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A surprising rollout result

Posted By: Timothy Chow
Date: Friday, 19 January 2024, at 1:39 p.m.

Paul Epstein posted the position below on the rec.games.backgammon newsgroup. The checker-play decision is interesting, but what I want to draw attention to is that when I performed a million-game rollout of the safe play of 5/3 5/2, XG told me that in 22 games, White lost a backgammon! (The percentage of backgammons is so low that you can't see it in the results below; I had to use the "Display Rollout Statistics" feature.)

The puzzle for you is, is XG misplaying something? Or does there really exist a perfect-play sequence that leads to White losing a backgammon after Blue plays 5/3 5/2?





White is Player 2

score: 0
pip: 122
Unlimited Game
Jacoby Beaver
pip: 25
score: 0

Blue is Player 1
XGID=-CCBbB-----------bagc-----:1:-1:1:32:0:0:3:0:10
Blue to play 32

1.Rollout15/3 5/2eq: +1.7182
Player:
Opponent:		100.00% (G:71.82% B:0.00%)
0.00% (G:0.00% B:0.00%)		Conf.: ±0.0000 (+1.7181...+1.7182)
Duration: 2 minutes 15 seconds
1 1000000 Games rolled with Variance Reduction.
Dice Seed: 271828
Moves: 3-ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.19.211.pre-release

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