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In loser chess, the rules are the same as in classical chess but for the following points:

  • The king behaves just as any normal piece and can be captured. There is no check, no checkmate, and the king being under attack is not an issue if you want to castle.

  • If you can capture a piece, you have to capture it.

  • If you can capture several pieces, you can choose to take whichever of them.

  • You win if and only if you are stalemated, i.e. either when your opponent has taken all your pieces or when none of your remaining pieces can move.

My friends Fabi and Mag love chess, but just for fun they also like to play a casual loser chess game once in a while. Indeed, when I met them yesterday evening they were in the middle of a fascinating loser chess game.

When I arrived, Black was thinking hard in this position:

enter image description here

White: Ke1, Qh1, Ra1, Bc1, Nb1, Pa2-b2-c2-d2-e3-f2-g2-h3
Black: Kd8, Qc8, Ra8, Be8, Bf8, Nb8, Ng8, Pa7-b7-c7-e7-f7-g7

Fabi, who was playing White, told me that he had just played his 15th move.

How did they reach this position?

Hint 1:

As Parseltongue explained in his answer, a good question to start with is: what black piece has taken the white rook ?

Hint 2:

We need 6 captures to reach this position. At least 4 of them are actually consecutive.

Hint 3:

The answer is unique, and even the move order is unique.

Hint 4:

The most active king has played exactly 3 more moves than the other king.

Hint 5:

Black never played ...Rh8-g8.

This puzzle is an entry in Fortnightly Topic Challenge #40: Retrograde Analysis.

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    $\begingroup$ This is so much more difficult than it initially looks, lol $\endgroup$
    – Jafe
    Oct 22, 2018 at 20:05
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    $\begingroup$ @Parseltongue The rules say "the king being or not under attack is not an issue if you want to castle". So it looks like castling is definitely allowed. $\endgroup$
    – Jafe
    Oct 22, 2018 at 20:28
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    $\begingroup$ @InstantSoup if you try Loser Chess yourself, that is clearly what you will want to do. But for the sake of this puzzle, I wouldn't claim that Fabi nor Mag are smart Loser Chess players. $\endgroup$
    – Evargalo
    Oct 23, 2018 at 16:55
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    $\begingroup$ Thanks for the clarification. It didn't seem possible if the players were any good. $\endgroup$
    – Instantsoup
    Oct 23, 2018 at 17:09
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    $\begingroup$ We basically worked together. I wish we could split the check mark! $\endgroup$
    – Instantsoup
    Oct 26, 2018 at 23:13

3 Answers 3

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Here is the solution:

1. h3 h5 2. e3 h4 3. Ne2 Rh5 4. Rh2 Rb5
5. Ng3 hxg3 6. Bxb5 gxh2 7. Bxd7+ Kxd7 8. Ke2 Qe8
9. Kf3 Ke6 10. Kg3 Bd7 11. Kxh2 Qc8 12. Qh1 Be8
13. Kg1 Kd7 14. Kf1 Kd8 15. Ke1

https://lichess.org/pXKoC1V0

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    $\begingroup$ Wow! We both found a 19 move solution using very similar ideas at the same time... I think I can incorporate the pawn idea of yours to get it down further! $\endgroup$
    – Parseltongue
    Oct 24, 2018 at 18:44
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    $\begingroup$ Actually move 13. d3, in your newest 19 move solution, is a mistake. The pawn has to take on e3 $\endgroup$
    – Parseltongue
    Oct 24, 2018 at 18:50
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    $\begingroup$ Yep. Edited a new response. This is so difficult. $\endgroup$
    – Instantsoup
    Oct 24, 2018 at 18:58
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    $\begingroup$ @Instantsoup, I need another one of your brain blasts. Try having the knight take the rook either on h2 or g1. There are some insane setups I've been experimenting with which allows this to happen $\endgroup$
    – Parseltongue
    Oct 25, 2018 at 21:51
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    $\begingroup$ Thanks for your contribution! I have removed your previous incorrect answers, so that the accepted answer shows only the correct answer. In the future, if you are submitting a new answer (substantially different from the previous), please submit it separately as another answer. Our goal is to make this easily readable and understandable by someone who finds this question years from now, so only the information that is currently relevant should be visible (you can always look at the edit history to see the rest). $\endgroup$
    – GentlePurpleRain
    Nov 1, 2018 at 16:27
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POST-MORTEM: Absolutely relieved to say that, after a week of this puzzle keeping me up at night, the solution was finally found in a multi-day collaboration with @instantsoup. InstantSoup found all the key insights all at once! Can't wait for the next puzzle!

Solution here: https://lichess.org/study/xQK1BQqn

The really insane thing about this puzzle, is that it was possible to get a 17, 18, 19, 20, and 21-move solution, each of which shared very few moves in common with the actual solution.

I'll keep working on this throughout the week. It's really, really difficult.

