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PROJECT EULER · #0982

The Third Dice

Statement only · UnsolvedOriginal problem ↗

Alice and Bob play the following game with two six-sided dice (numbered 1 to 6):

  1. Alice rolls both dice; she can see the rolled values but Bob cannot
  2. Alice chooses one of the dice and reveals it to Bob
  3. Bob chooses one of the dice: either the one he can see, or the one he cannot
  4. Alice pays Bob the value shown on Bob's chosen dice

Each player devises a (possibly non-deterministic) strategy. An example strategy for each player could be:

  • Alice chooses to reveal the dice with value closest to 3.5, or if both are equidistant she chooses randomly with equal probability
  • Bob chooses the revealed dice if its value is at least 4; otherwise he chooses the hidden dice

In fact, these two strategies together form a Nash equilibrium. That is, given that Bob is using his strategy, Alice's strategy minimises the expected payment; and given that Alice is using her strategy, Bob's strategy maximises the expected payment.

With these strategies the expected payment from Alice to Bob is 145364.027778.

To make the game more interesting, they introduce a third (six-sided) dice:

  1. Alice rolls three dice; she can see the rolled values but Bob cannot
  2. Alice chooses two of the dice and reveals both to Bob
  3. Bob chooses one of the three dice: either one of the two visible dice, or the one hidden dice
  4. Alice pays Bob the value shown on Bob's chosen dice

Supposing they settle on a pair of strategies that form a Nash equilibrium, find the expected payment from Alice to Bob, and give your answer rounded to six digits after the decimal point.

Write-up coming later

The complete problem is available here. An approach, code, and answer will be added later.