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Combinatorics Difficulty 6.8 National olympiad Find the answer

Three friends Archie, Billie, and Charlie play a game. At the beginning of the game, each of them has a pile of 20242024 pebbles. Archie makes the first move, Billie makes the second, Charlie makes the third and they continue to make moves in the same order. In each move, the player making the move must choose a positive integer nn greater than any previously chosen number by any player, take 2n2n pebbles from his pile and distribute them equally to the other two players. If a player cannot make a move, the game ends and that player loses the game.
\hspace{5px} Determine all the players who have a strategy such that, regardless of how the other two players play, they will not lose the game.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To solve the problem, let's understand the sequence and the rules of the game.

Initially, all players start with 20242024 pebbles. Each player's turn involves picking a unique number n n such that no player has previously picked, taking 2n 2n pebbles from their pile, and distributing those pebbles (i.e., n n to each of the other two players).

### Game Analysis:

1. Game Dynamics:
- If a player chooses an integer n n , they lose 2n 2n pebbles, and each of the other two players gains n n pebbles.
- The sum of pebbles among the three players remains constant at 3×2024=6072 3 \times 2024 = 6072 .

2. End Condition:
- A player loses the game if on their turn they cannot choose a higher number n n such that 2n 2n pebbles can be taken from their pile (i.e., their pebbles are fewer than 2n 2n ).

3. Pebbles Allocation:
- Let the player's pebbles count after various rounds be ai,bi,ci a_i, b_i, c_i , for Archie, Billie, and Charlie after round i i respectively.
- On Archie's turn in round 1: a1=20242n1 a_1 = 2024 - 2n_1 , b1=2024+n1 b_1 = 2024 + n_1 , c1=2024+n1 c_1 = 2024 + n_1 .

4. Strategy and Analysis:
- As each progressive round occurs, the number n n chosen must always be larger than the previous n n . This means the depletion in pebbles accelerates.
- The cumulative number of pebbles each player loses over the turns increases due to the growing n n .

### Strategy to Avoid Loss:

- Charlie's Position:
- Since Charlie plays last in each cycle, he gains from the choices of both Archie and Billie before making his own decision.
- If Archie and Billie are unable to make a move (meaning they cannot choose a larger n n than previously chosen and have 2n 2n pebbles), they lose.

- Charlie always has an advantage as he gets to benefit from two preceding loss functions (first by Archie's then by Billie's subtraction), which accumulate to hasten their depletion before his resources end.

Thus, Charlie has a strategy to avoid losing, assuming they optimally choose when n n runs closer to depleting Archie and Billie's resources:

Charlie \boxed{\text{Charlie}}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.