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Rollout and comments

Posted By: Timothy Chow
Date: Saturday, 27 May 2017, at 12:24 a.m.

In Response To: I wish I had five 6's to play (Timothy Chow)





White is Player 2

score: 0
pip: 161
Unlimited Game
pip: 164
score: 0

Blue is Player 1
XGID=-a-a-AD-C--AdDa--c-cb---B-:0:0:1:66:0:0:0:0:10
Blue to play 66

1.Rollout124/18(2) 11/5 8/2eq: +0.259
Player:
Opponent:		56.39% (G:13.82% B:0.48%)
43.61% (G:9.92% B:0.36%)		Conf.: ± 0.013 (+0.246...+0.272) - [100.0%]
Duration: 24 minutes 51 seconds
2.Rollout124/18(2) 13/7 11/5eq: +0.216 (-0.043)
Player:
Opponent:		55.09% (G:13.23% B:0.47%)
44.91% (G:9.45% B:0.38%)		Conf.: ± 0.013 (+0.203...+0.229) - [0.0%]
Duration: 26 minutes 52 seconds
3.Rollout124/18 13/7(2) 11/5eq: +0.133 (-0.126)
Player:
Opponent:		52.85% (G:16.67% B:0.94%)
47.15% (G:13.17% B:0.63%)		Conf.: ± 0.014 (+0.119...+0.147) - [0.0%]
Duration: 24 minutes 15 seconds
4.Rollout124/18(2) 13/7(2)eq: +0.102 (-0.157)
Player:
Opponent:		52.81% (G:12.13% B:0.40%)
47.19% (G:10.96% B:0.39%)		Conf.: ± 0.014 (+0.088...+0.116) - [0.0%]
Duration: 29 minutes 16 seconds
1 1296 Games rolled with Variance Reduction.
Dice Seed: 271828
Moves: 3-ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.19.208.pre-release

I have a few problems like this one in my files, but this is the first one where the bot play includes 8/2, and so it wasn't on my radar screen. The fact that slotting 13/7 leaves a double direct shot doesn't by itself disqualify it from consideration (see Jake's position below) but White has made her 5pt already so getting hit on our 7pt is costly.

The first position of this type that I recall encountering was Problem 24 in Robertie's 501 Essential Backgammon Problems (21$-55P-66):

Money Game
Blue to play 66
White147

Blue164
GNUBg Id: jM/BATDQc+QBMA:cAkbAAAAAAAA
XGID=-b---AD-C--AcD---c-e--b-B-:0:0:1:66:0:0:0:0:10

The bot play is 24/18 13/7(2) 11/5 (I think that this may be an exception to the "rule" not to separate your back checkers on the third roll if the opponent has just played 55). The most obvious alternative play, 24/18(2) 13/7 11/5, rolls out as a blunder, and perhaps surprisingly, the second-best play is 13/7(3) 11/5—a play not considered by Robertie, who regards three of the four 6's as "forced": 24/18, 13/7, 11/5.

Recently, in a Gammon Village article called "Hard Ten," Jake Jacobs posed the following position (21$-32S-66):

Money Game
Blue to play 66
White162

Blue164
GNUBg Id: 4HPkASLQc+QBMA:cAkbAAAAAAAA
XGID=-a--aAD-C--AdDa--c-e----B-:0:0:1:66:0:0:0:0:10

Here, the bot plays 24/18(2) 13/7 11/5, despite leaving a double direct shot. Jacobs mentions that Joe Dwek's Backgammon For Profit contains another problem of this type, but I don't own that book. Perhaps some BGO reader can post that position here, along with bot analysis.

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