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x + y Backgammon Puzzle -- Solution

Posted By: Nack Ballard
Date: Saturday, 4 September 2010, at 9:03 a.m.

In Response To: x + y Backgammon Puzzle -- Solution (pierrez)

Thanks, Pierre, for the clarification. I thought your "preamble" was just an explanation of the difficulties that you encountered as a consequence of your 3-or-more interpretation of White's roll caveat, and then you went on to describe your variant (in the form of three changes to mine).

If I understand correctly now, the puzzle you are proposing does retain minimum x + y as the primary objective, but with the following four changes:

  • all wins are bargammons
  • same Blue starting position for minimum and maximum pipcount difference
  • same sequence of dice for both sides in both configurations.
  • White never rolls smaller than a 3 (instead of 2) on either die.

For your variant (the 3+ instead of 2+ being the pivotal difference), I couldn't find a 16 value for x + y either. (Almost, but not quite.) Indeed, I believe I can prove 16 impossible. So, 18 it is.

I slightly prefer the solution pair below, because the smaller pipcount difference is closer to zero. (However, the difference in differences is 71 in both cases, so I can't claim it to be a clear improvement.)

White wins a Bar-G in 5 + 4 = 9 rolls39


eX ' ' ' ' ' ' ' ' ' ' '

 'bO2O1O '1O ' ' ' ' ' '

Blue wins a Bar-G in 5 + 4 = 9 rolls38


White wins a Bar-G in 5 + 4 = 9 rolls110


1X ' ' ' 'cX ' ' ' ' '1X

 'bO2O1O '1O ' ' ' ' ' '

Blue wins a Bar-G in 5 + 4 = 9 rolls38


I'll leave Blue's 9-roll (i.e., 5 + 4 roll) bargammoning sequence as an exercise for the reader. (Blue never rolls larger than a 2 on either die, White never rolls smaller than a 3 on either die.) There's a little trick.

I was unable to reduce that last pip (from 1 to zero) in the pipcount difference of the left-hand position while maintaining the 71 diff/diff, but it was fun trying. :)

Nack

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