are risk-neutral (utility is linear in money over the relevant range of $0 to $10). How many pure-strategy Nash equilibria (bid profiles where every player bids one value with certainty) does this game have?

Short answer

5,937,424,6025{,}937{,}424{,}602. The chest is worth $10 in expectation, so no one bids above 10. If two or more bid 10, everyone's payoff is 00 and no deviation helps, whatever the others bid in {0,…,10}\{0,\dots,10\}: that gives 1110−1010−10⋅10911^{10}-10^{10}-10\cdot10^{9} profiles. Otherwise the only equilibrium is everyone bidding 9 (a lone 10-bidder would drop to 9, and any lower top bid invites outbidding), adding 11.

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