6

An easier version of this

White has 20 available moves at the start of the game (from the standard position). After 1.h3 h5, white has 19 available moves. (The h pawn has lost the ability to move two squares, the knight can no longer move to h3, and the rook now has one new move, to h2.)

What is the longest sequence of moves whereby white has at least 1 available move fewer than on the previous turn?

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  • I'm also curious to hear whether this problem has been posed before. Coming up with original problems is hard :) Commented May 1, 2023 at 0:55

4 Answers 4

8

It seems to have been missed so far that f3 is also a possible reducing move it gains a king move, but loses both a knight and a pawn move. Therefore, we have a lot more choice and, in fact, a full 20 moves are possible:

[FEN ""]

1. a3 a6 2. f3 a5 3. h3 b6 4. b4 a4 5. b5 e6 6. g3 Bb4 7. g4 g5 8. Bg2 Bc3 9. Bf1 Qf6 10. f4 Qxf4 11. Rh2 Qxh2 12. h4 Qxg1 13. h5 Ke7 14. h6 Kf6 15. e3 Bxd2+ 16. Kxd2 Qf2+ 17. Kd3 Qxe3+ 18. Kc4 d5+ 19. Kb4 c5+ 20. bxc6 Qc5#
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  • And now the same as shortest proof game :-) (Just joking, how shall one avoid swapping a3/f3/h3?) Commented Apr 28, 2023 at 18:23
  • That's amazing. I did have F3 in my answer but didn't see b4. Commented Apr 29, 2023 at 22:15
  • I particularly love move 6, where white gains axb and Bg2 but loses g3, d3 and d4. Commented May 1, 2023 at 0:51
3

I think I can get 7 moves like this:

[FEN ""]
1. h3 h5 2. Rh2 h4 3. a3 a5 4. Ra2 a4 5. f3 c6 6. Kf2 Qb6+ 7. e3 Qxe3+
  1. h3 h5 (19)
  2. Rh2 h4 (18)
  3. a3 a5 (17)
  4. Ra2 a4 (16)
  5. f3 c6 (15)
  6. Kf2 Qb6+ (3)
  7. e3 Qxe3+ (1)
2

From the beginning position, in this case only a3 and h3 have a negative effect, and even with swapped colors, the opponent lacks one tempo to capture a rook. A check might be given, thought, thus a sequence might be -20- 1.a3 e6 -19- 2.h3 Qh4 -18- 3.d3 Bb4+ -6- 4.c3 Bxc3+ -4- 5.Nd2 Bxd2+ -3- 6.Qxd2 Qxf2+ -2- 7.Kd1. One move might be addable...Reeeeewan! :-)

1

I raise the bar to 8 moves, for Hauke.

[FEN ""]

1. a3 a5 2. Ra2 a4 3. h3 h5 4. Rh2 h4 5. f3 e6 6. Rh1 Bc5 7. Rh2 Bd4 8. Rh1 Bf2+
  1. a3 a5: 19
  2. Ra2 a4: 18
  3. h3 h5: 17
  4. Rh2 h4: 16
  5. f3 e6: 15
  6. Rh1 Bc5: 14
  7. Rh2 Bd4: 13
  8. Rh1 Bf2+: 1

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