Here's a solution in 23 moves. I'm sure there are ways to trivially reduce this, but I'm really time constrained

1. e3 Nh6 2. Nf3 Rg8 3. Bd3 d5 4. Bxh7 Bd7
5. Bxg8 Nxg8 6. O-O Qc8 7. Ne1 Kd8 8. Kh1 Bb5
9. Na3 Bxf1 10. Nb5 Bxb5 11. Nf3 d4 12. Nxd4 Bd7
13. Ne2 Bb5 14. Nc3 Be8 15. Nb1 Kd7 16. h3 Na6
17. Kh2 Nb8 18. Qh1 Na6 19. Kg1 Nb8 20. Kf1 Na6
21. Ke2 Nb8 22. Kd1 Kd8 23. Ke1

Here's a playthrough of the game: https://lichess.org/MSRNxs3w

EDIT 2: Alright, I just shaved the solution down to 21 moves (the previous solution was actually 23). Here is the new sequence:

1. e3 Nh6 2. Nf3 d5 3. Be2 Bd7 4. O-O Bc6 5. h3 Rg8 6. Kh2 Kd7 7. Ne1 Ke6 8. Na3 Qc8 9. Bd3 Bb5 10. Bxh7 Bxf1 11. Bxg8 Nxg8 12. Nb5 Bxb5 13.
Nf3 Be8 14. Qh1 d4 15. Nxd4+ Kd7 16. Kg1 Na6 17. Ne2 Nb8 18. Nc3 Na6
19. Nb1 Nb8 20. Kf1 Kd8 21. Ke1

Here is the link: https://lichess.org/eDLWyCfe#0

EDIT 3: I got it down to 20 moves... slow and steady:

1. e3 d5 2. Na3 Kd7 3. Ke2 Ke6 4. Nf3 Nh6
5. Ne1 Rg8 6. Kf3 Bd7 7. Bd3 Bb5 8. Bxh7 Ba6
9. Bxg8 Nxg8 10. Rf1 Bxf1 11. Nb5 Bxb5 12. h3 Bd7
13. Ke2 Be8 14. Nf3 d4 15. Nxd4+ Kd7 16. Qh1 Qc8
17. Ke1 Nh6 18. Ne2 Ng8 19. Nc3 Kd8 20. Nb1

The playthrough: https://lichess.org/2UT38oCl

EDIT 4: 19 moves. Introduction of completely new idea: the rook must be captured on h2!

1. e3 d5 2. Ne2 Nh6 3. Ng3 Rg8 4. h3 Kd7
5. Rh2 Ke6 6. Bd3 Bd7 7. Bxh7 Be8 8. Bxg8 Nxg8
9. Qe2 Qd6 10. Ne4 Qxh2 11. Nd6 Qxd6 12. Qf1 Qd7
13. Ke2 Qc8 14. Kd3 d4 15. Kxd4 Kd7 16. Kd3 Na6
17. Ke2 Nb8 18. Ke1 Kd8 19. Qh1

https://lichess.org/9ugXLNHh

EDIT 5: Down to 17 moves, but ends on black

1. e3 d5 2. Ne2 Kd7 3. h3 Ke6 4. Ng3 Nh6
5. Rh2 Qd6 6. Ne4 Qxh2 7. Nd6 Qxd6 8. Bd3 Rg8
9. Bxh7 Qd8 10. Bxg8 Nxg8 11. Ke2 Kd7 12. Kd3 d4
13. Kxd4 Na6 14. Qh1 Nb8 15. Kd3 Na6 16. Ke2 Nb8
17. Ke1 Ke8

https://lichess.org/PEln3GM6

EDIT 6: 18 move solution with a completely new idea... this has to be the breakthrough. Rook must be captured by the pawn on h2!

1. e3 { A00 Van't Kruijs Opening } d5 2. Nf3 Kd7 3. Bd3 h5 4. Ke2 Ke6 5. h3 Nf6 6. Rh2 Bd7 7. Qh1 Be8 8. Ke1 Kd7 9. Be2 d4 10. Nxd4 h4 11. Bd1 Qc8 12. Ne2 Rh5 13. Ng3 hxg3 14. Bxh5 gxh2 15. Qxh2 Nxh5 *

https://lichess.org/3CdrZNbS

One key thing to notice:

  • The Black bishop cannot move, and therefore cannot have taken the white Rook. This means that the white rook had to have been captured on f1 (likely by the white bishop) or h2.
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    $\begingroup$ Good job! In your final position the bK is on d7 instead of d8, but you can easily fix that with all the spare black moves. Indeed, 'who has taken the wR?' is the decisive question for solving this. $\endgroup$
    – Evargalo
    Oct 23, 2018 at 5:53
  • $\begingroup$ @Parseltongue I think the 20 move solution has an error at move 15, black would have to Qxd4. $\endgroup$
    – Instantsoup
    Oct 24, 2018 at 18:34
  • $\begingroup$ I think your 19 move is broken at 4 h3... would have to be followed by Bxh3. $\endgroup$
    – Instantsoup
    Oct 24, 2018 at 18:45
  • $\begingroup$ You're right, but there's a move order that resolves that (get king out first.) I'll post new solution soon $\endgroup$
    – Parseltongue
    Oct 24, 2018 at 18:51
  • $\begingroup$ I updated the end of your 17 move to fit the ending position of the board. lichess.org/RKxZBpuu $\endgroup$
    – Instantsoup
    Oct 24, 2018 at 19:24
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Parseltongue and InstantSoup joint efforts solved the problem.

Let me just add an animated diagram to show the sequence:

enter image description here

It is the only solution, and the move order is unique as well.

As a remark, a slightly better version of the problem is the one I published in 2006: put the bK on c8 and the bQ on d8, and search for a proof game in 15.0 moves, i.e. find the game that arrived in this position after Black's 15th move.

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